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Direct monitoring of active geohazards: Emerging geophysical tools for deep-water assessments
Abstract
Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing
communication links, information, and energy supplies. Despite their global importance, these
networks are vulnerable to damage by a number of natural seafloor hazards, including landslides,
turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution
reflection seismic and side-scan sonar, are commonly employed in geohazard assessments.
These conventional tools provide essential information for route planning and design; however, such
surveys provide only indirect evidence of past processes and do not observe or measure the geohazard
itself. As such, many numerical-based impact models lack field-scale calibration, and much
uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent
advances in technology now enable a step change in their understanding through direct monitoring.
We outline some emerging monitoring tools and how they can quantify key parameters for deepwater
geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on
evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic
Doppler current profiling provides new insights into the structure of turbidity currents, whereas
instrumented mobile sensors record the nature of movement at the base of those flows for the first
time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed
measurements of parameters such as strain across large areas of seafloor. These techniques provide
valuable new measurements that will improve geohazard assessments and should be deployed in a
complementary manner alongside conventional geophysical tools.
... On the other hand, it can provide an effective early warning for seabed instability caused by offshore oil and gas exploitation. Therefore, geotechnical engineering monitoring of offshore sites is becoming more and more common, such as using insitu pressure gauges and inclinometers to understand in-situ-specific slope stability issues [23]. In addition, multi-session seafloor surveys using high-resolution multibeam systems ...
... The submarine landslide in-situ investigation methods include geotechnical monitoring, repeated seafloor surveys, water column imaging, acoustic doppler current profilers (ADCP), mobile sensors, sub-surface timelapse, seismological networks, cabled systems, etc. (as shown in Figure 4). [23][24][25][26], at active pro-deltas [27,28], in areas with shallow water, and in large displacement conditions [29]. In addition to using the above-mentioned equipment to measure changes in seabed topography, instruments such as ocean bottom seismometers (OBS) can also be used for monitoring, providing information on the timing and nature of slope failure. ...
... On the other hand, it can provide an effective early warning for seabed instability caused by offshore oil and gas exploitation. Therefore, geotechnical engineering monitoring of offshore sites is becoming more and more common, such as using in-situ pressure gauges and inclinometers to understand in-situ-specific slope stability issues [23]. In addition, multi-session seafloor surveys using high-resolution multibeam systems reveal the magnitude and frequency of seafloor landslides in multiple systems around the world, such as in deep-water (200-300 m) submarine canyons [23][24][25][26], at active pro-deltas [27,28], in areas with shallow water, and in large displacement conditions [29]. ...
Submarine landslides have attracted widespread attention, with the continuous development of ocean engineering. Due to the recent developments of in-situ investigation and modelling techniques of submarine landslides, significant improvements were achieved in the evolution studies on submarine landslides. The general characteristics of typical submarine landslides in the world are analyzed. Based on this, three stages of submarine landslide disaster evolution are proposed, namely, the submarine slope instability evolution stage, the large deformation landslide movement stage, and the stage of submarine landslide deposition. Given these three stages, the evolution process of submarine landslide disaster is revealed from the perspectives of in-situ investigation techniques, physical simulation, and numerical simulation methods, respectively. For long-term investigation of submarine landslides, an in-situ monitoring system with long-term service and multi-parameter collaborative observation deserves to be developed. The mechanism of submarine landslide evolution and the early warning factors need to be further studied by physical modelling experiments. The whole process of the numerical simulation of submarine landslides, from seabed instability to large deformation sliding to the impact on marine structures, and economizing the computational costs of models by advanced techniques such as parallel processing and GPU-accelerators, are the key development directions in numerical simulation. The current research deficiencies and future development directions in the subject of submarine landslides are proposed to provide a useful reference for the prediction and early warning of submarine landslide disasters.
... For instance, Ercilla et al. (2021) and Camargo et al. (2019) conducted reviews on marine geohazards, but the uncertainties involved were not analysed. Clare et al. (2017) focused only on uncertainties in geohazard variables whose quantities are highly difficult to reduce due to lack of fieldscale validation. These authors limited their enquiries to landslides, flows, underground fluid and gas flows, and scour hazards; they did not explore in depth the sources of uncertainty. ...
... Talling et al. (2014) pointed out that, due to the infrequent nature of submarine landslides and the possibility that sedimentary records do not register the events due to reworking, the estimation of landslide frequencies, based on these records, is limited. Clare et al. (2017) mentioned that many numerical based models lack field-scale calibration, and much uncertainty exists about quantities such as the initial soil volumes involved. The authors added that the recurrence time of these events may be far too long to be captured by monitoring techniques. ...
... For instance, Bransby et al. (2010) and Bruschi et al. (2014) stated that considerable uncertainty exists on the mechanics of migration of sediments and depth such as sand waves. Clare et al. (2017) pointed out that the threshold condition for mobility or scour, rate of bedform migration, form and location of scour initiation, rate and extent of scour inception and evolution as the quantities for which uncertainty is large. Han, Chen, and Sun (2019) have mentioned that, due to the complexity of fluid flow, it is rather challenging to theoretically develop precise physical scouring models. ...
By exhaustively reviewing the literature related to marine geohazards, this paper reports on their uncertainties. Examples of marine geohazards include submarine landslides, fluid flows in the underground, scour events, and seabed gouging by ice. Key uncertain variables of interest to marine geohazard assessments are identified and structured by relating a framework defining the main generic components of any risk description to the task of describing risk in the marine geohazards field. Furthermore, issues related to the sources of uncertainty are scrutinised and some recommendations on how to address the identified large uncertainties in geohazard risk assessments are made. Specific considerations are proposed for analysing geohazards in the Arctic, where exploration and development activities are currently regaining momentum. Ultimately, based on the large uncertainties identified, we also strive to identify knowledge gaps to orientate scientific research efforts in the field of marine geohazards.
... Monitoring turbidity currents poses several challenges because deploying instruments on the deep seafloor is logistically challenging, flows may occur infrequently, and the powerful nature of flows can damage the instruments intended to measure them (e.g. Inman et al., 1976;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017;Lintern et al., 2019). Despite these challenges, several studies have prevailed to provide direct measurements of turbidity currents, including seminal field campaigns using point current meters (that measured velocity at one elevation in the water column), in settings ranging from active river-fed fjords (Hay et al., 1982Prior et al., 1987;Syvitski and Hein, 1991;Bornhold et al., 1994), lakes (Lambert and Giavanoli, 1988) and deep-sea submarine canyons (Inman et al. 1976;Shepard et al., 1977;Khripounoff et al., 2003Khripounoff et al., , 2009Vangriesheim et al., 2009). ...
... Modern turbidity current monitoring campaigns typically integrate multiple sensors and tools, such as multi-beam sonar (imaging the water column), optical back-scatter sensors (to detect suspended particles), acoustic monitoring transponders (to determine seafloor movement), sediment traps (to collect suspended sediment) (Lintern and Hill, 2010;Xu, 2011;Khripounoff et al., 2012;Hughes Clarke, 2016;Lintern et al., 2016;Clare et al., 2017;Paull et al., 2018;Lintern et al., 2019;Hage et al., 2019;Maier et al., 2019a&b). The tools that can be used to measure turbidity currents are partly covered by a number of reviews (Xu, 2011;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017). ...
... Modern turbidity current monitoring campaigns typically integrate multiple sensors and tools, such as multi-beam sonar (imaging the water column), optical back-scatter sensors (to detect suspended particles), acoustic monitoring transponders (to determine seafloor movement), sediment traps (to collect suspended sediment) (Lintern and Hill, 2010;Xu, 2011;Khripounoff et al., 2012;Hughes Clarke, 2016;Lintern et al., 2016;Clare et al., 2017;Paull et al., 2018;Lintern et al., 2019;Hage et al., 2019;Maier et al., 2019a&b). The tools that can be used to measure turbidity currents are partly covered by a number of reviews (Xu, 2011;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017). ...
Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (<<2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended modifications to improve design by simplifying mooring configurations, minimizing surface area and enhancing seafloor stability. Finally, we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as feet and ballast that can be ejected. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, in a wider range of settings.
... Monitoring turbidity currents poses several challenges because deploying instruments on the deep seafloor is logistically challenging, flows may occur infrequently, and the powerful nature of flows can damage the instruments intended to measure them (e.g. Inman et al., 1976;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017;Lintern et al., 2019). Despite these challenges, several studies have prevailed to provide direct measurements of turbidity currents, including seminal field campaigns using point current meters (that measured velocity at one elevation in the water column), in settings ranging from active river-fed fjords (Hay et al., 1982Prior et al., 1987;Syvitski and Hein, 1991;Bornhold et al., 1994), lakes (Lambert and Giavanoli, 1988) and deep-sea submarine canyons (Inman et al. 1976;Shepard et al., 1977;Khripounoff et al., 2003Khripounoff et al., , 2009Vangriesheim et al., 2009). ...
... Modern turbidity current monitoring campaigns typically integrate multiple sensors and tools, such as multi-beam sonar (imaging the water column), optical back-scatter sensors (to detect suspended particles), acoustic monitoring transponders (to determine seafloor movement), sediment traps (to collect suspended sediment) (Lintern and Hill, 2010;Xu, 2011;Khripounoff et al., 2012;Hughes Clarke, 2016;Lintern et al., 2016;Clare et al., 2017;Paull et al., 2018;Lintern et al., 2019;Hage et al., 2019;Maier et al., 2019a&b). The tools that can be used to measure turbidity currents are partly covered by a number of reviews (Xu, 2011;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017). ...
... Modern turbidity current monitoring campaigns typically integrate multiple sensors and tools, such as multi-beam sonar (imaging the water column), optical back-scatter sensors (to detect suspended particles), acoustic monitoring transponders (to determine seafloor movement), sediment traps (to collect suspended sediment) (Lintern and Hill, 2010;Xu, 2011;Khripounoff et al., 2012;Hughes Clarke, 2016;Lintern et al., 2016;Clare et al., 2017;Paull et al., 2018;Lintern et al., 2019;Hage et al., 2019;Maier et al., 2019a&b). The tools that can be used to measure turbidity currents are partly covered by a number of reviews (Xu, 2011;Talling et al., 2013;Puig et al., 2014;Clare et al., 2017). ...
Turbidity currents transport globally significant volumes of sediment and organic carbon into the deep-sea and pose a hazard to critical infrastructure. Despite advances in technology, their powerful nature often damages expensive instruments placed in their path. These challenges mean that turbidity currents have only been measured in a few locations worldwide, in relatively shallow water depths (<<2 km). Here, we share lessons from recent field deployments about how to design the platforms on which instruments are deployed. First, we show how monitoring platforms have been affected by turbidity currents including instability, displacement, tumbling and damage. Second, we relate these issues to specifics of the platform design, such as exposure of large surface area instruments within a flow and inadequate anchoring or seafloor support. Third, we provide recommended improvements to improve design by simplifying mooring configurations, minimising surface area, and enhancing seafloor stability. Finally we highlight novel multi-point moorings that avoid interaction between the instruments and the flow, and flow-resilient seafloor platforms with innovative engineering design features, such as ejectable feet and ballast. Our experience will provide guidance for future deployments, so that more detailed insights can be provided into turbidity current behaviour, and in a wider range of settings.
... Unlike mass-movement processes on land, where there are actual eyewitness accounts, a direct link between the trigger mechanisms and associated submarine slope failure events is difficult to establish as the slope regions are remotely located and largely unmonitored (Urlaub et al. 2013). Repeat bathymetry surveys have emerged as one of the promising tools for direct monitoring of potential marine geohazards in shallow and deep waters (Clare et al. 2017) and a detailed listing is presented as supplementary information (Table S1). Repeat multibeam surveys have led to the discovery of submarine landslides in the Oguué River submarine delta, Gabon (Biscara et al. 2012), Offshore Nice, France (Kelner et al. 2016), Monterey Canyon, California, USA (Smith et al. 2007), Fraser River Delta, Canada (Lintern et al. 2016), Squamish submarine delta, Canada (Hughes Clarke et al. 2012;Clare et al. 2016), Westerschelde Estuary, The Netherlands (Mastbergen et al. 2016), and Bute Inlet, British Columbia (Heijnen et al. 2020). ...
... The repeat bathymetry surveys have shown that dynamic processes operate over a few years and lead to variation in seafloor depths in different marine environments (Smith et al. 2007;Casalbore et al. 2011;Biscara et al. 2012;Hughes Clarke et al. 2012;Clare et al. 2016Clare et al. , 2017Kelner et al. 2016;Mastbergen et al. 2016;Lintern et al. 2016;Vendettuoli et al. 2019). The maximum variation in the seafloor is of the order of 15-20 m in most cases (Table S1). ...
Submarine mass wasting in continental margins poses a significant threat to offshore installations, submarine communication cables, and coastal communities due to tsunamis. Based on the analysis of time-lapse geophysical data, we report a very recent submarine slope failure and associated mass transport deposit (MTD) in the Krishna-Godavari (KG) basin, the Bay of Bengal that occurred between January 2009 and December 2015. The head scarp shows erosion of ~ 160 m thick sedimentary strata with an estimated volume of ~ 11 km3. The fan-shaped MTD, located at water depths of 950 to 1100 m, shows a spatial coverage of ~ 70 km2 (volume is ~ 2.2 km3) with a maximum thickness of ~ 60 m. This is one of the largest submarine slope failures reported from time-lapsed geophysical surveys. We analyzed potential triggers such as floods, earthquakes, and cyclones that could have caused the slope failure. We believe that Cyclone Helen, a category-1 storm, may have contributed to the observed slumping in unstable shelf/slope regions of the KG Basin in November 2013 as the Cyclone eye traversed over the head of the slump. However, we cannot rule out other factors such as the extreme flooding events (2010 and 2013) and a 6.0 magnitude earthquake in the Bay of Bengal in May 2014. The study shows that pre-conditioning of sediments is an important factor in the assessment of deepwater geohazards, and areas with relatively moderate external triggers are also at high risk.
... Oceans are of great importance for the living beings on the Earth as they provide abundant resources and space. The sparsity of valid observation methods leads to a biased recognition of oceans, limiting the availability of information on subsea geohazards [1], Earth's interior structure [2], underwater seismology [3], and oceanography [4]. Hence, exploiting marine observation techniques is necessary for long-term sustainable development. ...
... As shown in Figure 1c, both the transmitted and reflected beams interfering with each other in the cavity multiply, leading to precise fringes with high spectral resolution. The phase difference of the FPI is given as, (1) where is the wavelength of the incident light, is the refractive index of the cavity, and is the length of the cavity. When the perturbation is introduced to the cavity, the optical phase difference is influenced by the changes in the cavity length and refractive index, resulting in a shift in wavelength. ...
Submarine optical cables, utilized as fiber-optic sensors for seismic monitoring, are gaining increasing interest because of their advantages of extending the detection coverage, improving the detection quality, and enhancing long-term stability. The fiber-optic seismic monitoring sensors are mainly composed of the optical interferometer, fiber Bragg grating, optical polarimeter, and distributed acoustic sensing, respectively. This paper reviews the principles of the four optical seismic sensors, as well as their applications of submarine seismology over submarine optical cables. The advantages and disadvantages are discussed, and the current technical requirements are concluded, respectively. This review can provide a reference for studying submarine cable-based seismic monitoring.
... An overview of conventional tools for monitoring deposits of sediment transport processes and emerging geophysical tools to directly monitor offshore sediment transport. Adapted from Clare et al. (2017Clare et al. ( ). et al., 1999Hsu et al., 2008;Carter et al., 2009Carter et al., , 2012Carter et al., , 2014Cattaneo et al., 2012;Gavey et al., 2017;Pope et al., 2017a,b;Talling et al., 2021). However, cable breaks will not record less powerful (i.e. ...
... 10s per year) interact with near-bed currents will further complicate sediment transport and therefore represent a key setting for the focus of future direct field monitoring campaigns. Clare et al., 2017Clare et al., , 2020 should also be a focus of future work to enable measurements in some of the largest (>1000s of km) submarine canyons. ...
Submarine canyons globally incise 11 % of the seafloor on continental slopes, and provide conduits for sediment and associated particle transport from the shallow marine realm to the deep-sea. Turbidity currents are thought to be the primary mechanism of sediment transport. An individual turbidity current can transport more than ten times the annual sediment for all the world’s rivers at speeds of up to 20 ms−1 . These fast flow velocities coupled with high sediment concentration mean turbidity currents can be destructive; posing a threat to subsea infrastructure. Understanding the frequency, timing and magnitude of turbidity currents (and other mechanisms of sediment transport) is therefore integral to developing our knowledge of particle flux to the deep sea, as well as for geohazard assessments. Despite this, very few measurements of turbidity currents exist, and only over the past decade have technological advancements, such as the use of acoustic Doppler current profilers (ADCPs), been deployed to directly monitor flows in detail. However, deployments are typically of short duration (a few months), use instruments that record at low temporal resolution, or are located distal to the flow source, therefore missing parts of an annual or longer cycles of turbidity current activity. To quantify the frequency and timing of sediment transport in submarine canyons this thesis focuses on unprecedented temporally and spatially extensive geophysical monitoring datasets, specifically using ADCPs in physiographically-diverse settings to show that: (1) Turbidity current activity is primarily controlled by sediment supply. Once sediment is available only minor triggering mechanism are required to initiate a flow; (2) Periods of enhanced turbidity current activity can be anticipated and flow probability is predictable where both sediment supply and triggering mechanism can be quantified; and (3) Even in systems of turbidity current inactivity, the focusing of bottom currents and internal submarine canyon tides are capable of transporting particles to the deep-sea. These findings highlight the highly active and complex nature of sediment transport in submarine canyons.
