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Geophysical prelude to first exploitation of submarine massive sulphides
... In hydrothermal systems hosted within mafic to felsic volcanic rocks, the alteration of titanomagnetite to titanomaghemite, as well as the dissolution of titanomagnetite and subsequent formation of pyrite, by high-temperature fluids produces zones of anomalously low magnetic susceptibility and magnetization (11,12). Three-dimensional (3D) minimum-structure inverse modeling can be used to model the location and geometry of these anomalously low magnetism zones within the crust (10,13). Minimum-structure inversion of a magnetic field dataset constructs a 3D distribution of effective magnetic susceptibility in the seafloor that closely reproduces the measured survey data and that contains only sufficient features to reproduce those data (14). ...
... To date, magnetic modeling of the seafloor has been limited to the shallow subseafloor at hydrothermal discharge sites (10,11,13,26). Our study significantly improves upon the scale of these previous magnetic models by imaging the entire structure of a high-temperature convection column, including the identification of the top of the underlying magma chamber. ...
Hydrothermal fluid circulation beneath the seafloor is an important process for chemical and heat transfer between the solid Earth and overlying oceans. Discharge of hydrothermal fluids at the seafloor supports unique biological communities and can produce potentially valuable mineral deposits. Our understanding of the scale and geometry of subseafloor hydrothermal circulation has been limited to numerical simulations and their manifestations on the seafloor. Here, we use magnetic inverse modeling to generate the first three-dimensional empirical model of a hydrothermal convection system. High-temperature fluid-rock reactions associated with fluid circulation destroy magnetic minerals in the Earth’s crust, thus allowing magnetic models to trace the fluid’s pathways through the seafloor. We present an application of this modeling at a hydrothermally active region of the East Manus Basin.
... Even though von Herzen et al. (1996) and Cairns et al. (1996) conducted initial studies on electric measurements at the Trans-Atlantic Geotraverse (TAG) hydrothermal mount, it took over 10 years until severe interest in marine, highgrade, polymetallic deposits and their exploration was fuelled by rising resource prices. Kowalczyk (2008) performed first interdisciplinary studies, including bathymetric and magnetic surveys and a remotely operated vehicle (ROV)-based coil experiment, to search for conductive structures at the Solwara 1 hydrothermal field. Swidinsky et al. (2012) studied the sensitivity of transient loop sensors for submarine massive sulphide exploration. ...
... By the nature of marine offshore surveys, the time-cost-benefit equation is critical. This favours the development of multi-sensor platforms or steerable ROV/autonomous unmanned vehicle (AUV)-based designs, to refine and complement the measured data set and enhance geological interpretations (Kowalczyk, 2008;Lee et al., 2016;Bloomer et al., 2018). Here, a spatial separation between the transmitter/receiver and the metal components is constrained by the extremely limited space, if not completely unfeasible. ...
Electromagnetic loop systems rely on the use of non‐conductive materials near the sensor to minimize bias effects superimposed on measured data. For marine sensors, rigidity, compactness, and ease of platform handling are essential. Thus, commonly a compromise between rigid, cost‐effective, and non‐conductive materials (e.g. stainless steel versus fiberglass composites) needs to be found. For systems dedicated to controlled‐source electromagnetic measurements, a spatial separation between critical system components and sensors may be feasible, whereas compact multi‐sensor platforms, remotely operated vehicles, and autonomous unmanned vehicles require the use of electrically conductive components near the sensor. While data analysis and geological interpretations benefit vastly from each added instrument and multidisciplinary approaches, this introduces a systematic and platform immanent bias in the measured electromagnetic data. In this scope we present two comparable case studies targeting loop‐source electromagnetic applications in both time and frequency domain: the MARTEMIS time domain system trades the compact design for a clear separation of 15 m between an upper fiberglass frame, holding most critical titanium system components, and a lower frame with its coil and receivers. In case of the GOLDEN EYE frequency domain profiler, the compact and rigid design is achieved by a circular fiberglass platform, carrying the transmitting and receiving coils, as well as several titanium housings and instruments. In this study, we analyze and quantify the quasi‐static influence of conductive objects on time and frequency domain coil systems by applying an analytically and experimentally verified 3D finite element model. Moreover, we present calibration and optimization procedures to minimize bias inherent in the measured data. The numerical experiments do not only show the significance of the bias on the inversion results, but also the efficiency of a system calibration against the analytically calculated response of a known environment. The remaining bias after calibration is a time/frequency dependent function of seafloor conductivity, which doubles the commonly estimated noise‐floor from 1% to 2%, decreasing the sensitivity and resolution of the devices. By optimizing size and position of critical conductive system components (e.g. titanium housings) and/or modifying the transmitter/receiver geometry, we significantly reduce the effect of this residual bias on the inversion results as demonstrated by 3D‐modelling. These procedures motivate the opportunity to design dedicated, compact, low‐bias platforms and provide a solution for autonomous and remotely steered designs by minimizing their effect on the sensitivity of the controlled‐source electromagnetic sensor.