... Despite continuously emerging tools in marine technology, sampling techniques and in-situ monitoring still generally lag behind the technological advances in geophysical data acquisition (e.g. Clare et al., 2017). The lack of adequate sampling devices constrains our efforts to effectively sample weak layers of submarine landslides. ...
... Several previous studies suggest that morphometry and other characteristics may be similar between cohesive landslides across many orders of magnitude (e.g. Micallef et al., 2008;Moernaut and De Batist, 2011;Urgeles and Camerlenghi, 2013;Casas et al., 2016;Clare et al., 2017), allowing the extrapolation of information from small to larger landslides. Hence, it may be sensible to focus on smaller-scale landslides until deeper sampling is viable. ...
Submarine landslides pose a hazard to coastal communities as they can generate powerful tsunamis, and threaten critical offshore infrastructure such as seafloor cable networks that underpin global communications. Such events can be orders of magnitude larger than their onshore equivalents. Despite the hazard they pose, many aspects of submarine landslides remain poorly understood, such as why they fail on low angle (<2°), seemingly stable slopes. Many studies have proposed that failure on low slope angles, and the large areal extent of submarine landslides, may be controlled by the presence of laterally-extensive weak layers embedded within the slope stratigraphy, which precondition slopes to failure. Little remains known, however, about the characteristics and processes that control and form weak layers. We conducted a comprehensive review of published submarine landslide studies that examine failure planes and apparent weak layers associated with historical and ancient submarine landslides. Based on a new global landslide catalogue that comprises 64 case studies, this review aims to investigate the types of sediment that form weak layers and to understand the controls on their global variability. Existing classification schemes are based on mechanical process(es), and do not readily enable a diagnosis of weak layers from unfailed sediments. Here, a new and complementary classification of weak layers based on lithology is introduced. This classification enables weak layer recognition from sediment cores (including those sampling unfailed sediments), and allows us to attribute failure mechanisms to different environmental settings where distinct types of weak layers are more likely. The results show that failure planes usually form in the vicinity of an interface between distinct lithologies that together comprise a weak layer. The weak layers of 22 of our 64 case studies were related to characteristic sediment sequences within the slope stratigraphy, of which 19 were classified based on direct measurements from sediment cores and in-situ measurements: 16 weak layers were classified as siliciclastic, four as volcaniclastic, and two as fossiliferous sediment sequences. Only three submarine landsides were related to clay-dominated weak layers. In addition, failure along lithological contrasts was inferred for six case studies. Based on global depositional likely locations of these different types of weak layer can be inferred. These include oceanic gateways where long-term circulation can create distinct permeability interfaces within siliciclastic sequences, areas of high productivity where biogenic sediments may dominate, and regions that experience widespread ash fall from volcanic eruptions. We highlight that many submarine landslide studies have historically not collected sediment cores that characterise weak layers within intact sedimentary sequences and instead have focused on characterising the slope failure deposit. As weak layers can collapse or become heavily modified during failure, there is a widespread omission of key information required for geotechnical analysis to determine where and why certain slopes are predisposed to failure. We conclude by highlighting the need to combine detailed geotechnical measurements with sedimentological and geophysical analyses including grain-scaled observations (e.g. micro-Computed Tomography high-resolution 3D imagery), and emphasise the importance of a uniform workflow that will allow for a better comparison between individual studies.
... This research is supported by the Southampton Marine & Maritime Institute (SMMI) Leverhulme Trust "Understanding Maritime Futures" Doctoral Training Programme and is sponsored by Sonardyne International LTD installation and operation of the infrastructure [1], [2] These properties are typically obtained by combining wide area shipbased or underwater vehicle-based Multi-beam Echo Sounder (MBES), Side-scan Sonar (SSS) and sparse in-situ contact measurements (e.g. soil penetration tests, thermal and resistivity probes, stiffness and friction characterisation) or soil sample retrieval [3]. ...
... Neglecting the effects of hydrodynamic lift, the resulting net force can be calculated as the sum of the force due to seafloor currents F C and the difference between the gravity F G and buoyancy F B forces along the slope plane. The force limiting condition to remain stationary after landing on a surface with a slope θ can be represented by (3). ...
Determining the geotechnical seafloor properties needed to plan subsea infrastructure is time consuming and expensive as it requires soil sampling or in-situ contact mea- surements to be made using Remotely Operated Vehicles or ship based systems. To increase the efficiency of such surveys, we introduce a predictive framework for autonomous underwater vehicles (AUV) to determine locations where they can land and make contact measurements. We introduce a geotechnical measurability index that is computed using high, cm-resolution AUV observations. To address the small footprint of high- resolution AUV observations, our method infers the distribution of measurability onto more widely available remote sensed bathymetry that has resolutions of tens of centimeters to metres. Features are extracted from these low-resolution priors using an unsupervised Location-Guided Autoencoder. Geotechnical mea- surability maps are generated using a Bayesian Neural Network that combines these features with the geotechnical measurability calculated from high-resolution AUV observations to infer the measurability over a wide area. The framework is demonstrated using AUV structured light mapping data that was obtained from a 420 × 120m region of the Takuyo Daigo seamount. The data was artificially down sampled to simulate low resolution priors with sub-regions observed at high-resolution. The geotechnical measurability maps generated using the predictive framework preserve details that would otherwise be lost if the measurability index was calculated directly based on low resolution priors.
... Based on the mechanical properties and rheology of the processes, two main groups can be defined: (i) slides/slumps/spreads and (ii) gravity flows. These two groups, with important differences in their pre-and post-failure behavior, occur in all physiographic environments and are efficient transporters of sediment, organic carbon, nutrients, and pollutants [77][78][79][80][81]. They are scale-invariant processes that range greatly in size from the meter scale to many km across ( Figure 4). ...
... Scenarios can be affected by multiple hazardous features, some in a land-marine transition context, and the integration of offshore and onshore observations is essential. The figure was used as idea and base to create a new one including more information from [79]. ...
Offshore geological hazards can occur in any marine domain or environment and represent a serious threat to society, the economy, and the environment. Seismicity, slope sedimentary instabilities, submarine volcanism, fluid flow processes, and bottom currents are considered here because they are the most common hazardous processes; tsunamis are also examined because they are a secondary hazard generated mostly by earthquakes, slope instabilities, or volcanic eruptions. The hazards can co-occur and interact, inducing a cascading sequence of events, especially in certain contexts, such as tectonic indentations, volcanic islands, and canyon heads close to the coast. We analyze the key characteristics and main shortcomings of offshore geological hazards to identify their present and future directions for marine geoscience investigations of their identification and characterization. This review establishes that future research will rely on studies including a high level of multidisciplinarity. This approach, which also involves scientific and technological challenges, will require effective integration and interplay between multiscale analysis, mapping, direct deep-sea observations and testing, modelling, and linking offshore observations with onshore observations.
... Untuk menggelar kabel laut membutuhkan biaya yang tidak sedikit, demikian pula dengan perbaikan apabila terjadi putus di tengah laut. Perbaikan kabel laut bisa menghabiskan jutaan dollar dan waktu yang diperlukan saat perbaikan bisa menyebabkan gangguan (Clare et al., 2017). Kerusakan kabel optik pada kedalaman laut kurang dari 200 meter kemungkinan terjadi antara 65 % sampai dengan 75 % disebabkan sebagian besar oleh kegiatan penangkapan ikan dan pelayaran. ...
... Secara internasional belum ada standar survei rekayasa yang ditujukan untuk surveyor dalam pengerjaan survei dan pemetaan untuk proyek kabel optik bawah laut (Yin-can et al., 2018). Namun demikian sebagian besar perusahaan yang mengoperasikan kabel bawah laut bekerja berdasarkan standard dan sistem manajemen kualitas mengacu pada ISO 9000 dan ISO 9001 serta rekomendasi yang diterbitkan oleh the International Cable Protectiion Committee (ICPC) termasuk memilih jalur kabel, proteksi kabel dan perbaikan kabel (Clare et al., 2017). Survei pemetaan rute kabel bawah laut biasanya dilakukan atas dasar rute terpilih dari desktop study, yang tujuannya adalah menggunakan metode dan peralatan survei secara profesional untuk mendapatkan data batimetri, topologi dan fitur dasar laut (Seabed features and topology), dan parameter geoteknik (Geotechnical properties). ...
Tsunami merupakan kejadian alam yang dipengaruhi oleh aktifitasyang terjadi di dasar laut seperti gempa laut, gunung berapimeletus, dantanah longsor di dasar laut. Indonesia yang berada pada kawasan “Pacific ring of fire” merupakan negara yang rawan akan bencana tsunami. Saat ini, BPPT telah mengembangkan beberapateknologi untuk mendeteksi tsunami, salah satunya dengan Indonesia Cable Base Tsunamimeter (Ina-CBT). Teknologi ini dapat mendeteksi tsunami dengan menggunakan sensor di Ocean Bottom Unit (OBU) yang terpasang di dasar laut dan kemudian OBU dapat mengirimkan sinyal melalui kabel di dasar laut ke stasiun di darat. Pada tahun 2020 ini, BPPT berencana akan melakukan pemasangan empat kabel laut Indonesia (Ina-CBT) di beberapa lokasi yang memiliki potensi tsunami yang cukup besar yaitu segment Labuan Bajo,segment Rokatenda, segment Ibu Kota Negara (IKN) dan segment Cilacap – Krui.Untuk mendukung kegiatan Ina-CBT, perlu dilakukan survei pemetaan dasar laut untuk mengetahui kondisi dasar laut agar didapatkan lokasi penempatan kabel laut yang tepat dan terhindar dari bahaya (hazard) baik pada saat penggelaran maupun untuk keperluan pemeliharaan kabel. Penelitian ini menjelaskan prosedur pelaksanaan survei pemetaan bawah laut untuk perencanaan sistem kabel laut Indonesia (Ina-CBT).Prosedur pelaksanaan survei meliputi lingkup pekerjaan survei, metode survei, wahana dan peralatan survei, proses data hasil survei, dan laporan hasil survei. Pengembangan prosedur survei yang tepat diperlukan untuk menghasilkan data yang berkualitas.
... Untuk menggelar kabel laut membutuhkan biaya yang tidak sedikit, demikian pula dengan perbaikan apabila terjadi putus di tengah laut. Perbaikan kabel laut bisa menghabiskan jutaan dollar dan waktu yang diperlukan saat perbaikan bisa menyebabkan gangguan (Clare et al., 2017). Kerusakan kabel optik pada kedalaman laut kurang dari 200 meter kemungkinan terjadi antara 65 % sampai dengan 75 % disebabkan sebagian besar oleh kegiatan penangkapan ikan dan pelayaran. ...
... Secara internasional belum ada standar survei rekayasa yang ditujukan untuk surveyor dalam pengerjaan survei dan pemetaan untuk proyek kabel optik bawah laut (Yin-can et al., 2018). Namun demikian sebagian besar perusahaan yang mengoperasikan kabel bawah laut bekerja berdasarkan standard dan sistem manajemen kualitas mengacu pada ISO 9000 dan ISO 9001 serta rekomendasi yang diterbitkan oleh the International Cable Protectiion Committee (ICPC) termasuk memilih jalur kabel, proteksi kabel dan perbaikan kabel (Clare et al., 2017). Survei pemetaan rute kabel bawah laut biasanya dilakukan atas dasar rute terpilih dari desktop study, yang tujuannya adalah menggunakan metode dan peralatan survei secara profesional untuk mendapatkan data batimetri, topologi dan fitur dasar laut (Seabed features and topology), dan parameter geoteknik (Geotechnical properties). ...
Tsunami merupakan kejadian alam yang dipengaruhi oleh aktifitasyang terjadi di dasar laut seperti gempa laut, gunung berapimeletus, dantanah longsor di dasar laut. Indonesia yang berada pada kawasan “Pacific ring of fire” merupakan negara yang rawan akan bencana tsunami. Saat ini, BPPT telah mengembangkan beberapateknologi untuk mendeteksi tsunami, salah satunya dengan Indonesia Cable Base Tsunamimeter (Ina-CBT). Teknologi ini dapat mendeteksi tsunami dengan menggunakan sensor di Ocean Bottom Unit (OBU) yang terpasang di dasar laut dan kemudian OBU dapat mengirimkan sinyal melalui kabel di dasar laut ke stasiun di darat. Pada tahun 2020 ini, BPPT berencana akan melakukan pemasangan empat kabel laut Indonesia (Ina-CBT) di beberapa lokasi yang memiliki potensi tsunami yang cukup besar yaitu segment Labuan Bajo,segment Rokatenda, segment Ibu Kota Negara (IKN) dan segment Cilacap – Krui.Untuk mendukung kegiatan Ina-CBT, perlu dilakukan survei pemetaan dasar laut untuk mengetahui kondisi dasar laut agar didapatkan lokasi penempatan kabel laut yang tepat dan terhindar dari bahaya (hazard) baik pada saat penggelaran maupun untuk keperluan pemeliharaan kabel. Penelitian ini menjelaskan prosedur pelaksanaan survei pemetaan bawah laut untuk perencanaan sistem kabel laut Indonesia (Ina-CBT).Prosedur pelaksanaan survei meliputi lingkup pekerjaan survei, metode survei, wahana dan peralatan survei, proses data hasil survei, dan laporan hasil survei. Pengembangan prosedur survei yang tepat diperlukan untuk menghasilkan data yang berkualitas.
... Nevertheless, geotechnical monitoring of offshore sites is becoming more commonplace, such as the deployment of in situ piezometers and tiltmeters to understand slope stability issues at specific locations (Figure 12.7; e.g., Strout & Tjelta, 2005;Stegmann et al., 2012;Clare et al., 2017). Besides, repeated seafloor surveys using highresolution multibeam systems revealed not just the scale but also the frequency of submarine landslides in several systems worldwide (e.g., along the Nice slope (Kelner et al., 2014); at active pro-deltas (Hughes Clarke, 2016; Obelcz et al., 2017); in deepwater submarine canyons Smith et al., 2007)). ...
... ASV image from asvglobal.com. Source: AfterClare et al. (2017). ...
Submarine landslides have been identified in almost all ocean basins worldwide. The largest submarine landslides occur on very shallow slopes and can be far larger than any terrestrial landslide. Submarine landslides can produce tsunami whose far‐reaching effects can rival those produced by earthquake‐tsunamis and threaten increasingly populated coastlines. Even small landslides can damage very expensive and critically important offshore infrastructure, such as pipelines used for oil and gas recovery, and telecommunication cables that now carry over 95% of digital data traffic. A better understanding of submarine landslide processes, including triggering mechanisms, preconditioning factors, timing, and frequency as well as dynamics of submarine landslide, and their consequences are of clear societal and economic importance. Despite their importance, many fundamental submarine landslide processes are still poorly understood. We currently have many studies that have mapped and sampled submarine landslide deposits; however, in order to fill outstanding but key knowledge gaps, future studies may have to go beyond this in order to unravel processes governing submarine landslides with even more interdisciplinary approaches. This chapter provides a very short review about submarine landslide studies, with emphasis on the emerging needs in future landslide research.
... These time-lapse surveys can provide a major advance in understanding of the rate and nature of seafloor change in different settings. Previous examples of marine time-lapse surveys include studies of estuaries (Mastbergen et al., 2016), submarine deltas (Hill et al., 2008;Casalbore et al., 2011;Biscara et al., 2012;Clare et al., 2017;Lintern et al., 2016), continental slopes (Kelner et al., 2016), deep-sea submarine canyons (Smith et al., 2007;Xu et al., 2008;Paull et al., 2018;Mountjoy et al., 2018), submarine channels in fjords (Conway et al., 2012;Normandeau et al., 2014;Gales et al., 2018), and lakes (Corella et al., 2016;Silva et al., 2018). These time-lapse datasets cover seven or fewer repeat surveys, over timescales of months to decades (Table S1), which is much less frequent than the rate at which sediment transport events occur. ...
... As well as carrying globally important volumes of sediment, these flows transport organic carbon, oxygenated waters, nutrients and contaminants that accumulate within submarine channels and downslope at their terminal lobes or submarine fans (Galy et al., 2007;Kao et al., 2010;Gwiazda et al., 2015;Hughes et al., 2015). The often-powerful nature of turbidity currents poses a significant hazard to critical seafloor infrastructure (Carter et al., 2014), which also makes direct monitoring challenging (Inman et al., 1976;Clare et al., 2017). There is a paucity of direct measurements of turbidity currents (Talling et al., 2015), so one typically has to make inferences of past flows based upon the deposits ('turbidites') that are left behind (Hubbard et al., 2014;Jobe et al., 2017). ...
Turbidity currents are powerful flows of sediment that pose a hazard to critical seafloor infrastructure and transport globally important amounts of sediment to the deep sea. Due to challenges of direct monitoring, we typically rely on their deposits to reconstruct past turbidity currents. Understanding these flows is complicated because successive flows can rework or erase previous deposits. Hence, depositional environments dominated by turbidity currents, such as submarine channels, only partially record their deposits. But precisely how incomplete these deposits are, is unclear. Here we use the most extensive repeat bathymetric mapping yet of any turbidity current system, to reveal the stratigraphic evolution of three submarine channels. We re-analyze 93 daily repeat surveys performed over four months at the Squamish submarine delta, British Columbia in 2011, during which time >100 turbidity currents were monitored. Turbidity currents deposit and rework sediments into upstream-migrating bedforms, ensuring low rates of preservation (median 11%), even on the terminal lobes. Large delta-lip collapses (up to 150,000 m3) are relatively well preserved, however, due to their rapidly emplaced volumes, which shield underlying channel deposits from erosion over the surveyed timescale. The biggest gaps in the depositional record relate to infrequent powerful flows that cause significant erosion, particularly at the channel-lobe transition zone where no deposits during our monitoring period are preserved. Our analysis of repeat surveys demonstrates how incomplete the stratigraphy of submarine channels can be, even over just 4 months, and provides a new approach to better understand how the stratigraphic record is built and preserved in a wider range of marine settings.
... Discussed in 3.3% of the analyzed articles, this category of marine geohazard is related to environments exposed to hydrodynamic forcing (wind-driven, tidal, and thermohaline currents), capable of generating mobile bedforms [138,139]. Mobile bedforms are of critical engineering importance in the placement of submarine pipelines and cables [140]. According to [141], the presence of mobile bedforms implies specific challenges because the stability of a marine pipeline, which is exposed to lateral currents, is one of the major concerns of the pipeline engineer. ...
... More recently, there is a trend towards studies with social perspectives and networks of seafloor observatories for continuous geohazards monitoring [35,54,[217][218][219]. Therefore, direct monitoring represents a complementary manner, alongside conventional techniques in which environmental conditions call for deployment and provide enhanced confidence regarding geohazard assessments [139]. The technology involved in seafloor observatories is developing and should generate continuous data that will complement knowledge that concerns geohazards episodic events that are (in general) recorded in response to cable breaks [53,97,220,221]. ...
Marine geohazard research has developed during recent decades, as human activities intensified towards deeper waters. Some recent disastrous events (e.g., the 2004 Indian Ocean and 2011 Japan tsunamis) highlighted geohazards socioeconomic impacts. Marine geohazards encompass an extensive list of features, processes, and events related to Marine Geology. In the scientific literature there are few systematic reviews concerning all of them. Using the search string 'geohazard*', this bibliometric-based review explored the scientific databases Web of Science and Scopus to analyze the evolution of peer-reviewed scientific publications and discuss trends and future challenges. The results revealed qualitative and quantitative aspects of 183 publications and indicated 12 categories of hazards, the categories more studied and the scientific advances. Interdisciplinary surveys focusing on the mapping and dating of past events, and the determination of triggers, frequencies, and current perspectives of occurrence (risk) are still scarce. Throughout the upcoming decade, the expansion and improvement of seafloor observatories' networks, early warning systems, and mitigation plans are the main challenges. Hazardous marine geological events may occur at any time and the scientific community, marine industry, and governmental agencies must cooperate to better understand and monitor the processes involved in order to mitigate the resulting unpredictable damages.
... For submarine landslides, a development mode that is nurtured over a long period of time, the monitoring technology includes submarine pore pressure monitoring (Strout and Tjelta 2005), submarine erosion and siltation monitoring (Wright et al. 1986), and long-term submarine borehole monitoring (Cacchione et al. 2006), etc. Meanwhile, submarine deformation and sliding monitoring can also play a role in its suddenness (Clare et al. 2017). Submarine pore pressure monitoring technology is primarily achieved through the use of pore pressure monitoring probes, such as seafloor geotechnical probes (Xu et al. 2021) and pressure gauges developed by the French Institute of Oceanography. ...
Submarine landslides, as a significant marine geological phenomenon, pose substantial risks to underwater construction projects such as offshore oil and gas exploitation, submarine pipelines and cables, offshore wind power installations, and even coastal engineering. However, existing reviews on submarine landslides primarily focus on summarizing their triggering mechanisms and flow characteristics from the perspective of the landslides themselves, while rarely addressing their impact on ocean engineering applications, which lacks practical significance. Therefore, following an in-depth analysis of the fundamental characteristics, mechanisms of occurrence, and relevant influencing factors of submarine landslides, this study provides an in-depth exploration into the potential impact of submarine landslides on marine infrastructure through illustrative examples of past submarine landslide incidents. Additionally, it proposes risk assessment methodologies as well as monitoring and mitigation strategies for related projects. The key findings from our research are as follows: (1) There is an urgent need to establish a unified classification standard for types of submarine landslides instead of continuously refining their categorization; (2) The primary distinction between marine engineering and land-based engineering lies in the complex marine dynamic environment that necessitates considerations such as geological conditions, environmental factors, and structural design; (3) Current measures to prevent submarine landslide disasters in marine engineering primarily focus on avoiding hazardous areas during the design and exploration stages, with limited emphasis on post-landslide disaster prevention and mitigation measures; (4) Existing field investigation techniques for studying submarine landslides predominantly concentrate on basic research aimed at identifying already existing occurrences but lack industrial applicability due to being mostly experimental single-object monitoring approaches; (5) Numerical models used to simulate submarine landslides often oversimplify the phenomenon's complexity hindering practical project applications. Henceforth, it is crucial to consider macro–micro interconnected effects when simulating the evolution of underwater landslide movements.
... They involve diverse groups of complex sedimentary processes that are recognised as significant geomorphic processes on slopes (Densmore et al., 1998). Submarine slope instabilities are considered one of the main offshore geohazards since, through the effects of dragging, burial, undermining or direct impact, they can damage infrastructure, such as communication cables or pipelines (Clare et al., 2017;Ercilla et al., 2021, among others). Landslide-generated tsunamis are also real threats to coastal communities (Clare et al., 2019;Ercilla et al., 2021;Harbitz et al., 2014). ...
In this work, over 3620 km2 from the Palomares continental slope, which is located in the W. Mediterranean Sea, was analysed to quantify the impact of recent mass movements on this margin. A total of 936 landslides were identified, mapped and characterised by defining several morphometric variables that outline the accumulated impact of landslides equivalent to 918 km2 and 10.34 km3 of eroded sediment on the continental slope. The smallest event area was 0.0014 km2, whereas the largest event area was 32.48 km2. Smaller scars with a higher headwall gradient tend to dominate when the environment is steeper, and major mass movements are located on open slopes and structural highs. However, the slight or null correlations between variables indicate that a wide range of sizes may occur on any slope gradient and at any depth.
The Palomares continental slope is intensively affected by mass movements. Compared with other passive margins (e.g., the U.S. Atlantic continental margin), landslides mobilised a limited amount of sediment, although it is comparable to other Mediterranean areas where small- to moderate-sized events are characteristic.
The cumulative size distribution can be defined by a power-law function that describes events larger than 0.7 km2 with an exponent of α = 1.269. These results are consistent with those of other published inventories, including onshore cases. This result allows us to assume that the scale-invariant properties of the events are mapped. Scale-invariant properties can be explained by different models; self-organised criticality (SOC) is probably the most assumed by the scientific community, although alternative models may be nominated. Each model has important implications in terms of the landslide distribution and long-term landslide history of any slope. Alternative scenarios, such as submarine slopes, with more precise landslide inventories may contribute to new hazard assessment models that consider scaling exponents derived from size–frequency distributions.
... Conventional geophysical tools for analyzing submarine landslides and rapid sedimentation (modified fromClare et al. 2017;Shan et al. 2022). ...
Submarine landslides are a critical geological process that significantly influences seafloor stability and geological evolution, making them a focal point of geological disaster research. Particularly, rapid sedimentation as a crucial preconditioning factor for submarine landslide initiation is increasingly recognized in contemporary research. However, no comprehensive review currently examines the correlation between rapid sedimentation and submarine landslides. To elucidate the interaction mechanisms between submarine landslides and rapid sedimentation and to provide a new theoretical foundation for the prediction and prevention of geological hazards, the study categorizes three primary sedimentation patterns in marine environments and establishes a detailed historical database of sedimentation-related submarine landslide events. Moreover, the paper systematically reviews and critiques the existing research methods and advancements in addressing rapid sedimentation-related submarine landslides, and a bibliometric analysis of recent research progress is conducted. Finally, the study highlights several challenges in the study of rapid sedimentation and proposes future recommendations, aiming to provide robust support for the in-depth investigation of submarine landslides. ARTICLE HISTORY
... Although long-term, high-density, multi-scale and multi-method observations are costly, these in-situ investigations offer the most valuable and timely data for validating the physical and mathematical models. Further, with the exponentially growing volumes of marine data, there is a strong need to create efficient databases (such as (Clare et al., 2017;Ercilla et al., 2021;Vanneste et al., 2014). bathymetric, submarine landslide, and coastline databases) with a homogeneous standard for identification, interpretation, and characterization. ...
Marine geohazards related to geological features and processes in coastal and offshore environments can cause damage to health, environment, field installations, or loss of life and assets. Given increasing human activities in marine environments and population density on coasts, threats to society from marine geohazards are serious. However, the understanding and awareness of marine geohazards are still severely limited. This paper is aimed to highlight the advances and challenges on marine geohazards. Here, we present an overview of marine geohazard research to discuss the past, present, and future of marine geohazards based on bibliometric analysis and our understanding from an engineering geologist's perspective. This paper summarizes marine geohazard research in the development phase, geographic authorship distribution, related areas and disciplines, keywords, and subjects of focused interest. Trends of significance are synopsized from past events (over the last three decades), present processes, and future (foretelling) marine geohazards. Finally, we provide three recommendations from scientific research and safeguarding society perspectives: (1) intense involvement of engineering geologists, (2) integrated multidisciplinary marine geohazard research, and (3) collaborative efforts in marine geohazard mitigation. There is still a long way to go in protecting the public and infrastructure from marine geohazards.
... One of the advantages of using RTK GNSS is that smaller cm-level measurement uncertainties are achievable (Odijk 2017). In hydrography, this may benefit activities where measurement precision is a critical parameter, such as when charting for tighter underkeel clearance modelling (MNZ 2020) and seabed change-detection monitoring (Clare et al. 2017, Schimel et al. 2015. ...
... Biaya Pemasangan kabel fiber optik bawah laut membutuhkan anggaran yang tidak sedikit, serta biaya perawatan jika ada kendala ditengah instalasi (Clare et al., 2017). Risiko Kerusakan kabel serat optik 65% hingga 75% disebabkan aktivitas penangkapan ikan dan aktivitas pelayaran pada kedalaman di bawah 200 meter. ...
p>Indonesia adalah salah satu negara di dunia yang sering diguncang bencana alam, salah satunya tsunami. Menjadi penting bagi Indonesia untuk memiliki detektor tsunami kabel atau Indonesia Cable Based Tsunameter (Ina-CBT). Pengembangan Teknologi Tsunami Early Warning System di Indonesia (InaTEWS), Teknologi Ina-TEWS terdiri dari: Ina-BUOY, Ina-CBT dan Tomografi. Ina-CBT adalah teknologi sensor deteksi Tsunami yang bekerja mengukur naik atau turunnya permukaan air laut melalui perubahan tekanan air bawah laut. Cable Based Tsunameter (CBT) efektif dalam melacak kemungkinan tsunami yang terjadi akibat gempa tektonik dan dapat mendeteksi perubahan gelombang kecil di bawah permukaan air. Untuk mendukung pengelolaan Ina-CBT yang berkelanjutan, Ina-CBT membutuhkan dukungan dari sisi regulasi. Penelitian ini menyajikan rumusan strategi pengelolaan berkelanjutan untuk penguatan ekosistem pengoperasian Ina-CBT dan pelembagaan sistem pengelolaan instrumentasi Ina-CBT. </p
... The Storegga Slide, for example, located off the Norwegian coast is considered amongst the largest landslide on Earth , with a volume of 2400 to 3200 km 3 , an area of ~95,000 km 2 , and a run out distance of nearly 400 km (Haflidason et al., 2004;Bryn et al., 2005). The environmental and economic significance of subaqueous landslides has been investigated over the last twenty years, with a particular focus on the risk to nearshore and offshore infrastructures, and tsunami hazard modelling investigations (Talling et al., 2014;Clare et al., 2017). The advent of higher resolution geophysical techniques would allow the investigation of seafloors and the bottom of lakes with the same resolution that is possible on land (Savini et al., 2022), for example by using airborne geophysics, and thus improve the mitigation strategies of subaqueous landslides especially for vulnerable coastal communities. ...
Subaqueous spreading, a type of extensional mass transport that is characterized by a ridge and trough morphology, has been documented globally but is poorly understood. Subaqueous spreading is observed on gently inclined surfaces (typically <3°) when sediment bodies experience a sudden reduction of shear strength along their basal plane during clay softening or liquefaction of sands or silty sand sediment. Historically, spreading has been associated with very large landslides, but many unknown aspects of these mass movements have yet to be clarified. Does spreading influences the large catastrophic failure? What are the sedimentological and morphological aspects that contribute in initiating this process? These are some of the research questions that spurred the present work. Here, we introduce a database that incorporates information from thirty-two case studies, and use this to provide key insights into the sedimentary and morphological aspects of subaqueous spreading that will assist in the identification of spreading elsewhere. We find that subaqueous spreading is most common along passive glacial margins, but is also observed along active margins. The occurrence of contourites interlayered with glaciogenic deposits is, in most cases, associated with landslides (or landslide complexes) with spreading morphology. The database shows that seismic loading is commonly suggested to be the dominant trigger mechanism, although more geotechnical observations and modelling analysis would be needed to support this conclusion. We compare subaqueous spreading with terrestrial spreading, in particular to earthquake-related lateral spreading and clay landslides. We find that subaqueous spreading shares the same driving processes and potentially also some of the trigger mechanisms that are associated with the terrestrial spreading cases. Future work will be required to address the association between spreading and its occurrence on some of the largest landslides on Earth, its development mechanism, and its potential hazard implications.
... Untuk menggelar kabel laut membutuhkan biaya yang tidak sedikit, demikian pula dengan perbaikan apabila terjadi putus di tengah laut. Perbaikan kabel laut dapat menghabiskan jutaan dollar dan menyebabkan gangguan listrik [1]. ...
... The volumes associated with surface changes are obtained by integrating the difference in depth over the areas of interest. This approach has been used to detect and monitor co-seismic seafloor displacements (Fujiwara et al., 2017), volcanic eruptions (Caress et al., 2012;Bosman et al., 2014;Ercilla et al., 2021), landslides and their morphological evolution Biscara et al., 2012;Casalbore et al., 2012;Kelner et al., 2016), submarine deltas (Hill et al., 2008;Clare et al., 2017), canyons (Guiastrennec-Faugas et al., 2020) and channels in fjords (Conway et al., 2012;Normandeau et al., 2014;Gales et al., 2019). These data sets include repeated surveys performed over months to decades and are used to characterize specific phenomena ( Figure 5.4.). ...
The publication presents an overview of the type, distribution and impact of marine geohazards in European coastal regions and on several Blue Economy sectors. It highlights novel scientific approaches that broaden our understanding of these hazards. The document points towards relevant actions that would ensure the development of effective risk-mitigation and science-based management practices and policies, such as mapping of the seabed and including geohazards in maritime spatial planning policies, thereby contributing to protect coastal population and economic activities at sea.
... Furthermore, it is important to address the evolution and depositional character of deep-water systems in any offshore activity. Gravity flows in deep-water channels can impact deep-water infrastructure such as submarine cables, pipelines, or the foundations of offshore wind farms (Baker et al., 2016;Clare et al., 2017;Schneider & Senders, 2010). ...