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... Consequently, there has been an ongoing development of marine EM systems for this purpose. In 2008, Ocean Floor Geophysics Inc. (OFG) developed and patented the first EM system to successfully map the limits of conductive near-surface mineralization in a survey of an SMS deposit at Solwera1 [2]. Since then, towed coincident loop time domain systems have been developed. ...
... To mitigate these problems, reduce costs on ship and crew to deploy the system, and to enable the simultaneous collection of high-resolution complementary data (MBES bathymetry, SSS, SBP, ocean chemistry, magnetometry), an AUV-CSEM concept was developed to (1) use battery powered transmitters placed on the seafloor to generate electrical fields that interact with the subsurface geology; (2) integrate the EM receiver electrodes into the Fukada AUV "Deep1" to measure the resulting electric fields; and (3) eventually use these E-field measurements to generate 3-D subsurface conductivity distributions over the survey area. This paper will summarize the highlights of two AUV-CSEM engineering tests performed in 2015 and 2016 by OFG, Fukada, and Scripps to resolve technical uncertainties and advance the use of CSEM technology with a survey AUV as a platform. ...
... Consequently, there has been an ongoing development of marine EM systems for this purpose. In 2008, Ocean Floor Geophysics Inc. (OFG) developed and patented the first EM system to successfully map the limits of conductive near-surface mineralization in a survey of an SMS deposit at Solwera1 [2]. Since then, towed coincident loop time domain systems have been developed. ...
... To mitigate these problems, reduce costs on ship and crew to deploy the system, and to enable the simultaneous collection of high-resolution complementary data (MBES bathymetry, SSS, SBP, ocean chemistry, magnetometry), an AUV-CSEM concept was developed to (1) use battery powered transmitters placed on the seafloor to generate electrical fields that interact with the subsurface geology; (2) integrate the EM receiver electrodes into the Fukada AUV "Deep1" to measure the resulting electric fields; and (3) eventually use these E-field measurements to generate 3-D subsurface conductivity distributions over the survey area. This paper will summarize the highlights of two AUV-CSEM engineering tests performed in 2015 and 2016 by OFG, Fukada, and Scripps to resolve technical uncertainties and advance the use of CSEM technology with a survey AUV as a platform. ...
... Seafloor massive sulphide (SMS) deposits are a potential source of base and precious metal resources, and lately interest in exploring for and economically exploiting these deposits has been increasing (Kowalczyk 2008). Transient electromag- Figure 1 Illustration of the Coil2Dipole configuration, consisting of a horizontally suspended square frame housing a loop transmitter that is towed by a ship. ...
... Remote two-component electric dipole receivers are deployed on the seafloor. (Kowalczyk 2008;Hölz et al. 2015a;Asakawa et al. 2016;Nakayama and Saito 2016). Marine CSEM systems typically employ a ship-towed electric dipole transmitter and electric dipole receivers placed on the seafloor. ...
We study a new marine electromagnetic configuration which consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulfide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modeling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly‐buried conductive material at up to ∼100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few 10s of meters. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modeling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulfide deposits. This article is protected by copyright. All rights reserved
... This is a lot simpler than flying over a conductive body at a well-documented test site (which is usually far away). Kowalczyk (2008) describes an innovative system for collecting controlled source EM data on the sea floor for massive sulphide (black smoker) exploration. These systems must acquire data in very rough and rugged terrain, and the electric field measurements commonly used in marine controlled source electromagnetics (mCSEM) would not be feasible. ...