Distal slope and basin depositional systems in deep waters of the Pará-Maranhão Basin, Equatorial Brazil, are investigated using a high-resolution 3D seismic volume, borehole data and multispectral satellite imagery. A Neogene calciclastic submarine fan and a series of channel-levee systems are analysed at water depths of 100 m to 3,500 m. Channel-levee systems have sinuous and straight morphologies and are of different sizes. Their origin is related to turbidity flows sourced and funnelled from the carbonate shelf to submarine canyons and gullies, as well as from areas with marked slope instability. A mixed calciclastic-siliciclastic sediment input is recognised with autochthonous calcarenites and calcilutites comprising the bulk of sediment on the mid and outer continental shelf. Minor amounts of siliciclastic sediment sourced from small rivers occur on the inner shelf. Sedimentation processes of a distally steepened carbonate ramp are discussed considering a general depositional setting dominated by fluctuations in relative sea level. Cross-sectional and planar parameters of mixed calciclastic-siliciclastic channel-levee systems are compared to their siliciclastic counterparts. Morphological results show similarities between calciclastic and siliciclastic channel-levee systems. As a corollary, three types of channel-levee systems are described: (1) channels related to calciclastic submarine fans, (2) low-sinuosity, aggradational channels, and (3) high-sinuosity channels.
... earthquakes, floods, sediment loading, slope instability or tsunamis (Alcantara et al., 2010;Arai et al., 2013;Tappin, 2018;Schulten et al., 2019). Inherent complexity and volatility within the submarine density flows pose a significant challenge to our understanding, thus direct flow monitoring, laboratory experiments and analysis of the internal character of their deposits are all crucial to our comprehension of these submarine processes and aid predictions of some of their behaviors (Marr et al., 2001;Baas and Best, 2002;Sumner et al., 2008;Clare et al., 2017). ...
Textural and structural features of submarine mass density flow sediments are a record of depositional flow
evolution and hydrodynamics. Record of changes in such flow movement are difficult or impossible to detect,
especially in fine sediments, without quantitative grain size analysis. This study shows an automated measurement
system of mineral liberation analysis MLA applied for the first time to grain size distribution. The analysis
concerns heavy minerals extracted from very fine- and fine-grained sandstones of the Cergowa Beds (Lower
Oligocene) in the Polish and Slovak Outer Carpathians, which macroscopically show unclear textural trends and
represent deposits of steady and uniform turbidity currents, possibly initiated by hyperpycnal effluents. The MLA
analysis showed that the tested material represents sediments deposited by flows with developing turbulence.
Simultaneously, the study verified that a turbulence damping contributes to size-density sorting during transport
and demonstrated that maximum rather than average grain size is controlled by flow concentration, therefore
reflects flow hydrodynamics. Besides, MLA measurements recognised variations in flow motion recorded by three
types of grain-size breaks: (i) in high-density flow when capacity-controlled conditions are changing so that high
energy flow erodes the previously deposited material; (ii) at a sharp interface between high-concentration basal
layer and the overriding flow with lower concentration, and (iii) at a transition between low- and high-density
flows during deposition of the Bouma interval Tb or between Tb and Tc. The MLA test study appears to be
promising because this method increases objectivity of large amount of data collection, therefore provides significant
statistical representation. Besides, this automated system reduces human errors, bias and tedious manual
analyses, thus opening new perspectives in analysis of detrital sediments and interpretation of depositional
processes.
... Subaqueous mass movements are common processes in marine as well as in lacustrine environments, capable of mobilizing and transporting large volumes of sediments from submerged slopes to deep basins (Masson et al., 2006;Urgeles and Camerlenghi, 2013;Sammartini et al., 2019;Mountjoy et al., 2020;Strasser et al., 2020). Subaqueous slope instabilities and ensuing gravity flows represent a significant hazard to offshore and near-shore environments, being able to damage offshore infrastructures (Piper et al., 1999;Mosher et al., 2010;Carter et al., 2014;Clare et al., 2017) and to generate devastating tsunamis (Masson et al., 2006;Glimsdal et al., 2016;Watt et al., 2019;Williams et al., 2019), for which mass movements are the second most frequent trigger Løvholt et al., 2020). Direct and real-time observations of slope instabilities are rare and challenging, and therefore, most of our understanding of subaquatic mass movement processes derives from investigating the final products of the instability process, i.e. mass-transport deposits (MTDs). ...
Subaquatic mass movements are common in marine and lacustrine environments, but due to their barely predictable nature, direct observations of these processes are limited so that knowledge is only indirectly obtained by investigating the resulting mass-transport deposits (MTDs). Most research focuses on the most common frontally emergent slides, fast-moving events able to generate turbidity currents and tsunamis. Geohazards of frontally confined slides and mechanisms behind their typical fold-and-thrust deformation structures are however still poorly understood.
We investigate frontally confined MTDs in Lake Lucerne (Switzerland) by integrating bathymetric and high-resolution seismic data with geotechnical information derived from in situ Cone Penetrometer Tests and short core analysis. Investigated MTDs consist of three units: i) a mass-slide deposit, located at the base of the slope consisting of a coherent slope sequence, ii) a fold-and-thrust system developed in basin sediments, and iii) an overrunning mass flow deposit, consisting of remolded slope sediments. The deformed and thrusted basin sediments show higher undrained shear strength compared to the undisturbed basin sequence. We propose that this strengthening is caused by lateral compression leading to fluid expulsion in the high-plasticity basin sediments by the bulldozing sliding mass. Relative kinematic indicators document that the fold-and-thrust deformation structures occur rapidly. Thus, they should be considered in tsunami hazard analysis. Furthermore, our data highlight that the slope angle of the gliding surface and basin topography are key controlling factors for slope stability and propagation of basin-plain deformations, respectively. Our integrated study supports and refines propagation models proposed in marine environments, revealing the potential of investigating smaller-scale easier-to-access MTDs in lakes.
... Subsea infrastructure, including communication and energy cables, oil and gas rigs and pipelines, are potentially at risk of damage and destruction due to these seafloor landslides, which could be triggered by storms, earthquakes, tsunamis and local processes that destabilise the seabed. Avoidance of hazardous areas is preferred but is not always an option (Clare et al. 2017). It is important to understand the processes that may cause seafloor instabilities in order to mitigate the risk posed by marine geohazards to strategically important, offshore infrastructure (Campbell 1999). ...
The three-dimensional transport of sediments released by the main rivers in the Colombian Caribbean basin is investigated using numerical model simulations. Different types of sediments (fine sands, very fine sands and coarse silts) were tracked by implementing SedimentDrift software, a subclass of the OpenDrift open-source trajectory framework. The simulations were forced with climatological winds from ERA-5 and currents from the Copernicus Marine Environment Monitoring Service (CMEMS). In situ measurements from the area were utilised in the evaluation of the forcing fields. The diagnostic analysis of ERA-5 and CMEMS at the evaluated stations in the Colombian basin led to the conclusion that these datasets are reliable; hence, they can be used for several oceanographic and coastal engineering studies. The sediment transport of non-cohesive particles at each river mouth is subject to the variability of local hydrodynamics, morphological features and the grain size, which determines the settling velocities. These factors were considered and evaluated in detail. Given the lack of available in situ information, the performance of the Lagrangian model is evaluated by comparing the resulting simulations with previous studies reported in the area for river plume dynamics. However, this study presents an initial analysis of the pathways that sediments of different sizes follow, from the river mouth to the seafloor. This approach is convenient given that the Colombian rivers deliver large amounts of sediment of various grain sizes, there are insufficient in situ data to assess the seasonal cycle of sediment transport and many aspects of the sediment dynamics are not fully resolved. The resulting simulations provide evidence of the importance of high-resolution ocean current data, given that this is the main factor determining the trajectories and dispersion patterns at seasonal scales.
... The identification, characterisation and assessment of geohazards such as shallow gas, fluid flow and seepage involves a multidisciplinary approach utilising a range of site investigation techniques (Cevatoglu et al., 2015;Clare et al., 2017;Vanneste et al., 2014). This study aims to integrate multi-scale geophysical datasets in order to develop a geological framework to study potential fluid migration pathways from deeper stratigraphy or source rocks to shallow gas accumulations, and thereafter seepage at seafloor in the Irish Sea. ...
Shallow gas accumulation in unconsolidated Quaternary sediments, and associated seepage at the seafloor, is widespread in the north Irish Sea. This study integrates high-resolution seafloor bathymetry and sub-surface geophysical data to investigate shallow gas accumulations and possible fluid (gas and/or liquids) migration pathways to the seafloor in the northern part of the Irish Sea. Shallow gas occurs broadly in two geological settings: the Codling Fault Zone and the Western Irish Sea Mud Belt. The gas has been recognised to accumulate in both sandy and muddy Quaternary marine near-surface sediments and is characterised by three characteristic sub-bottom acoustic features: i) enhanced reflections, ii) acoustic turbid zones, and iii) acoustic blanking. The seepage of shallow gas at the seafloor has resulted in the formation of morphological features including methane-derived authigenic carbonates, seabed mounds and pockmarks. In many instances, the evidence for this gas as biogenic or thermogenic in origin is inconclusive. Two distinct types of pockmarks are recorded in the Western Irish Mud Belt: pockmarks with a relatively flat centre, and pockmarks with a central mound. Based on our observation and existing models, we infer that the formation of a carbonate crust at the seabed surface is needed as a precursor for the creation of such mounds within pockmarks. The formation processes are interpreted to be different for sandy versus muddy sediments, due to variability in erodibility and sealing capacities of the substrate. We suggest that the origin of these features is linked to the presence of deeper hydrocarbon source rocks with existing and reactivated faults forming fluid migration pathways to the surface. This in turn could indicate a mixed thermogenic-biogenic origin for seep-related structures in the study area. These features have significant implications for the future development of offshore infrastructure including marine renewable energy as well as for seabed ecology and conservation efforts in the Irish Sea.
... Improved ability to acquire high spatial and temporal resolution marine datasets (e.g. Khripounoff et al., 2003;Xu et al., 2004;Paull et al., 2011;Babonneau et al., 2013;Maier et al., 2013;Hughes Clarke, 2016;Azpiroz-Zabala et al., 2017;Carvajal et al., 2017;Clare et al., 2017; This article is protected by copyright. All rights reserved Maier et al., 2019) over the past decade has led to novel insights into turbidity current occurrence and seafloor morphodynamics (Covault et al., 2017;Hage et al., 2018;Mountjoy et al., 2018;Paull et al., 2018;Vendettuoli et al., 2019). ...
Deep‐water deposits are important archives of Earth’s history including the occurrence of powerful flow events and the transfer of large volumes of terrestrial detritus into the world’s oceans. However the interpretation of depositional processes and palaeoflow conditions from the deep‐water sedimentary record has been limited due to a lack of direct observations from modern depositional systems. Recent seafloor studies have resulted in novel findings, including the presence of upslope‐migrating bedforms such as cyclic steps formed by supercritical turbidity currents that produce distinct depositional signatures. This study builds on process‐to‐product relationships for cyclic steps using modern and ancient datasets by providing sedimentological and quantitative, three‐dimensional architectural analyses of their deposits, which are required for recognition and palaeoflow interpretations of sedimentary structures in the rock record. Repeat‐bathymetric surveys from two modern environments (Squamish prodelta, Canada, and Monterey Canyon, USA) were used to examine the stratigraphic evolution connected with relatively small‐scale (average 40 to 55 m wavelengths and 1.5 to 3.0 m wave heights) upslope‐migrating bedforms interpreted to be cyclic steps within submarine channels and lobes. These results are integrated to interpret a succession of Late Cretaceous Nanaimo Group deep‐water slope deposits exposed on Gabriola Island, Canada. Similar deposit dimensions, facies and architecture are observed in all datasets, which span different turbidite‐dominated settings (prodelta, upper submarine canyon and deep‐water slope) and timescales (days, years or thousands of years). Bedform deposits are typically tens of metres long/wide, <1 m thick and make up successions of low‐angle, backstepping trough‐shaped lenses composed of massive sands/sandstones. These results support process‐based relationships for these deposits, associated with similar cyclic step bedforms formed by turbidity currents with dense basal layers under low‐aggradation conditions. Modern to ancient comparisons reveal the stratigraphic expression of globally prevalent, small‐scale, sandy upslope‐migrating bedforms on the seafloor, which can be applied to enhance palaeoenvironmental interpretations and understand long‐term preservation from ancient deep‐water deposits.
... Quantitative characterization of shallow marine sediments combining in situ geotechnical measurements and very high resolution (VHR, frequencies > 200 Hz) seismic data is of first interest for assessing marine geohazards such as slope stability, shallow gas, or gas hydrates (Kvalstad, 2007;Vanneste et al., 2014;Clare et al., 2017;Badhani et al., 2020a). In deep-water environments, 2D VHR seismic acquisitions performed at the sea surface suffer from major limitations such as the recording of out-of-plane reflections due to a large first Fresnel zone and a lack of penetration Ker et al., 2010). ...
Very High-Resolution (VHR) marine seismic reflection helps to identify and characterize potential geohazards occurring in the upper part (300 m) of the sub-seafloor. Whereas the lateral and vertical resolutions achieved in shallow water depth (<200 m) using conventional surface-towed technology are adequate, these resolutions quickly deteriorate at greater water depths. SYSIF (SYstème SIsmique Fond), a multichannel deep-towed seismic system, has been designed to acquire VHR data (frequency bandwidth [220-1050 Hz] and vertical resolution of 0.6 m) at great water depths. However, the processing of deep-towed multichannel data is challenging as both the source and receivers are constantly moving with respect to each other according to the towing configuration. We present a new workflow that allows the application of conventional processing algorithms to extended deep-towed seismic datasets. First, a relocation of the source and receivers is necessary to obtain a sufficiently accurate acquisition geometry. Variations along the profile in the depth of the deep-towed system result in a complex geometry where the source and receiver depth vary separately and do not share the same acquisition datum. We designed a dedicated datuming algorithm to shift the source and receivers to the same datum. The procedure thus allows the application of conventional processing algorithms to perform both velocity analysis and depth imaging and therefore allowing access to the full potential of the seismic system. We successfully applied this methodology to deep-towed multichannel data from the western Black Sea. In particular, the derived velocity model highlights shallow gas charged anticline structures with unrivaled resolution.
... Careful attention to the impact of landslides on marine infrastructure is required at these sites. Even small slides have the potential to damage infrastructure (Forsberg et al. 2016;Clare et al. 2017). Small Holocene slides have been observed in all channels of the fjord. ...
Douglas Channel is a 140 km-long fjord system on Canada's west coast where steep topography, high annual precipitation and glacially over-deepened bathymetry have resulted in widespread slope failures. A 5 year project involving numerous marine expeditions to the remote area produced a comprehensive assessment of the magnitude and frequency of slope failures in the region. A classification scheme is presented based on morphology and failure mechanism: (1) debris flows are the most common in all parts of the fjord – they are often small with a subaerial component where fjord wall slope is very high or tend to exceed volumes of 10 ⁶ m ³ where fjord wall slope is lower, allowing for accumulation of marine sediments; (2) large failures of oversteepened glacial sediments occurring at transgressive moraines and glaciomarine plateaus following deglaciation – the largest is at Squally Channel with an estimated volume of 10 ⁹ m ³ ; (3) fjord wall failures that involve bedrock slump or rock avalanche; (4) translation of marine sediments; (5) composite/other slides; and (6) two scallop-shaped sackungen, or deep-seated gravitational slope deformations of granodiorite with volumes exceeding 60 × 10 ⁶ m ³ . The postglacial marine sedimentary record shows evidence of large-scale slope failures of all styles that were especially active following deglaciation. The Holocene marks a transition to a lower frequency and change to primarily debris flows and smaller rock slides. Slope failures that may be capable of generating tsunamis and may be damaging to coastal infrastructure have occurred in all parts of Douglas Channel through much of the Holocene. Here we present a morphological analysis with volume estimates and age control using multibeam bathymetry, high-resolution sub-bottom data and sediment cores. The study details an extensive analysis of slope failures in a fjord network that can be extended to other fjord environments.
... Persistent challenges in sampling directly from sediment-laden flows in many environments, and subsequently sampling their deposits, have resulted in continued debate regarding the fidelity to which sedimentary deposits record sediment transport processes (e.g., Hodgson et al., 2018). Modern sediment transport processes in the deep sea have been especially difficult to observe, measure, and sample in submarine canyon environments (e.g., Paull et al., 2010Paull et al., , 2018Talling et al., 2015) because of great water depths, and the often-unpredictable timing, and destructive potential of some powerful flows (e.g., Harris and Whiteway, 2011;Xu, 2011;Xu et al., 2014;Clare et al., 2017). Turbid mixtures of sediment and seawater are driven down submarine canyons by density differences between the flow and surrounding seawater. ...