... Combrinck et al. (2009b) give an example of geological and structural mapping of a covered area in Namibia, and Finn et al. (2010) describe how regional AEM can be used to assist in mapping overburden thickness that can then be used to guide subsequent exploration programs. Espinosa-Corriols and Kowalczyk (2008) document the collection of a number of geophysical data sets in the Quesnel porphyry belt and conclude that the airborne VTEM survey is useful for mapping the overburden thickness. A fixed-wing frequency-domain survey of the Isle of Wight was used to show that the measured conductivity could be diagnostic of the lithologies on the geological map (Beamish 2011). ...
In the period from 2008 to 2012, the topic of electromagnetic (EM) induction methods applied to mineral exploration has been the subject of more than 50 papers in journals and more than 300 extended abstracts presented at conferences (about 100 of which contain developments worthy of mentioning). Most of the work at the universities has been on modelling, inversion and data processing, and most of this material is published in the refereed literature. However, academia has also undertaken work on system geometry changes, system calibration and sensor design. There have been papers describing new systems developed for mineral exploration and case histories describing the use of EM methods to directly discover mineral deposits or to map the geology. Most of this work is by the service companies and mining companies and reported in the unrefereed literature. Since 2008, the pace of development of helicopter time-domain systems has slowed and more effort has been directed to developing natural source magnetic systems and to modelling and inverting this data. A number of studies comparing the results from natural source methods with the results from artificial source methods conclude that the natural source methods can see large-scale geological structures usually when there is a weak conductivity contrast with the surrounding material, but the natural source methods are unable to see small features that have a very large conductivity contrast with the country rock. Hence, they are not a good detector of mineral deposits unless one is looking for a large porphyry system.
... For the exploration targeting marine mineral resources, including polymetallic sulfides, cobalt-rich ferromanganese crusts, and manganese nodules, ROVs and AUVs are commonly used for underwater work. Various types of geophysical surveys have successfully deployed ROVs and AUVs; e.g. a sea-floor EM survey for polymetallic sulfides deposits (Kowalczyk 2008), a TDEM, Spontaneous potentials (SP), and Magneto-Impedance (MI) magnetometer system suspended from an ROV , and a sea-floor DC resistivity EM survey (Goto et al. 2013). Japan Agency for Marine-Earth Science and Technology (JAMSTEC) succeeded in the undersea structural survey for the first time in 2018 by operating multiple AUVs (Japan Agency for Marine-Earth Science and Technology 2018). ...
Breakthrough technologies for mineral exploration are discussed from two perspectives. The first perspective is intended to discuss the important factors required for exploration technologies, derived deductively from a review of the role and expectations of exploration in the mining industry and the current situation of the mining industry. The second perspective is intended to discuss the common characteristics of breakthrough technologies for mineral exploration derived inductively from a review of specific examples of technological breakthroughs that actually brought about innovation to exploration: e.g. induced polarization (IP); airborne electromagnetics (EM); airborne gravity gradiometry (AGG); spectroscopic methods; global navigation satellite system (GNSS); unmanned survey platform; and neural networks/deep learning. The specific issues to be solved as breakthroughs in exploration technology in the near future are summarized as follows:
(1)
Significant improvement in economic efficiency, namely: labor-saving, automated or unmanned operation, e.g. unmanned aerial vehicle (UAV)-based exploration and automatic spectroscopic scanning of drill cores; higher accessing capability, e.g. improvement of various airborne exploration techniques including airborne EM, AGG, and especially airborne IP; higher work efficiency and productivity, e.g. automatic data processing by neural networks/deep learning; and lower cost to implement, e.g. less expensive platforms such as UAVs.
(2)
To obtain information that is really needed for exploration, specifically: IP effect of sulfide minerals associated with mineralization, i.e. practical spectral IP (SIP); geophysical characterization of the deep underground, e.g. enhancement of superconducting quantum interference device (SQUID)-based time-domain electromagnetic (TDEM); and removal of effects of the surface layer, e.g. very conductive deeply-weathered overburden and younger volcanics rich in magnetic minerals.
(3)
To obtain information that could not be obtained by the conventional methods, specifically: distribution of endmember minerals related to mineralization, i.e. hyperspectral mapping with high spatial resolution.
... There is a resistivity contrast between highly conductive SMS and relatively resistive surrounding rocks (Cairns et al., 1996;Von Herzen et al., 1996;Kowalczyk, 2008;Spagnoli et al., 2016;Safipour et al., 2018;Ishizu et al., 2019). Marine CSEM methods are able to detect SMS deposits because of this resistivity contrast. ...
Three-dimensional (3D) marine controlled-source electromagnetic (CSEM) surveys for mapping hydrocarbons use dozens of ocean-bottom electric (OBE) receivers deployed in a grid pattern and several transmitter towlines. This study considers seafloor massive sulfides (SMS) exploration and the horizontal survey scale of SMS is a few kilometers, which is small compared to hydrocarbon surveys of tens of kilometers. If we apply a 3D CSEM survey using a receiver deployment on grids to map SMS, high survey costs will be incurred despite the small survey size. We newly propose a cost-effective 3D marine CSEM survey that uses fewer receivers than the survey with a receiver deployment on grids to reduce survey costs for SMS. This proposed CSEM survey uses a line of OBE receivers in the center of the survey area and several transmitter towlines. Numerical tests demonstrate that the proposed survey (7 OBE receivers) using 80% fewer receivers than the survey with a receiver deployment on grids (35 OBE receivers) is able to accurately map SMS, obtaining a similar performance to that of the receiver deployment on grids. Then, we explored SMS in the Ieyama hydrothermal area off Okinawa, southwest Japan, using the proposed 3D CSEM survey with a line of six OBE receivers and three transmitter towlines. The resulting 3D resistivity distribution from the observed data highlights three potential SMS zones consisting of 0.2 ohm-m low resistivity embedded into 1 ohm-m sediment.
... W hile to our knowledge no geophysical work has been performed in this area beyond regional satellite gravity & magnetic imaging as indicated in Figure 9, various marine massive sulfide deposits have been investigated using gravity, magnetic, electrical, electromagnetic, self-potential and reflection/refraction seismic methods outside of the Arabian Peninsula (e.g. Kowalczyk 2008;Safipour et al., 2017Safipour et al., , 2018Gehrmann et al. 2019). The positive results of these studies suggest that analogous marine geophysical work in the Red Sea to better understand and characterize these unique metalliferous SEDEX muds will occur in the near future. ...
... Although the idea of applying EM methods to detect SMS deposits has been introduced for decades [12], the focus of marine EM surveys has been on seafloor resistive targets associated with hydrocarbon reservoirs or gas hydrates within the last decades [13][14][15], and suitable marine EM systems for SMS exploration are still in active development [3,11]. Until recently, several pilot EM surveys have been conducted to detect the structures of SMS and demonstrated their applicability in SMS exploration (e.g., [2,4,5,7,[16][17][18][19][20]. ...
Seafloor massive sulfide (SMS) deposits have attracted growing interest and become the focus of current seafloor mineral exploration. One key challenge is to delineate potential SMS accumulations and estimate their quantity and quality for prospective resource mining. Recently, geophysical electromagnetic methods which are routinely used for land-based mineral exploration are being adapted to detect and assess SMS occurrences by imaging their conductivity distributions. However, the rough seafloor topography and electrical anisotropy of the seafloor formations encountered in practical surveys pose challenges for reliable data interpretation, and recent studies have revealed that the rough bathymetry could cause measurable distortions. Here, we consider a fixed-offset marine controlled-source electromagnetic method (CSEM) for SMS exploration, and investigate the effects of electrical anisotropy of sedimentary formations through numerical simulations for marine CSEM surveys aiming at conductive targets in the shallow regions of the seafloor. Numerical results demonstrate that the presence of electrical anisotropy could impose significant influence on fixed-offset marine CSEM data and suggest that the distortions should be sufficiently accounted for reliable data interpretation, thus lending confidence to subsequent quantification of available SMS minerals.
... However, they had low sensitivity to near-seafloor structures because of the limited number of receivers and their survey configuration. Some new equipment has been designed for the near-seafloor resistivity mapping but with limited penetration depth of less than several meters (Kowalczyk, 2008). Other new surveys have deeper penetration to tens to hundreds of meters (Constable et al., 2018;Imamura et al., 2018;Müller et al., 2018;Safipour et al., 2017;Safipour et al., 2018). ...