Submarine canyons are conduits for episodic and powerful sediment density flows (commonly called turbidity currents) that move globally significant amounts of terrestrial sediment and organic carbon into the deep sea, forming some of the largest sedimentary deposits on Earth. The only record available for most turbidity currents is the deposit they leave behind. Therefore, to understand turbidity current processes, we need to determine the degree to which these flows are represented by their deposits. However, linking flows and deposits is a major long-standing scientific challenge. There are few detailed measurements from submarine turbidity currents in action, and even fewer direct measurements that can be compared to resulting seabed deposits. Recently, an extensive array of moorings along Monterey Canyon, offshore California, took measurements and samples during sediment density flow events, providing the most comprehensive dataset to date of turbidity current flows and their deposits. Here, we use sediment trap samples, velocity measurements, and seafloor cores to document how sand is transported through a submarine canyon, and how the transported sediment is represented in seafloor deposits. Sediment trap samples from events contain primarily fine to medium-grained sand with sharp bases, normal grading, and muddy tops. Sediment captured from the water column during the flow shows normal grading, which is broadly consistent with the initial peak and waning of flow velocities measured at a single height within the flow, and may be enhanced by collapsing flows. Flow events contain coarser sand concentrated toward the seafloor and larger grain sizes on the seafloor or in the dense near-bed layer, possibly representative of stratified flows. Although flow velocity varies, sand grain sizes in sediment traps are similar over distances of 50 km down-canyon, suggesting that grain size is an unfaithful record of down-canyon changes in maximum flow speeds. Sand transported within flow events and sampled in sediment traps is similar to sand sampled from the seafloor shortly after the events, but traps do not contain pebbles and gravel common in seabed deposits. Seabed deposits thus appear to faithfully record the sand component that is transported in the water column during sub-annual turbidity currents.
... Despite these limitations, our understanding of particulate density fractionation during transport and deposition provides a basis for an initial predictive assessment of their distribution within individual deposits. From the ancient record (e.g., Kneller and Branney, 1995;Talling et al., 2012b) and from observations in modern environments (e.g., Smith et al., 2007;Biscara et al., 2012;Clare et al., 2017;Hage et al., 2018;Mountjoy et al., 2018;Stevenson et al., 2018) we know that some individual event deposits (e.g., by gravity flows) can be many meters thick; hence, the assertion that plastics are only present in the first few centimeters of sediment below the seafloor is unlikely to hold (c.f. Martin et al., 2017). ...
An estimated 8.3 billion tons of non-biodegradable plastic has been produced over the last 65 years. Much of this is not recycled and is disposed into the natural environment, has a long environmental residence time and accumulates in sedimentary systems worldwide, posing a threat to important ecosystems and potentially human health. We synthesize existing knowledge of seafloor microplastic distribution, and integrate this with process-based sedimentological models of particle transport, to provide new insights, and critically, to identify future research challenges. Compilation of published data shows that microplastics pervade the global seafloor, from abyssal plains to submarine canyons and deep-sea trenches (where they are most concentrated). However, few studies relate microplastic accumulation to sediment transport and deposition. Microplastics may enter directly into the sea as marine litter from shipping and fishing, or indirectly via fluvial and aeolian systems from terrestrial environments. The nature of the entry-point is critical to how terrestrially sourced microplastics are transferred to offshore sedimentary systems. We present models for physiographic shelf connection types related to the tectono-sedimentary regime of the margin. Beyond the shelf, the principal agents for microplastic transport are: (i) gravity-driven transport in sediment-laden flows; (ii) settling, or conveyance through biological processes, of material that was formerly floating on the surface or suspended in the water column; (iii) transport by thermohaline currents, either during settling or by reworking of deposited microplastics. We compare microplastic settling velocities to natural sediments to understand how appropriate existing sediment transport models are for explaining microplastic dispersal. Based on this analysis, and the relatively well-known behavior of deep-marine flow types, we explore the expected distribution of microplastic particles, both in individual sedimentary event deposits and within deep-marine depositional systems. Residence time within certain deposit types and depositional environments is anticipated to be variable, which has implications for the likelihood of ingestion and incorporation into the food chain, further transport, or deeper burial. We conclude that the integration of process-based sedimentological and stratigraphic knowledge with insights from modern sedimentary systems, and biological activity within them, will provide essential constraints on the transfer of microplastics to deep-marine environments, their distribution and ultimate fate, and the implications that these have for benthic ecosystems. The dispersal of anthropogenic across the sedimentary systems that cover Earth’s surface has important societal and economic implications. Sedimentologists have a key, but as-yet underplayed, role in addressing, and mitigating this globally significant issue.
... Numerous studies have demonstrated the key role that MTCs play in (a) continental margin construction, (b) petroleum systems development and (c) geohazard prediction (e.g. Posamentier & Kolla, 2003;Weimer & Shipp, 2004;Moscardelli, Wood, & Mann, 2006;Armitage & Stright, 2010;Meckel, 2011;Clare et al., 2017). Qualitative descriptions of the structure and kinematic indicators within MTCs are well-documented by many seismic-reflection and outcrop-based works (e.g. ...
Strain style, magnitude, and distribution within mass-transport complexes (MTCs) is important for understanding the process evolution of submarine mass flows and for estimating their runout distances. Structural restoration and quantification of strain in gravitationally-driven passive margins have been shown to approximately balance between updip extensional and downdip compressional domains; such an exercise has not yet been attempted for MTCs. We here interpret and structurally restore a shallowly buried (c. 1500 mbsf) and well-imaged MTC, offshore Uruguay using a high-resolution (12.5 m vertical and 15x12.5 m horizontal resolution) 3D seismic-reflection survey. This allows us to characterise and quantify vertical and lateral strain distribution within the deposit. Detailed seismic mapping and attribute analysis shows that the MTC is characterised by a complicated array of kinematic indicators, which vary spatially in style and concentration. Seismic-attribute extractions reveal several previously undocumented fabrics preserved in the MTC, including internal shearing in the form of sub-orthogonal shear zones, and fold-thrust systems within the basal shear zone beneath rafted-blocks. These features suggest multiple phases of flow and transport directions during emplacement. The MTC is characterised by a broadly tripartite strain distribution, with extensional (e.g. normal faults), translational and compressional (e.g. folds and thrusts) domains, along with a radial frontally emergent zone. We also show how strain is preferentially concentrated around intra-MTC rafted-blocks due to kinematic interaction between these features and the underlying basal shear zone. Overall, and even when volume loss within the frontally emergent zone is excluded, a strain difference between extension (1.6-1.9 km) and contraction (6.7-7.3 km) is calculated. We attribute this to a combination of distributed, sub-seismic, ‘cryptic’ strain, likely related to de-watering, grain-scale deformation, and related changes in bulk sediment volume. This work has implications for assessing MTCs strain distribution and provides a practical approach for evaluating structural interpretations within such deposits.
... Field experiments using distributed acoustic sensing along fiber-optic cables in Alaska and California found a high degree of correlation with earthquake measurements acquired with a conventional seismometer and that only a minimal degree of cable-sediment coupling is required for P and S wave detection (Blum et al., 2010). This demonstrated application of fiber-optic cables thus opens up many further possibilities for indirectly measuring other active seafloor processes such as volcanic activity, tsunamis, slope collapses, sediment transport, and fluid expulsion and for detecting a wide range of seismic events, in a similar manner to recent monitoring efforts using hydrophones, ocean bottom seismometers, and broadband seismic arrays (Burtin et al., 2011;Caplan-Auerbach et al., 2014;Clare et al., 2017;Kimura, 2017aKimura, , 2017bLin et al., 2010;Lindsey et al., 2017;Sgroi et al., 2014). Clearly, calibration will be required for each of these processes and in a range of ocean settings, but the potential application is extremely promising. ...
Full text available at: https://eprints.soton.ac.uk/425079/1/Final_paper_in_MTSJ.pdf
Distributed optical fiber sensors provide new opportunities for monitoring the
marine environment. We review the physical foundations of this sensor technology
and discuss how it can be applied to radically augment the networks of subsea sensors that help monitor fundamental marine processes and to complete our understanding of local, regional, and global interactions in this environment.
Keywords: distributed fiber-optic sensors, subsea infrastructure monitoring, seabed
stability, turbidity currents, subsea seismic activity
... For high-and ultra-high-resolution marine seismic reflection surveying, generating broad bandwidth source waveforms with useful energy content in the kHz frequency range is seen as a critical part of modern source design (Duarte et al, 2017). Commercial operations are often targeting engineering-scale imaging of the subsurface to aid in engineering design (e.g., Evans, 2011) or geohazard assessment (e.g., Clare et al, 2017), which requires decimetre-resolution imaging and penetration depths of 10s to (in some cases) 100s of metres. In addition, there is a growing interest in the application of seismic inversion methods to shallow marine geophysical data with an aim to provide more effective integration of geological, geotechnical and geophysical data (e.g., Vardy et al, 2017). ...
Seismic attenuation is an important parameter to understand and parameterise, with implications for survey design, processing, interpretation, and inversion. Here we present a brief summary of the physical theory underpinning our understanding of attenuation and illustrate how this can be used to compensate for the loss of energy within a data processing workflow, preserving vertical resolution down-trace. Furthermore, we discuss the potential uses of attenuation as a parameter for characterising the nature of the subsurface, suggesting that attenuation should be perceived in a more positive light and seen as another parameter (alongside amplitude, phase, and move-out) that provides valuable quantitative information.
The near-seabed multichannel seismic exploration systems have yielded remarkable successes in marine geological disaster assessment, marine gas hydrate investigation, and deep-sea mineral exploration owing to their high vertical and horizontal resolution. However, the quality of deep-towed seismic imaging hinges on accurate source-receiver positioning information. In light of existing technical problems, we propose a novel array geometry inversion method tailored for high-resolution deep-towed multichannel seismic exploration systems. This method is independent of the attitude and depth sensors along a deep-towed seismic streamer, accounting for variations in seawater velocity and seabed slope angle. Our approach decomposes the towed line array into multiline segments and characterizes its geometric shape using the line segment distance and pitch angle. Introducing optimization parameters for seawater velocity and seabed slope angle, we establish an objective function based on the model, yielding results that align with objective reality. Employing the particle swarm optimization algorithm enables synchronous acquisition of optimized inversion results for array geometry and seawater velocity. Experimental validation using theoretical models and practical data verifies that our approach effectively enhances source and receiver positioning inversion accuracy. The algorithm exhibits robust stability and reliability, addressing uncertainties in seismic traveltime picking and complex seabed topography conditions.
Gravity sensing is a valuable technique used for several applications, including fundamental physics, civil engineering, metrology, geology, and resource exploration. While classical gravimeters have proven useful, they face limitations, such as mechanical wear on the test masses, resulting in drift, and limited measurement speeds, hindering their use for long-term monitoring, as well as the need to average out microseismic vibrations, limiting their speed of data acquisition. Emerging sensors based on atom interferometry for gravity measurements could offer promising solutions to these limitations, and are currently advancing towards portable devices for real-world applications. This article provides a brief state-of-the-art review of portable atom interferometry-based quantum sensors and provides a perspective on routes towards improved sensors.
By describing critical tasks in quantifying uncertainty using geohazard models, we analyse some of the challenges involved. Under the often-seen condition of very limited data and despite the availability of recently developed sophistications to parameterise models, a major challenge that remains is the constraining of the many model parameters involved. However, challenges also lie in the credibility of predictions required in the assessments, the uncertainty of input quantities, and the conditional nature of the quantification on the choices and assumptions made by analysts. Addressing these challenges calls for more insightful approaches that are yet to be developed; however, clarifications and reinterpretations of some fundamental concepts together with practical simplifications may be required first, and these are discussed in this paper. The research aims at strengthening both the foundation of geohazard risk assessments and its practice.
Sediment remobilization of seafloor strata is linked to the early stages of sediment burial, diagenesis and fluid migration in different geological settings. It can impact the depositional architecture of a sedimentary basin by promoting local and widespread erosion while, in parallel, lead to an overall redistribution of near-seafloor strata (the mass movement per se). It can also generate relatively deep sediment injections, fluid-flow features and associated sediment extrusions. Sediment remobilization plays an important role in hydrocarbon-rich basins. Mass transport complexes and deposits can contain reservoirs intervals or constitute competent seal units. Sediment injections can form either reservoirs or comprise routes for fluid migration (sand injectites). Furthermore, the existence of deep hydrocarbon reservoirs is often associated with fields of mud volcanoes. This chapter highlights sediment remobilization processes as being significant due to their societal, economic and ecological impact as both geohazards and hydrocarbon indicators. While associated with hydrocarbon shows and prolific accumulations at depth, some of these processes can be also damaging to infrastructure, local populations and marine life. Finally, mass movement on continental slopes, volcanic islands or seamounts can trigger catastrophic tsunamis.
Textural and structural features of submarine mass density flow sediments are a record of depositional flow evolution and hydrodynamics. Record of changes in such flow movement are difficult or impossible to detect, especially in fine sediments, without quantitative grain size analysis. This study shows an automated measurement system of mineral liberation analysis MLA applied for the first time to grain size distribution. The analysis concerns heavy minerals extracted from very fine- and fine-grained sandstones of the Cergowa Beds (Lower Oligocene) in the Polish and Slovak Outer Carpathians, which macroscopically show unclear textural trends and represent deposits of steady and uniform turbidity currents, possibly initiated by hyperpycnal effluents. The MLA analysis showed that the tested material represents sediments deposited by flows with developing turbulence. Simultaneously, the study verified that a turbulence damping contributes to size-density sorting during transport and demonstrated that maximum rather than average grain size is controlled by flow concentration, therefore reflects flow hydrodynamics. Besides, MLA measurements recognised variations in flow motion recorded by three types of grain-size breaks: (i) in high-density flow when capacity-controlled conditions are changing so that high energy flow erodes the previously deposited material; (ii) at a sharp interface between high-concentration basal layer and the overriding flow with lower concentration , and (iii) at a transition between low- and high-density flows during deposition of the Bouma interval Tb or between Tb and Tc. The MLA test study appears to be promising because this method increases objectivity of large amount of data collection, therefore provides significant statistical representation. Besides, this automated system reduces human errors, bias and tedious manual analyses, thus opening new perspectives in analysis of detrital sediments and interpretation of depositional processes.
The sparsity of permanent seismic instrumentation in marine environments often limits the availability of subsea information on geohazards, including active fault systems, in both time and space. One sensing resource that provides observational access to the seafloor environment are existing networks of ocean bottom fiber optic cables; these cables, coupled to modern distributed acoustic sensing (DAS) systems, can provide dense arrays of broadband seismic observations capable of recording both seismic events and the ambient noise wavefield. Here, we report a marine DAS application which demonstrates the strength and limitation of this new technique on submarine structural characterization. Based on ambient noise DAS records on a 20 km section of a fiber optic cable offshore of Moss Landing, CA, in Monterey Bay, we extract Scholte waves from DAS ambient noise records using interferometry techniques and invert the resulting multimodal dispersion curves to recover a high resolution 2D shear-wave velocity image of the near seafloor sediments. We show for the first time that the migration of coherently scattered Scholte waves observed on DAS records can provide an approach for resolving sharp lateral contrasts in subsurface properties, particularly shallow faults and depositional features near the seafloor. Our results provide improved constraints on shallow submarine features in Monterey Bay, including fault zones and paleo-channel deposits, thus highlighting one of many possible geophysical uses of the marine cable network.
Turbidity currents transport prodigious volumes of sediment to the deep sea. But there are very few direct measurements from oceanic turbidity currents, ensuring they are poorly understood. Recent studies have used acoustic Doppler current profilers (ADCPs) to measure velocity profiles of turbidity currents. However, there were no detailed measurements of sediment concentration, which is a critical parameter because it provides the driving force and debate centers on whether flows are dilute or dense. Here we provide the most detailed measurements yet of sediment concentration in turbidity currents via a new method using dual‐frequency acoustic backscatter ADCP data. Backscatter intensity depends on size and concentration of sediment, and we disentangle these effects. This approach is used to document the internal structure of turbidity currents in Congo Canyon. Flow duration is bimodal, and some flows last for 5–10 days. All flows are mainly dilute (<10 g/L), although faster flows contain a short‐lived initial period of coarser‐grained or higher‐concentration flow within a few meters of the bed. The body of these flows tends toward a maximum speed of 0.8–1 m/s, which may indicate an equilibrium in which flow speeds suspend available sediment. Average sediment concentration and flow thickness determine the gravitational driving force, which we then compared to average velocities. This comparison suggests surprisingly low friction values, comparable to or less than those of major rivers. This new approach therefore provides fundamental insights into one of the major sediment transport processes on Earth.
The consequences of subaqueous landslides have been at the forefront of societal conscience more than ever in the last few years, with devastating and fatal events in the Indonesian Archipelago making global news. The new research presented in this volume demonstrates the breadth of ongoing investigation into subaqueous landslides, and shows that while events like the recent ones can be devastating, they are smaller in scale than those Earth has experienced in the past. Understanding the spectrum of subaqueous landslide processes, and therefore the potential societal impact, requires research across all spatial and temporal scales. This volume delivers a compilation of state-of-the-art papers covering regional landslide databases, advanced techniques for in situ measurements, numerical modelling of processes and hazards.