Plain Language Summary
Hydrothermal circulation of seawater through the permeable ocean crust engenders formation of seafloor massive sulfide (SMS) deposits, which present high potential for metal mining. Geophysical surveys using modes such as electrical and electromagnetic methods have revealed that SMS deposits exhibit lower resistivity than the surrounding host rock. However, because detailed geophysical images of internal structures of SMS deposits are lacking, the spatial distribution of SMS deposits and the evolutionary processes of SMS deposits remain unclear. For this study, we applied a deep‐towed marine electrical resistivity tomography (ERT) system to capture detailed images of electrical resistivity structures of SMS deposits in the Iheya North hydrothermal field, Okinawa Trough, southwestern Japan. An optimal sub‐seafloor resistivity section reveals a semi‐layered structure consisting of double low‐resistivity SMS layers: exposed and deep‐seated ones. Between the SMS layers, a cap rock layer is recognized as a moderately resistive zone. This detailed structure offers an explanation of how the SMS deposits accumulate: Hydrothermal fluids upwelling from the deep crust are trapped by less‐permeable cap rock, which results in the precipitation and accumulation of SMS deposits below the cap rock. Then fluids passing through the cap rock to the seafloor produce SMS deposits on the seafloor.
... 海底熱水鉱床調査における深海曳航式電気探査の有効性評価 石須慶一 * ・Chatchai Vachiratienchai ** ・Weerachai Siripunvaraporn *3,*4 後藤忠徳 *,*5 ・笠谷貴史 *6,*7 ・岩本久則 *6,*8 (Tornos et al., 2015) 。海底熱水鉱床 の利用のため，世界各国が技術開発を始めており，カナ 論 文 ダの企業等はパプアニューギニアの領海などで鉱床探査 をすでに開始している (Lipton, 2012) 。海底下の熱水や 海底熱水鉱床の鉱物は，低比抵抗を示すため，海底電気・ 電磁探査はそれらの探査に有効であると考えられ，熱水 域での地下構造調査に用いられてきた(例えば Cairns et al., 1996;Von Herzen et al., 1996) 。商用の海底熱水鉱床の 調査においても海底電磁探査が用いられており， 例えば， Kowalczyk(2008) and Morrison, 1979) ，有限要素法(e.g., Rijo, 1977;Uchida and Murakami, 1990) (Constable et al., 1987;deGroot-Hedlin and Constable, 1990) ...
Seafloor Massive Sulfide (SMS) deposits generated by hydrothermal circulation of sea-water are focused as a new mineral resource. Electric/electromagnetic surveys have been conducted for investigations of SMS deposits. The results revealed that SMS deposits have low resistive features. Therefore, the resistivity structures are useful for imaging distributions of SMS deposits. However, the previous studies in hydrothermal areas have employed simple instruments. More sophisticated and regional-survey techniques for obtaining resistivity structures of SMS deposits have not been established. Here, we focus on a marine deep-towed Direct Current (DC) resistivity survey, and evaluate the effectiveness for imaging sub-seafloor resistivity structures in hydrothermal area. We developed two-dimensional (2D) inversion software for the towed DC resistivity survey, based on the finite-difference method in the forward modeling and the Occam's inversion algorithm, and discussed the effectiveness with numerical simulations.
For evaluating effectiveness of the deep-towed DC resistivity survey, hypothetical models including low resistive anomalies were prepared. With respect to heights of the towed system, depths and thicknesses of the imbedded anomalies, various hypothetical data sets were generated. After the application of our inversion to those data sets, we found that our technique clearly recovered the anomalies, whose depths are less than 40 m and thicknesses are more than 20 m when the length of towed cable is about 180m and the towed height is less than 15 m. Often in deep-sea environment, observation errors of towed height, cable tilt and seawater resistivity are not negligible; therefore, we also examined effects on the inversion results by these errors. As a result, the conditions when the inverted models were distorted by the errors were clarified together with the degree of distortions. Moreover, we carried out inversion tests using synthetic data generated from a realistic model including complex seafloor topography. An exposed SMS deposit on the seafloor was clearly recovered; however, a deep-buried SMS deposit could not be recovered. From these synthetic tests, we conclude that 2D inversion of the towed DC resistivity survey (with cable length of 180 m) is useful for exploring low resistive anomalies situated within 40 m below the seafloor. Thus, this method is suitable for regional survey of near-seafloor resources such as SMS deposits.