Underwater communication cables transport large amounts of sensitive information between countries. This fact converts these cables into a critical infrastructure that must be protected. Monitoring the underwater cable environment is rare and any intervention is usually driven by cable faults. In the last few years, several reports raised issues about possible future malicious attacks on such cables. The main objective of this operational research and analysis (ORA) paper is to present an overview of different commercial and already available marine sensor technologies (acoustic, optic, magnetic and oceanographic) that could be used for autonomous monitoring of the underwater cable environment. These sensors could be mounted on different autonomous platforms, such as unmanned surface vehicles (USVs) or autonomous underwater vehicles (AUVs). This paper analyses a multi-threat sabotage scenario where surveying a transatlantic cable of 13,000 km, (reaching water depths up to 4000 m) is necessary. The potential underwater threats identified for such a scenario are: divers, anchors, fishing trawls, submarines, remotely operated vehicles (ROVs) and AUVs. The paper discusses the capabilities of the identified sensors to detect such identified threats for the scenario under study. It also presents ideas on the construction of periodic and permanent surveillance networks. Research study and results are focused on providing useful information to decision-makers in charge of designing surveillance capabilities to secure underwater communication cables.
Current methods for characterizing near-surface marine sediments rely on extensive coring/penetrometer testing and correlation to seismic facies. Little quantitative information is regularly derived from geophysical data beyond qualitative inferences of sediment characteristics based on seismic
facies architecture. Even these fundamental seismostratigraphic nterpretations can be difficult to correlate with lithostratigraphic data due to inaccuracies in the time-to-depth conversion of geophysical data and potential loss and/or compression of high-porosity and under-consolidated seafloor material during direct sampling. To complicate matters further, when quantitative information is derived from marine geophysical data, it often describes the sediments using terminology (e.g., acoustic impedance and seismic quality factor) that is impenetrable to geologists and engineers. In
contrast, for hydrocarbon prospecting, reservoir characterization using quantitative inversion of geophysical data has developed enormously over the past 20 years or more. Impedance and amplitude-versus-angle inversion techniques are now commonplace, whereas computationally expensive waveform inversions are gaining traction, and there is a well-developed interface between these geophysical and reservoir engineering fields via rock physics.
In this paper, we collate and review the different published inversion methods for high-resolution geophysical data. Using several case study examples spanning a broad range of depositional environments, we assess the current state of the art in remote characterization of shallow sediments from a multidisciplinary viewpoint, encompassing geophysical, geological, and geotechnical angles. By identifying the key parameters used to characterize the subsurface, a framework is developed whereby geological, geotechnical, and geophysical characterizations of the subsurface can be
related in a less subjective manner. As part of this, we examine the sensitivity of commonly derived acoustic properties (e.g., acoustic impedance and seismic quality factor) to more fundamentally important soil properties (e.g., lithology, pore pressure, gas saturation, and undrained shear strength), thereby facilitating better integration between geological, geotechnical, and geophysical data for improved mapping of sediment properties. Ultimately, we present a number of ideas for future research activities in this field.<br/
Autonomous Underwater Vehicles (AUVs) have a wide range of applications in marine geoscience, and are increasingly being used in the scientific, military, commercial, and policy sectors. Their ability to operate autonomously of a host vessel makes them well suited to exploration of extreme environments, from the World’s deepest hydrothermal vents to beneath polar ice sheets. They have revolutionized our ability to image the seafloor, providing higher resolution seafloor mapping data than can be achieved from surface vessels, particularly in deep water. This contribution focuses on the major advances in marine geoscience that have resulted from AUV data. The primary applications are i) submarine volcanism and hydrothermal vent studies, ii) mapping and monitoring of low-temperature fluid escape features and chemosynthetic ecosystems, iii) benthic habitat mapping in shallow- and deep-water environments, and iv) mapping of seafloor morphological features (e.g. bedforms generated beneath ice or sediment-gravity flows). A series of new datasets are presented that highlight the growing versatility of AUVs for marine geoscience studies, including i) multi-frequency acoustic imaging of trawling impacts on deep-water coral mounds, iii) collection of high-resolution seafloor photomosaics at abyssal depths, and iii) velocity measurements of active submarine density flows. Future developments in AUV technology of potential relevance to marine geoscience include new vehicles with enhanced hovering, long endurance, extreme depth, or rapid response capabilities, while development of new sensors will further expand the range of geochemical parameters that can be measured.
Rivers and turbidity currents are the two most important sediment transport processes by volume on Earth. Various hypotheses have been proposed for triggering of turbidity currents offshore from river mouths, including direct plunging of river discharge, delta mouth bar flushing or slope failure caused by low tides and gas expansion, earthquakes and rapid sedimentation. During 2011, 106 turbidity currents were monitored at Squamish Delta, British Columbia. This enables statistical analysis of timing, frequency and triggers. The largest peaks in river discharge did not create hyperpycnal flows. Instead, delayed delta-lip failures occurred 8–11 h after flood peaks, due to cumulative delta top sedimentation and tidally-induced pore pressure changes. Elevated river discharge is thus a significant control on the timing and rate of turbidity currents but not directly due to plunging river water. Elevated river discharge and focusing of river discharge at low tides cause increased sediment transport across the delta-lip, which is the most significant of all controls on flow timing in this setting.
Field observations of turbidity currents remain scarce, and thus there is continued debate about their internal structure and how they modify underlying bedforms. Here, I present the results of a new imaging method that examines multiple surge-like turbidity currents within a delta front channel, as they pass over crescent-shaped bedforms. Seven discrete flows over a 2-h period vary in speed from 0.5 to 3.0 ms-1. Only flows that exhibit a distinct acoustically attenuating layer at the base, appear to cause bedform migration. That layer thickens abruptly downstream of the bottom of the lee slope of the bedform, and the upper surface of the layer fluctuates rapidly at that point. The basal layer is inferred to reflect a strong near-bed gradient in density and the thickening is interpreted as a hydraulic jump. These results represent field-scale flow observations in support of a cyclic step origin of crescent-shaped bedforms.
This paper describes the measurement and modeling of turbidity currents (TC) in the Congo Canyon and the application of that data to design of a telecommunications cable. There are few direct measurements of TCs largely because the measurement instruments are often damaged. Indeed, this is only the second set of measurements ever recorded in the Congo Canyon, and the only set that shows the propagation of TCs through multiple sites. Four moorings were deployed in 2013 along a 500 km stretch of the Canyon and measured roughly a dozen TCs during a 5-month period. The largest event reached 150 cm/s. Two-D and 3-D numerical models were set up and calibrated using the measurements. The 3-D model shows significant along- and cross-channel variations and these results were used to select optimal crossing locations. The models were then integrated with cable data to permit stress and fatigue analysis at selected cable installation locations, permitting both cable route and cable type to be evaluated.
Scour poses a significant risk to infrastructure placed on mobile seabeds. Seabed mobility is common on the North West Shelf of Australia, in parts of the North Sea, and also occurs in the deepwater Gulf of Mexico, due to loop currents. Scour can undermine structures and, for shallow-skirted mudmat foundations, there can be significant consequences including excessive settlements, tilt and loss of bearing and sliding capacity. However, scour mitigation via engineered protection is costly, and to be avoided if possible.
This paper describes a new quantitative risk-based approach for assessing the susceptibility of subsea infrastructure to scour processes. This probabilistic scour assessment accommodates measurable uncertainties in metocean and seabed conditions, using new characterization techniques. The approach allows operators and owners to better assess the optimum strategy to address scour risk, selecting from mitigation during installation or in-service monitoring, prediction and remediation.
The paper describes (i) best practice approaches for assessing scour susceptibility and propagation rates with and without engineered protection, (ii) new methods for determining the applicable seabed and metocean inputs, (iii) a probabilistic framework for encompassing uncertainties, and (iv) how this approach can be applied in project applications.
Our probabilistic method of assessing and presenting scour risk produces a distribution of estimates of scour depth and time rate. By capturing and quantifying the full range of uncertainties, this method facilitates decision-making by showing the range of potential outcomes and allowing the associated costs and consequences to be evaluated. This approach is superior to deterministic ‘worst case’ calculations, which are often used to assess scour susceptibility.
In summary, this paper provides operators and owners with an improved methodology to unlock Capex and Opex savings through more accurate and informed scour assessments.
Subsea pipeline networks with components such as Wye-pieces and Pipeline End Manifolds (PLEMs) require protection from mechanical impacts. Pipeline scour is well understood and scour at solid caissons/gravity base foundations and piled foundations has been studied (Whitehouse, 1998; Whitehouse, et al., 2011; Sumer and Fredsoe, 2002). The scouring at small subsea gravity protection structures has received less attention than those with piles (Ottesen Hansen, 1997; Fog and Krogh, 2001) and hence there is uncertainty about predicting scour at such structures (Whitehouse, et al., 2010).
The design of subsea structures requires a composite assessment of scour and stability for the protected asset and the protection structure itself. A closed structure may lead to the formation of a scour footprint that is quite similar to a solid caisson or gravity base. For an open structure, the contribution to scour interaction from the different components is more difficult to assess.
Large-scale mobile bed laboratory experiments evaluated scour for a surface laid pipeline, with a spur and valves, and a generic subsea protection structure with mudmats and a perimeter skirt. Bathymetry mapping was achieved with a high precision underwater laser scanner. The experiments evaluated the contributions to scour arising from the different subsea components.
Sediment mobility was a key factor, with the perimeter skirt able to resist scour over part of its length when the mobility was low/moderate, whereas in a high mobility environment the skirt was completely undermined. The foundation beams were underscoured quite quickly and the eroded sediment was flushed out through the perforated mudmats, highlighting the importance of mudmat design. The scour pattern and rate was similar with or without the pipeline in place. The underscouring of the beams caused settlement and tilting of the structure, highlighting the importance of assessing scour and scour countermeasures for these types of structure. The scour assessment needs to be integrated into the structural, geotechnical and functional performance study for any structure and the new results illustrate the importance of understanding effects related to sediment mobility, the prevailing direction of currents, the design of mudmats and skirts, and the need for additional scour countermeasures.
The A.D. 2011 Tohoku-Oki Mw 9 earthquake ruptured the megathrust up to the Japan Trench with a large displacement and caused a catastrophic tsunami. This study is the first to use short-lived radioisotopes, including those emitted by the damaged nuclear reactors at the Fukushima Daiichi nuclear power plant (Japan), to document the remobilization of the upper few centimeters of sediment as a highly significant process triggered by the earthquake and its aftershocks. Targeting the post-earthquake environment allowed characterization of the sedimentary signature of this event for a better understanding of paleoearthquakes in Japan and other tectonically active boundary areas. The results stem from 23 piston cores recovered by the 2013 expedition NT13-19 of the Japan Agency for Marine-Earth Science and Technology. We document submarine homogeneous muddy flow deposits that were triggered by ground motion in 2011. They are highly enriched with excess (xs) xs210Pb, requiring only centimeters-deep sediment remobilization over large areas of the seafloor. Some contain 134Cs and 137Cs radioisotopes derived from the Fukushima nuclear reactors, indicating that sedimentation persisted for at least 30 days after the main shock. We found these deposits at all sampling sites in an ~5000 km2 area of the seafloor in 4000-6000 m of water depth. The study area extends for ~260 km parallel to the strike of the trench. The thickness of this "Tohoku layer" (3-200 cm) increases toward the zone of maximum megathrust slip, where deposits are thickest. These results demonstrate that the shaking of the seafloor above large megathrust ruptures near the trench remobilized surficial unconsolidated sediment for hundreds of kilometers. The characteristics of these deposits may typify deposits resulting from large fault slips like that of the Tohoku-Oki earthquake, but also other earthquake deposits, contributing to their identification in the sedimentary record globally.
Flow slides form a major threat to flood defences along coastlines and riverbanks in the Netherlands. Due to the uncertainties with respect to the process in combination with the severity of the consequences and costs for prevention measures, there is a need to improve existing models for prediction or occurrence of, and damage by, flow slides. One of the key questions to be answered is whether slope failure by a flow slide is caused by up-slope migrating breaches or by static liquefaction. Although fundamentally different mechanisms, both result in a flowing sand-water mixture or turbidity current that eventually redeposits on a gentle slope. Over the last decades numerical models have been developed for both mechanisms, based on flume experiments. Upscaling these experiments is complex, as scaling rules are different for the various processes involved. To evaluate the failure mechanism on a natural scale, validate numerical models and test new technology to monitor the occurrence of flow slides, a large, controlled field test was performed.
The west Ghanaian continental slope is incised at the shelf break by multiple gullies that coalesce downslope to formbroader canyons. Evidence for past turbidity current activity includes the presence of upslope migrating, kilometre-scale bedforms within such canyons, and smaller bedforms on the interfluves. The larger bedforms are inferred to relate to high density turbidity current activity focused within a canyon thalweg, with more dilute plumes of sediment overspilling onto the interfluves. These are interpreted to have been formed by supercritical turbidity currents which can pose a credible threat to seafloor structures - particularly pipelines. Piston cores acquired within canyons show evidence of past high density (sand-rich) and low density (mudrich) turbidity currents. The results of geochronological analyses show that turbidity current magnitude appears to have been controlled by climatic changes over the past 25 thousand years. High density turbidity current activity “switched off” approximately 17,000 years ago, coincident with sea level rise and the restriction of sediment delivery to the shelf. Hence, the canyons can be accepted as a viable route for pipelines or placement of structures. Bedforms on the interfluves are interpreted as static features during the present day and only present a topographic consideration. The geotechnical implications of laterally and vertically variable turbidity current-controlled deposition are illustrated, including sand-rich deposits within canyon bases, heterogeneous facies on the flanks, and clay-rich sediment on the interfluves. We document how an integrated approach has reduced the perceived risk of high density turbidity currents to a proposed field development and has also been used to understand the field-wide variability in soil conditions.
As human pressure on the marine environment increases, safeguarding healthy and productive seas increasingly necessitates integrated, time- and cost-effective environmental monitoring. Employment of a Wave Glider proved very useful for the study of sediment transport in a shallow sandbank area in the Belgian part of the North Sea. During 22 days, data on surface and water-column currents and turbidity were recorded along 39 loops around an aggregate-extraction site. Correlation with wave and tidal-amplitude data allowed the quantification of current- and wave-induced advection and resuspension, important background information to assess dredging impacts. Important anomalies in suspended particulate matter concentrations in the water column suggested dredging-induced overflow of sediments in the near field (i.e., dynamic plume), and settling of finer-grained material in the far field (i.e., passive plume). Capturing the latter is a successful outcome to this experiment, since the location of dispersion and settling of a passive plume is highly dependent on the ruling hydro-meteorological conditions and thus difficult to predict. Deposition of the observed sediment plumes may cause habitat changes in the long-term.
Quantifying marine methane fluxes of free gas (bubbles) from the seafloor into the water column is of importance for climate related studies, for example, in the Arctic, reliable methodologies are also of interest for studying man-made gas and oil leakage systems at hydrocarbon production sites. Hydroacoustic surveys with singlebeam and nowadays also multibeam systems have been proven to be a successful approach to detect bubble release from the seabed. A number of publications used singlebeam echosounder data to indirectly quantify free gas fluxes via empirical correlations between gas fluxes observed at the seafloor and the hydroacoustic response. Others utilize the hydroacoustic information in an inverse modeling approach to derive bubble fluxes. Here, we present an advanced methodology using data from splitbeam echosounder systems for analyzing gas release water depth (> 100 m). We introduce a new MATLAB-based software for processing and interactively editing data and we present how bubble-size distribution, bubble rising speed and the model used for calculating the backscatter response of single bubbles influence the final gas flow rate calculations. As a result, we highlight the need for further investigations on how large, wobbly bubbles, bubble clouds, and multi-scattering influence target strength. The results emphasize that detailed studies of bubble-size distributions and rising speeds need to be performed in parallel to hydroacoustic surveys to achieve realistic mediated methane flow rate and flux quantifications.
Turbidity currents, and other types of submarine sediment density flow, redistribute more sediment across the surface of the Earth than any other sediment flow process, yet their sediment concentration has never been measured directly in the deep ocean. The deposits of these flows are of societal importance as imperfect records of past earthquakes and tsunamogenic landslides and as the reservoir rocks for many deep-water petroleum accumulations. Key future research directions on these flows and their deposits were identified at an informal workshop in September 2013. This contribution summarizes conclusions from that workshop, and engages the wider community in this debate. International efforts are needed for an initiative to monitor and understand a series of test sites where flows occur frequently, which needs coordination to optimize sharing of equipment and interpretation of data. Direct monitoring observations should be combined with cores and seismic data to link flow and deposit character, whilst experimental and numerical models play a key role in understanding field observations. Such an initiative may be timely and feasible, due to recent technological advances in monitoring sensors, moorings, and autonomous data recovery. This is illustrated here by recently collected data from the Squamish River delta, Monterey Canyon, Congo Canyon, and offshore SE Taiwan. A series of other key topics are then highlighted. Theoretical considerations suggest that supercritical flows may often occur on gradients of greater than, similar to 0.6 degrees. Trains of up-slope-migrating bedforms have recently been mapped in a wide range ofmarine and freshwater settings. They may result from repeated hydraulic jumps in supercritical flows, and dense (greater than approximately 10% volume) near-bed layers may need to be invoked to explain transport of heavy (25 to 1,000 kg) blocks. Future work needs to understand how sediment is transported in these bedforms, the internal structure and preservation potential of their deposits, and their use in facies prediction. Turbulence damping may be widespread and commonplace in submarine sediment density flows, particularly as flows decelerate, because it can occur at low (< 0.1%) volume concentrations. This could have important implications for flow evolution and deposit geometries. Better quantitative constraints are needed on what controls flow capacity and competence, together with improved constraints on bed erosion and sediment resuspension. Recent advances in understanding dilute or mainly saline flows in submarine channels should be extended to explore how flow behavior changes as sediment concentrations increase. The petroleum industry requires predictive models of longer-term channel system behavior and resulting deposit architecture, and for these purposes it is important to distinguish between geomorphic and stratigraphic surfaces in seismic datasets. Validation of models, including against full-scale field data, requires clever experimental design of physical models and targeted field programs.
Submarine landslides on open continental slopes can be prodigious in scale. They are an important process for global sediment fluxes, and can generate very damaging tsunamis. Submarine landslides are far harder to monitor directly than terrestrial landslides, and much greater uncertainty surrounds their preconditioning factors and triggers. Submarine slope failure often occurs on remarkably low (< 2°) gradients that are almost always stable on land, indicating that particularly high excess pore pressures must be involved. Earthquakes trigger some large submarine landslides, but not all major earthquakes cause widespread slope failure. The headwalls of many large submarine landslides appear to be located in water depths that are too deep for triggering by gas hydrate dissociation. The available evidence indicates that landslide occurrence is either weakly (or not) linked to changes in sea level or atmospheric methane abundance, or the available dates for open continental slope landslides are too imprecise to tell. Similarly, available evidence does not strongly support a view that landslides play an important role in methane emissions that cause climatic change. However, the largest and best-dated open continental slope landslide (the Storegga Slide) coincides with a major cooling event 8,200 years ago. This association suggests that caution may be needed when stating that there is no link between large open slope landslides and climate change.
Turbidity currents are volumetrically the most important process for moving sediment in submarine environments. They may travel at high speeds, thereby posing a threat to important and expensive seafloor infrastructure. Despite their importance, we still know little about their flow dynamics because direct monitoring is challenging and consequently rare. Additionally, the few settings in which monitoring has been feasible, have generally involved a single instrument approach, either measuring flow velocity, sediment concentration or grain size. Here we present results issued from a multi-instrument study where a single turbidity current was observed with several instruments at the same location and time using different measuring frequencies. Three types of geophysical sensors were deployed from a single vessel moored over a turbidity current channel on the Squamish Delta in British Colombia, Canada. First, two 500 kHz multibeam sonars suspended from the bow of the ship imaged the incoming turbidity current and documented its interaction with the crescentic bedforms on the channel thalweg. Second, a 600 kHz downward-looking Acoustic Doppler Current Profiler (ADCP) lowered from the back of the ship provided vertical profiles of velocity through time. Third, a 1.0-24.0 kHz Chirp profiler enabled for the first time imaging of the dense near-bed zone of the turbidity current, which has so far been largely impenetrable using higher frequency sonar and ADCP instruments. Besides the stationary deployment, a repetitive multibeam survey was also performed using a moving vessel in order monitor temporal evolution of the seafloor morphology resulting from turbidity currents. By combining the measurements from each system, a single turbidity current was characterised in unusually high resolution. This current was 6 to 8 meters thick and at least 40 meters wide according to the multibeam sonars. The ADCP measured a front speed of around 1.5 m/s, higher than the internal velocities which decreased from 1.1 m/s to 0.3 m/s over 4 minutes. The fast-moving flow was followed by at least 15 minutes in which there was a sediment cloud in the water column that was visible both from the multibeam sonars and the ADCP. The Chirp sensor revealed that the incoming current excavated a thin layer of sediment from the seafloor. Conclusions from this experiment are that each instrument has its own benefits. Their combination and deployment over a single fixed location are therefore needed if we are to characterise turbidity currents in detail. This provides new methodological insights on how to monitor turbidity currents in general.
Submarine turbidity currents create some of the largest sediment accumulations on Earth, yet there are few direct measurements of these flows. Instead, most of our understanding of turbidity currents results from analyzing their deposits in the sedimentary record. However, the lack of direct flow measurements means that there is considerable debate regarding how to interpret flow properties from ancient deposits. This novel study combines detailed flow monitoring with unusually precisely located cores at different heights, and multiple locations, within the Monterey submarine canyon, offshore California, USA. Dating demonstrates that the cores include the time interval that flows were monitored in the canyon, albeit individual layers cannot be tied to specific flows. There is good correlation between grain sizes collected by traps within the flow and grain sizes measured in cores from similar heights on the canyon walls. Synthesis of flow and deposit data suggests that turbidity currents sourced from the upper reaches of Monterey Canyon comprise three flow phases. Initially, a thin (38–50 m) powerful flow in the upper canyon can transport, tilt, and break the most proximal moorings and deposit chaotic sands and gravel on the canyon floor. The initially thin flow front then thickens and deposits interbedded sands and silty muds on the canyon walls as much as 62 m above the canyon floor. Finally, the flow thickens along its length, thus lofting silty mud and depositing it at greater altitudes than the previous deposits and in excess of 70 m altitude.
Infiltration of shallow soils by naturally occurring hydrocarbons has been documented in several deepwater environments worldwide. The potential for significant soil modification, such as the development of authigenic carbonates and alteration of the generally expected background geotechnical properties may provide constraints to flowline routing, foundation installation and engineering lifetime performance. This paper presents a review of the current state of knowledge of the authors with special reference to recent investigations in deepwater offshore Angola, and a suggested method for identification, characterization and prediction. Interpretation of 3D exploration seismic, enhanced by AUV (Chirp) data facilitates an initial identification of areas prone to hydrocarbon infiltration and the vertical and spatial extent of potential soil modification. A first pass geotechnical characterisation is developed by targeted seabed CPTs and soil sampling. The generation of an integrated predictive model requires a multidisciplinary advanced testing programme, including geophysical, geotechnical, geochemical and geological analyses. Once the extent, nature, and formative processes of hydrocarbon-related soil modification are understood, an assessment can be made of the challenges posed to a field development. This provides the necessary input to foundation and /or routing feasibility and determines if there is requirement to mitigate, through avoidance or design.
Turbidity currents pose a serious hazard to expensive oil and gas seafloor installations, especially in deep-water where mitigation, re-routing or repair is costly and logistically challenging. These sediment-laden flows are hazardous because they can be exceptionally powerful (up to 20 m/s), and can flow for long distances (>100s km) over several days duration, causing damage over vast areas of seafloor. Even less powerful flows (~1-2 m/s) can damage seafloor equipment, or break strategically important submarine telecommunication cables. The consequences of turbidity currents impacting seafloor structures depends on the velocity, duration, direction of impact and, perhaps most crucially, the sediment concentration (or density) of the flow. While some recent studies have successfully monitored turbidity currents in deep-water, imaging flow properties close to the seafloor has proven problematic. We present innovative approaches to the quantification of the velocity and sediment concentration of dense near-bed layers that provide new insights into this important aspect of turbidity current flow.
Firstly, we describe a novel experimental setup that is capable of measuring near-bed sediment concentration in dense (>10% volume by concentration) flows. Density contrasts are measured using Electrical Resistivity Tomography – a technique initially developed for geophysical characterisation of subsurface reservoirs. Velocity is measured using Ultrasonic Doppler Velocity Profiling and concentration is characterized using an Ultra High Concentration Meter. Secondly, we outline some recently developed geophysical approaches for the quantification of sediment concentration and velocity for real-world flows based on recent work in fjords, estuaries and deep-sea canyons. This includes integrated moored deployments of Acoustic Doppler Current Profilers, Multibeam Sonars, and a novel Chirp array. We outline some limitations and advantages of these methods. Finally, we underline the value and importance of establishing multiple field-scale test sites in a variety of settings, including deep-water, that will enhance the industry's understanding of turbidity current hazards.
Our results demonstrate the importance of near-bed dense layers for turbidity current interaction with seafloor structures. Density contrasts and pressure build up at the base of a flow may lead to uplift, undermining and loss of support, dragging, or pipeline rupture; hence quantification of this layer is crucial for hazard assessment.
Measurements of sediment concentration within turbidity currents are incredibly rare, and yet are a vital input for any numerical model that aims to predict sediment transport by turbidity currents in deep-water settings. Currently it is necessary to infer densities and velocities; however, such inferences are poorly calibrated against experimental or real world data. Our measurements underline the importance of understanding near-bed dense layers.
The global network of subsea fibre-optic cables plays a critical role in the world economy and is considered as strategic infrastructure for many nations. Sediment density flows have caused significant disruption to this network in the recent past. These cable breaks represent the only means to actively monitor such flows over large oceanic regions. Here, we use a global cable break database to analyse tropical cyclone triggering of sediment density flows worldwide over 25 yrs. Cable breaking sediment density flows are triggered in nearly all areas exposed to tropical cyclones but most occur in the NW Pacific. They are triggered by one of three sets of mechanisms. Tropical cyclones directly trigger flows, synchronous to their passage, as a consequence of storm waves, currents and surges. Cyclones also trigger flows indirectly, with near-synchronous timing to their passage, as a consequence of peak flood discharges. Last, cyclones trigger flows after a delay of days as a consequence of the failure of large volumes of rapidly deposited sediment. No clear relationship emerges between tropical cyclone activity (i.e. track, frequency and intensity) and the number of sediment density flows triggered. This is a consequence of the short period of observation. However, expansion of the cable network and predicted changes to cyclone activity in specific regions increases the likelihood of increasing numbers of damaging flows.
Time-series bathymetric data acquired from 1967 to 2011 are used to evaluate the morphological evolution of the continental shelf and upper continental slope off the city of Nice (SE France, Ligurian Sea). Mapping in water depths of 0–300 m was undertaken to identify the changing morphology of landslide scars and their erosive chutes. Quantitative Digital Terrain Model (DTM) comparisons reveal areas of erosion and deposition over intervals of 5–8 years. Sediment remobilization events on the upper slope (above depths of 200 m) are frequent and significant; landslide scars with volumes > 25000 m3 can occur with frequencies of < 8 years. Shelf break retreat toward the coastline can reach 60 m over 7–8 years where the continental shelf is wider (> 200 m). Periods of quiescence (1980–1990 and 2006–2011) are seen to alternate with periods when rapid retrogressive failure increase sediment volumes eroded from the upper slope-shelf transition by an order of magnitude (1999–2006). Temporal variations in landslide activity were correlated to several potential triggering factors that individually would not induce failures, including earthquake activity, rapid deposition of fine-grained sediments on a steep slope, and rainfall leading to fresh groundwater circulation below the shelf. This 4D bathymetric study suggests that over the last 50 years the most important factor triggering landslides offshore Nice is overpressure due to freshwater outflows.
At least 17 subsea telecommunications cables cross the Gaoping Canyon and Manila Trench system in the Strait of Luzon between Taiwan and the Philippines. There, cable breaks record rapid (5–16 ms− 1), long run-out (> 300 km) sediment density flows triggered by earthquakes and typhoons. Four major cable-breaking events have occurred in the last decade. In 2006, the Pingtung ML = 7.0 earthquakes formed up to 3 individual flows, some of which ran-out for up to 460 km. In 2009, Typhoon Morakot generated two sediment flows; the first was triggered by hyperpycnal river discharge, whereas a second flow formed 3 days later, possibly due to failure of recently deposited flood sediment in upper Gaoping Canyon. A flow in 2010 formed during a swarm of ML = 3–5 earthquakes that followed the ML = 6.4 Jiashian earthquake. Finally, Typhoon Soudelor of 2015 caused the Gaoping River to form an hyperpycnal plume that failed to break cables at < 2600 m depth, but broke at least 6 cables in deeper water.
The expansion of offshore renewable energy infrastructure and the need for trans-continental shelf power transmission require the use of submarine high-voltage (HV) cables. These cables have maximum operating surface temperatures of up to 70°C and are typically buried 1-2 m beneath the seabed, within the wide range of substrates found on the continental shelf. However, the heat flow pattern and potential effects on the sedimentary environments around such anomalously high heat sources in the near-surface sediments are poorly understood. We present temperature measurements from a 2-D laboratory experiment representing a buried submarine HV cable, and identify the thermal regimes generated within typical unconsolidated shelf sediments-coarse silt, fine sand and very coarse sand. We used a large (2 × 2.5 m²) tank filled with water-saturated spherical glass beads (ballotini) and instrumented with a buried heat source and 120 thermocouples to measure the time-dependent 2-D temperature distributions. The observed and corresponding Finite Element Method simulations of the steady state heat flow regimes and normalized radial temperature distributions were assessed. Our results show that the heat transfer and thus temperature fields generated from submarine HV cables buried within a range of sediments are highly variable. Coarse silts are shown to be purely conductive, producing temperature increases of >10°C up to 40 cm from the source of 60°C above ambient; fine sands demonstrate a transition from conductive to convective heat transfer between cf. 20 and 36°C above ambient, with >10°C heat increases occurring over a metre from the source of 55°C above ambient; and very coarse sands exhibit dominantly convective heat transfer even at very low (cf. 7°C) operating temperatures and reaching temperatures of up to 18°C above ambient at a metre from the source at surface temperatures of only 18°C. These findings are important for the surrounding near-surface environments experiencing such high temperatures and may have significant implications for chemical and physical processes operating at the grain and subgrain scale; biological activity at both microfaunal and macrofaunal levels; and indeed the operational performance of the cables themselves, as convective heat transport would increase cable current ratings, something neglected in existing standards.
This study was initiated because underwater mass movements in Lake Mjøsa, Norway had caused utility pipeline breakages. Multibeam bathymetry, sub bottom profiler data and samples were acquired to allow morphological interpretations of the processes leading to the mass movements. The underwater slopes of the lake generally show gradients of 15–20°, but exceed 30° in places. The sediment thickness above acoustic basement interpreted from sub bottom profiler data show that the accumulation rate in the central lake basin is about 2 mm/year. Numerous channels are seen on the bathymetry as well as several slide scarps that are about 2 m high. The channels are interpreted to be caused by dense water cascading, probably induced by winter cooling. Calculations based on geotechnical tests of samples indicate that sediment layers in excess of 2 m have a factor of safety less than 1.5 on slopes above 30°. Triggering of slides may thus be spontaneous due to sedimentation, but may also be triggered by oversteepening due to erosional channeling.
Turbidity currents are one of the main sediment transport processes on Earth, yet are notoriously difficult to monitor directly. This article presents the first direct and high bandwidth observation of a turbidity current using a cabled seafloor observatory. On 5 June 2012, a platform on Ocean Networks Canada, located in 107 m of water on the Fraser River delta slope, was displaced downslope and severed from its data cable. The platform weighed about 1000 kg in water. The event took place during high river discharge, high tides and rapid sediment accumulation on adjacent upslope areas of the seafloor. Data recorded as it tumbled down slope allow a reconstruction of the flow, which is inferred to have been an unconfined turbidity current. Lines of evidence indicate that the flow came in as a bed hugging wedge, and built up to between 1 m and 4 m in height as the head passed through. Comparison with laboratory data, suggest that the flow was initially supercritical. While the adjacent slope offset to the north clearly exhibits change over an annual resurvey period, the bathymetry directly at the event location show no resolvable change over a period from seven months before the event to one month after. Sediment cores collected after the event were pervasively biototurbated and they contain no obvious deposit connected with this event. The remarkable aspects of this research follow. The flow was powerful enough to carry a 1 tonne platform and sever a heavily armoured cable. The current occurred on the unchannelized open slope. The powerful event failed to cause discernible seabed elevation change. The flow was triggered by tidal conditions. The event was detected by a purpose-designed cabled observatory, thus providing high bandwidth data and also alerting researchers in real time to mount follow-on investigations. This article is protected by copyright. All rights reserved.
Multisensor piezometer probes were deployed at four different sites in the Mississippi Delta in water depths ranging from 13.5 to 43.6 m with sensor penetration depths of up to 15.6 meters. Absolute and differential pressure sensors were used to measure pore water pressure and excess pressures, respectively. The free water column pressure was measured with absolute pressure sensors. Pore pressures induced by probe insertion were determined as well as ambient excess pore pressures following the time-dependent decay of induced pressures. Significant differences in the pore pressures and related geotechnical properties were found between East Bay and Main Pass sediments. Generally higher probe insertion pressures and lower ambient excess pore pressures were characteristic of Main Pass compared to East Bay. Probe insertion pressures (Ui) were found to correlate well with the undrained shear strength (Su) of the sediments, indicating reasonably good agreement with the predicted relation: Ui = 6Su as suggested by an earlier study42. Using this relationship undrained shear strengths were calculated and compared with measured values.