... Resistivity can be a useful index for detecting the amount of ore deposits since the deposits is identified as low resistivity. The previous studies reported low resistivity anomalies around active hydrothermal areas (Cairns et al., 1996;Kowalczyk, 2008;Von Herzen et al., 1996). However, these sub-seafloor structures are simplified or have limited depth penetrations, and the detailed structures have not been revealed clearly due to hard operations on the complex topography at hydrothermal region. ...
... Since the early discovery of a black-smoker complex in 1978 on the East Pacific Rise at 21°N, speculations and expectations have been driven about the potential and perspectives of mining seafloor massive sulfide (SMS) deposits in the deep-ocean. With a worldwide accelerating industrialization, emerging markets, increased commodity prices and metal demand, and advance- ments in deep-water mining and extraction technologies, mining of SMS may become economically feasible in the near future (Kowalczyk, 2008). However, we still know little about the resource potential of SMS deposits, and the development of geophysical methods for an assessment of their spatial extent, composition, and inner structure is crucial to derive a proper assessment of their economic value. ...
We present an integrated near-surface mapping approach for marine mineral resources and explain field EM data with petrophysical and geochemical sulfide ore analyses.
(http://fb.eage.org/publication/content?id=94767) // Introduction:
Since the early discovery of a black-smoker complex in 1978 on the East Pacific Rise at 21°N, speculations and expectations have been driven about the potential and perspectives of mining seafloor massive sulfide (SMS) deposits in the deep-ocean. With a worldwide accelerating industrialization, emerging markets, increased commodity prices and metal demand, and advancements in deep-water mining and extraction technologies, mining of SMS may become economically feasible in the near future. However, we still know little about the resource potential of SMS deposits, and the development of geophysical methods for an assessment of their spatial extent, composition, and inner structure is crucial to derive a proper assessment of their economic value. Novel geophysical mapping techniques and exploration strategies are required to locate extinct and buried clusters of SMS deposits, away from the active vent fields and of larger economic potential, but difficult to find and sample by conventional methods.
In 2015 the International Seabed Authority (ISA) assigned an exploration license for polymetallic sulfide deposits to the German Federal Institute for Geosciences and Natural Resources (BGR) in a specified area comprising 100 patches, each 10 × 10 km in size, distributed along the Central and Southeastern Indian Ridge. The challenge to acquire high resolution near-surface electromagnetic (EM) data in such geologically and morphologically complex mid-ocean ridge environments has been addressed by our recent development of the deep-sea profiler GOLDEN EYE that utilizes a frequency-domain electromagnetic (FDEM) central loop sensor, of 3.3 m diameter (Müller et al. 2016). This system has been used in 2015 and 2017 to map active and relict hydrothermal vent fields in the SMS license areas. Aside from technological developments, this paper discusses new data processing routines and methods to unravel the conductivity-depth-distribution, induced polarization and magnetic susceptibility, and joint interpretation with geochemistry as key elements to map and evaluate SMS deposits.
... First SMS studies using electromagnetic (EM) methods revealed high-conductivity zones associated with the TAG field, but lacked in structural resolution (e.g., Cairns et al., 1996). More recent studies have addressed the Solvara 1 field (Kowalczyk 2008), the Okinawa hydrothermal area (Nakayama and Saito 2016), and the Palinuro volcanic complex (Hölz et al. 2015), using compact controlled source EM loop systems with rather small footprint, either passively towed or ROV-mounted. Our own EM studies address potential SMS sites in the German license areas for marine polymetallic sulphides along the Central and Southeastern Indian Ridges Müller et al. 2016). ...
A new deep-sea frequency domain electromagnetic (FDEM) sounding system named GOLDEN EYE has been developed and applied to map active and relict hydrothermal vent fields in the German license areas at the Central and Southeastern Indian Ridges. The spectral electromagnetic data reveal electric conductivity distributions in the shallow subsurface (approx. 0-20 m below seafloor) and allow estimation of induced polarization and magnetic susceptibility of outcropping and buried seafloor massive sulfide (SMS) structures. In-situ mapping results are cross-analyzed with petrophysical data measured on rock samples using electrical impedance spectroscopy and rock magnetics. We will discuss persistent questions like: How electromagnetic methods serve to estimate the resource potential of SMS deposits? Which petrophysical properties are most meaningful and do they work as proxies for rock mineralogy? What are the uncertainties and how to unravel conductivity-depth-distribution, induced polarization and magnetic susceptibility in such geological and morphological complex deep-sea, mid-ocean ridge environments?