The offshore industry is moving to depths > 1000 m, along or in the proximity of the continental slopes. The geohazard assessment for offshore installations is an increasingly important engineering activity for a number of current and near-to-come projects. In these circumstances a sequence of dedicated engineering tasks are requested from the early stage of the project, including: - identification of the potential failure modes of the seabed soils and relevant triggering mechanisms; - definition of the probability of occurrence for each failure mode that might interfere with seafloor infrastructures; - analysis of the structural response to loads or imposed displacements, and relevant integrity check. In general, infrastructures dedicated to offshore exploration, production and transportation of hydrocarbons, are designed to meet very stringent safety targets. International consensus was reached in the early 1990 on these targets, after analytical code-based performance analyses, reviews of field experience and comparisons with other disciplines. Indeed, deep and ultra-deep waters where geohazards might be relevant, would mean difficult remedial intervention works in case of incidents, so that the envisaged design criteria are definitely safe for life and often quite demanding at the design stage. In this paper reference is made to offshore pipelines transporting hydrocarbons over long distances, crossing seabeds affected by geohazards, in particular to projects recently completed as Blue Stream and near-to-come as Medgaz and Iran to India. For these strategic infrastructures, reliabilitybased limit state design guidelines currently in force, require that both geohazard specific load effects and pipeline strength capacity are well described in terms of relevant parameters and modelling. Particularly uncertainty measures influencing load occurrence and relevant effects must be known with a suitable degree of confidence to allow rationally based decision on pipeline routing and protection measures, if any and where. Working with theoretical superposition of tails of probabilistic distributions of load and capacity, as required in the probabilistic design or in the calibration of partial safety Factors for Loads and Resistance in the relevant Design formats (LRFD), requires care and a good reference basis for comparison. Structural reliability based design, targeting a failure probability of 10-4 or 10-6 per year, can hardly be based on load occurrence and relevant effects characterized by a large (greater than 0.3) coefficient of variation (load roughness). Unfortunately this is the case for infrastructures along the continental slopes affected by geohazards. Evidence of uncertainty is given by the engineering models dealing with the load transfer capacity from a typical plastic mass (soil and water) flow, running downhill, e.g. triggered by an earthquake, and impacting on a pipeline resting on the seabed, whether free spanning or partially embedded.
The oceans cover 70% of the terrestrial surface, and exert a pervasive influence on the Earth's environment but their nature is poorly recognized. Knowing the ocean's role deeply and understanding the complex, physical, biological, chemical and geological systems operating within it represent a major challenge to scientists today. Seafloor observatories offer scientists new opportunites to study multiple, interrelated natural phenomena over time scales ranging from seconds to decades, from episodic to global and long-term processes."Seafloor Observatories" poses the important and apparently simple question, "How can continuous and reliable monitoring at the seafloor by means of Seafloor Observatories extend exploration and improve knowledge of our planet?" The book leads the reader through: The present scientific challenges to be addressed with seafloor observatoriesthe technical solutions for their architecturean excursus on worldwide ongoing projects and programmessome relevant scientific multidisciplinary resultsanda presentation of new and interesting long-term perspectives for thecoming years.Current results will yield significant improvements and exert a strong impact not only on our present knowledge of our planetbut also on human evolution.
Horizontal strains were measured with high precision during Crossrail tunnel excavation in central London by means of distributed fibre-optic sensors embedded in shallow transverse and longitudinal surface trenches. Validation of these measurements against manual micrometer stick readings demonstrates the benefits of the fibre-optic sensing: high precision at small strains; significantly smaller spatial resolution; and the possibility for automatised measurements over larger spans. In order to further capitalise on these advantages, two additional potential applications of the fibre-optic measurements were investigated: (a) independent assessment of the vertical ground displacements using transverse strain measurements and analytical settlement trough models; and (b) early prediction of the vertical and horizontal ground displacements, using transverse and longitudinal strain measurements and analytical settlement trough models. While understanding that a single case study can only form a basis for limited conclusions, both applications do appear to be feasible.
Using beach ball–like detectors, researchers set out to determine how sediments, which could contain toxic contaminants, travel through submarine canyons to greater depths.
For decades, oceanographers have gone to sea to study limited portions of the ocean for short periods of time. In the 1950s, the reach and duration of research in the oceans was extended, utilizing limited bandwidth satellite systems for surficial imaging and communications.
Turbidite records along ocean margins and in lake basins are increasingly used as paleoseismic proxies. However, the slope remobilization processes that formed the earthquake-related turbidity currents are poorly understood and, therefore, it is difficult to assess the consistency of turbidite paleoseismic records. We analyzed the sedimentary imprint of the giant AD 1960 (Mw 9.5) Chile earthquake in four South-Central Chilean lakes. Mass-transport deposits and turbidites were compared by interpreting reflection-seismic profiles, sedimentary facies, volume balances and proxies for turbidite composition (i.e. grain size, radionuclides, sediment color, organic matter). Sediment lightness was used as a proxy for organic matter content. Comparison of lightness of turbidites and the underlying sediments shows that earthquake-triggered turbidites in the four lakes result from remobilization of a thin veneer (on average about 5 cm) of slope sediments and do not result from disintegration of subaquatic landslides. Statistical analysis of 34 turbidites in a new 5200 year sedimentary record does not show any correlation between inter-event time and turbidite thickness (and thus volume). This means that inter-event slope recharging is not a dominant factor governing turbidite recurrence and that initiation of turbidity currents by remobilization of surficial sediments is different to slope failure related to subaquatic landslides. We conclude that surficial slope sediment remobilization forms a valid mechanism that explains why turbidite paleoseismic records can be of excellent continuity and high sensitivity. Moreover, based on correlations between seismic intensity, turbidite volume and stratigraphic remobilization depth, we propose that surficial slope sediment remobilization allows for turbidite records containing information about paleo-earthquake intensity.
An active fjord head delta in Squamish British Columbia, was selected as the location for a repetitive multibeam survey program to monitor temporal evolution of the prodelta morphology. Daily resurveys in 2011 established the style and extent of submarine mass movements, their typical periodicity and the conditions associated with the most active periods. This has now been followed by an hourly resurvey program in 2012 during those most active periods to actually witness the progression of activity immediately preceding, during and subsequent to a singular event. The delta front in depths of 20–50 m is often the apparent start point for trains of sequential erosive and depositional events associated with upslope bedform migration along prodelta channels. Heavy targets on the channel floors were monitored in 2011 and indicated rare, abrupt down channel displacements of a few hundred metres, indicating that a small subset of events involved bulk translation of the seabed. In 2011, a bottom-mounted ADCP beyond one channel mouth recorded clear turbidity current events for a subset of the channel bedform migration periods. In 2012, using multibeam water column imaging and a rapidly dipping towed optical backscatter probe, the evolution of a descending suspended sediment plume below the overlying river plume was monitored on an hourly basis. Towards low water, that descending plume was seen to occasionally feed a near seabed higher suspended sediment layer. On the development of this layer, the water column imaging revealed a thin basal flow that lasted about an hour and corresponded directly with the period of migration of the channel floor bedforms. Delta-lip failures are associated with the upslope end of about half of the bedform trains suggesting an alternate initiating mechanism.
The development and application of acoustic and optical technologies and of accurate positioning systems in the past 30 years have opened new frontiers in the submarine canyon research communities. This paper reviews several key advancements in both technology and science in the field of currents in submarine canyons since the 1979 publication of Currents in Submarine Canyons and Other Sea Valleys by Francis Shepard and colleagues. Precise placements of high-resolution, high-frequency instruments have not only allowed researchers to collect new data that are essential for advancing and generalizing theories governing the canyon currents, but have also revealed new natural phenomena that challenge the understandings of the theorists and experimenters in their predictions of submarine canyon flow fields. Baroclinic motions at tidal frequencies, found to be intensified both up canyon and toward the canyon floor, dominate the flow field and control the sediment transport processes in submarine canyons. Turbidity currents are found to frequently occur in active submarine canyons such as Monterey Canyon. These turbidity currents have maximum speeds of nearly 200 cm/s, much smaller than the speeds of turbidity currents in geological time, but still very destructive. In addition to traditional Eulerian measurements, Lagrangian flow data are essential in quantifying water and sediment transport in submarine canyons. A concerted experiment with multiple monitoring stations along the canyon axis and on nearby shelves is required to characterize the storm-trigger mechanism for turbidity currents.
Subaqueous sediment density flows are one of the volumetrically most important processes for moving sediment across our planet, and form the largest sediment accumulations on Earth (submarine fans). They are also arguably the most sparely monitored major sediment transport processes on our planet. Significant advances have been made in documenting their timing and triggers, especially within submarine canyons and delta-fronts, and freshwater lakes and reservoirs, but the sediment concentration of flows that run out beyond the continental slope has never been measured directly. This limited amount of monitoring data contrasts sharply with other major types of sediment flow, such as river systems, and ensure that understanding submarine sediment density flows remains a major challenge for Earth science. The available monitoring data define a series of flow types whose character and deposits differ significantly. Large (> 100 km3) failures on the continental slope can generate fast-moving (up to 19 m/s) flows that reach the deep ocean, and deposit thick layers of sand across submarine fans. Even small volume (0.008 km3) canyon head failures can sometimes generate channelised flows that travel at > 5 m/s for several hundred kilometres. A single event off SE Taiwan shows that river floods can generate powerful flows that reach the deep ocean, in this case triggered by failure of recently deposited sediment in the canyon head. Direct monitoring evidence of powerful oceanic flows produced by plunging hyperpycnal flood water is lacking, although this process has produced shorter and weaker oceanic flows. Numerous flows can occur each year on river-fed delta fronts, where they can generate up-slope migrating crescentic bedforms. These flows tend to occur during the flood season, but are not necessarily associated with individual flood discharge peaks, suggesting that they are often triggered by delta-front slope failures. Powerful flows occur several times each year in canyons fed by sand from the shelf, associated with strong wave action. These flows can also generate up-slope migrating crescentic bedforms that most likely originate due to retrogressive breaching associated with a dense near-bed layer of sediment. Expanded dilute flows that are supercritical and fully turbulent are also triggered by wave action in canyons. Sediment density flows in lakes and reservoirs generated by plunging river flood water have been monitored in much greater detail. They are typically very dilute (< 0.01 vol.% sediment) and travel at < 50 cm/s, and are prone to generating interflows within the density stratified freshwater. A key objective for future work is to develop measurement techniques for seeing through overlying dilute clouds of sediment, to determine whether dense near-bed layers are present. There is also a need to combine monitoring of flows with detailed analyses of flow deposits, in order to understand how flows are recorded in the rock record. Finally, a source-to-sink approach is needed because the character of submarine flows can change significantly along their flow path.
Continental slope sediment failures around the epicentre of the 1929 ‘Grand Banks’ earthquake have been imaged with the SAR (Système Acoustique Remorqué) high-resolution, deep-towed sidescan sonar and sub-bottom profiler. The data are augmented by seismic reflection profiles, cores and observations from submersibles. Failure occurs only in water depths greater than about 650 m. Rotational, retrogressive slumps, on a variety of scales, appear to have been initiated on local steep areas of seabed above shallow (5–25 m) regional shear planes covering a large area of the failure zone. The slumps pass downslope into debris flows, which include blocky lemniscate bodies and intervening channels. Clear evidence of current erosion is found only in steep-sided valleys: we infer that debris flows passed through hydraulic jumps on these steep slopes and were transformed into turbidity currents which then evolved ignitively. Delayed retrogressive failure and transformation of debris flows into turbidity currents through hydraulic jumps provide a mechanism to produce a turbidity current with sustained flow over many hours.
Autonomous underwater vehicles (AUV) are untethered submarine robots that can be used to carry out deepwater mapping and seabed-characterization surveys (seafloor to 150-m depth). AUV surveys are used in the marine petroleum industry for various exploration, environmental, geohazard, and engineering applications. Typically, AUVs are launched from a mother ship. They execute a preprogrammed survey pattern and are recovered, and the survey data are downloaded for analysis. Data on water depth, geomorphology, stratigraphy, and structure ranging downward to 150 m below the seafloor routinely are collected using a variety of sensing technology. Principal survey tools include a multibeam echo sounder to provide water column, bathymetric, and seafloor-reflectivity data; a side-scan sonar to provide high-resolution seafloor imagery; and a subbottom profiler (1- to 24-kHz seismic-reflection tool) to show subseafloor stratigraphy and structural features. Other AUV survey tools can include still cameras, lidar scanners, magnetometers, geochemical sensors (CO2, CH4, PAH, dissolved oxygen), and temperature and salinity sensors. AUV surveys help deepwater exploration and development to proceed efficiently and safely and thus have become an indispensable tool for deepwater operations.
Over 25,300 seabed pockmarks were mapped from the Rosetta Channel region of the Western Nile Deep Sea Fan (NDSF) using concurrent High Resolution 2D, Chirp profiler and multibeam bathymetry data which spans the Holocene–Pleistocene period. Within the region, a pockmark field containing >13,800 pockmarks was analysed using spatial statistics to determine the distribution of pockmarks within the field. Pockmarks within the field are small (∼16 m diameter), shallow (∼0.5 m deep) circular depressions which formed within the last ∼ 6500 years. The fluid source for the field is identified as an accumulation/generation of gas beneath a hemipelagic seal c. 20–40 ms beneath the seabed. The position of the pockmarks is shown to be unrelated to the depth to the fluid source and an irregular high amplitude acoustic anomaly which is tentatively interpreted as a possible carbonate precipitate of biogenic microbial activity. Statistical spatial analysis of the field confirms the distribution of pockmarks is not random. An exclusion zone surrounding each individual pockmark is identified. The exclusion zone is a unique minimum radius around each pockmark which is not penetrated by any other pockmark. The exclusion zone works in unison with Self-Organised Criticality (SOC) in determining the spatial distribution of pockmarks within the field. The exclusion zone is interpreted as a pockmark “drainage cell”. A conceptual model for a pockmark drainage cell is proposed whereby pockmark formation dissipates a radius/area of fluid and overpressure, thereby preventing the formation of another pockmark within that cell. Consequently, pockmarks are observed to separate or produce anti-clustering tendencies within the field.
Field measurements of oceanic turbidity currents, especially diluted currents, are extremely rare. We present a dilute turbidity current recorded by instrumented moorings 14.5 km apart at 1300 and 1860 m water depth. The sediment concentration within the flow was 0.017%, accounting for 18 cm/s gravity current speed due to density excess. Tidal currents of similar to 30 cm/s during the event provided a "tailwind" that assisted the down-canyon movement of the turbidity current and its sediment plume. High-resolution velocity measurements suggested that the turbidity current was likely the result of a local canyon wall slumping near the 1300 m mooring. Frequent occurrences, in both space and time, of such weak sediment transport events could be an important mechanism to cascade sediment and other particles, and to help sustain the vibrant ecosystems in deep-sea canyons.
We present a numerical model developed to rationally predict the trajectory, the run-out distance and frontal velocity of pulsed submarine mass gravity flows, and illustrate how it forms an essential building block of an integrated geohazards assessment for deep offshore pipeline and cable developments. The model is inspired from Center-of-Mass approaches initially proposed for pulsed snow avalanches, but specifically adapted to the treatment of submarine sediment density flows. We outline how the modeling approach fits into the broader classes of possible modeling strategies and discuss the implications in terms of CPU requirements for field applications. The model relies on simple mass and momentum balance relations for the frontal part of the sediment density flow, with additional terms accounting e.g. for ambient mixing, sediment detrainment or added mass. Depending on the sediment type various rheologies and friction relations can be adopted. Herein we consider a shear resistance derived from a Herschel-Bulkley rheology and a Chezy-type entraining shear relation.
The model may be used to predict the spatial and temporal development of a sediment density flow from inception to runout on a full 3D bathymetry. Because it writes as a system of ordinary (rather than partial) differential equations, it is solved efficiently and is more amenable to repeated simulations than depth-integrated Finite Element models which may rapidly become CPU-prohibitive for field applications. The approach is therefore advantageously used at the stage of a geohazards risk assessment, during which many distinct scenarios may need to be evaluated. The sensitivity of model outputs to differing trigger locations, failed volumes corresponding to e.g. a design seismic scenario, and soil parameters, can be readily investigated, and model outputs possibly combined into an integrated map of maximum velocities and runout distances. These parameters allow estimating the magnitude and duration of forces impacting on submarine pipelines and cables, which can then be used as inputs for a verification of structural integrity based on a pipe/soil interaction model.
The adequacy of the approach is first tested on small-scale laboratory experiments of pulsed debris flows down a slope, including documented experiments from the literature previously used for validation purposes. Then, its application at field scale is illustrated for a hypothetical case of a localized slope failure on the steep flank of a sloping submarine canyon.
We illustrate the typical force signal imposed on a pipeline from recent laboratory experiments on debris flows impacting an instrumented pipe section. We then outline how the outputs of the flow modeling exercise may provide input for a quantitative assessment of potential damage and displacement endured by a submarine pipeline laid along a prescribed route. Impact forces estimated from the simulated conditions of the density flow upon impact are fed into a 3D finite element software modeling the structural response of the pipeline, using adequate pipe-soil interaction parameters and lay conditions.