... The investigators showed that Open Access *Correspondence: komori.shogo@aist.go.jp the IP effect makes the sulfide particles very conductive at high frequencies and perfectly insulating at low frequencies. In general, electromagnetic investigations are believed to be effective in finding submarine sulfide deposits and have been conducted in order to reveal conductive anomalies related to the presence of deposits (e.g., Kowalczyk 2008;Okada et al. 2017). However, the IP effect makes it possible for sulfide-bearing sediments to be both conductive and insulating, depending on the frequency of the current. ...
... Such methods can reveal the two-dimensional or three-dimensional distribution of subsurface electrical conductivity, which is sensitive to the occurrence of metal-containing materials such as sulphide minerals. These methods, which are being developed for use in marine environments [5][6][7][8] , might also be used effectively in the ocean, but they are difficult to handle as tools for initial surveys without detailed information of the target area. ...
We conducted a self-potential survey at an active hydrothermal field, the Izena hole in the mid-Okinawa Trough, southern Japan. This field is known to contain Kuroko-type massive sulphide deposits. This survey measured the self-potential continuously in ambient seawater using a deep-tow array, which comprises an electrode array with a 30-m-long elastic rod and a stand-alone data acquisition unit. We observed negative self-potential signals not only above active hydrothermal vents and visible sulphide mounds but also above the flat seafloor without such structures. Some signals were detectable >50 m above the seafloor. Analysis of the acquired data revealed these signals’ source as below the seafloor, which suggests that the self-potential method can detect hydrothermal ore deposits effectively. The self-potential survey, an easily performed method for initial surveys, can identify individual sulphide deposits from a vast hydrothermal area.
... Magnetic and electromagnetic methods have been applied in the search for SMS deposits (Wolfgram et al., 1986;Cairns et al., 1996;Kowalczyk, 2008;Tao et al., 2013;Caratori Tontini et al., 2014;Ligi et al., 2014;Szitkar et al., 2015). A less commonly used method is the self-potential (SP) method, which looks for naturally occurring anomalies in electrical potential, which can arise from buried conductive bodies (e.g., massive sulfides, graphite shear zones) (Sato and Mooney, 1960) or from streaming potentials caused by fluid flow (e.g., groundwater) (Revil and Jardani, 2013). ...
The self-potential (SP) method detects naturally occurring electric fields, which may be produced by electrically conductive mineral deposits, such as massive sulfides. Recently, there has been increasing interest in applying this method in a marine environment to explore for seafloor massive sulfide (SMS) deposits, which may contain economic resources of base and precious metals. Although SMS sites that are associated with active venting and are not buried under sediment cover are known to produce an SP signal, the effectiveness of the method at detecting inactive and sediment-covered deposits remained an outstanding question. We built an instrument capable of recording SP data in a marine setting. We carried out a test of the instrument at the Palinuro Seamount in the Tyrrhenian Sea. Palinuro is one of only a few known sites containing an SMS occurrence that is buried under sediment and not associated with active hydrothermal venting, although diffuse seepage of hydrothermal fluids is known to occur at the site. Elevated electric field strengths recorded in and near the site of previously drilled massive sulfide samples are on the order of 1-3 mV/m. A second zone of high field strengths was detected to the north of the drilling area where gravity coring later confirmed the existence of massive sulfides. Our observations indicate that an SP signal can be observed at the site of SMS mineralization even when the mineralized zone is shallowly buried and active hydrothermal venting is not present. These observations could aid in the planning of future marine research expeditions that use the SP method in the exploration of seafloor massive sulfides. © 2017 Society of Exploration Geophysicists. All rights reserved.
... The investigators showed that Open Access *Correspondence: komori.shogo@aist.go.jp the IP effect makes the sulfide particles very conductive at high frequencies and perfectly insulating at low frequencies. In general, electromagnetic investigations are believed to be effective in finding submarine sulfide deposits and have been conducted in order to reveal conductive anomalies related to the presence of deposits (e.g., Kowalczyk 2008;Okada et al. 2017). However, the IP effect makes it possible for sulfide-bearing sediments to be both conductive and insulating, depending on the frequency of the current. ...
Submarine hydrothermal deposits are one of the promising seafloor mineral resources, because they can store a large amount of metallic minerals as sulfides. The present study focuses on the electrical properties of active modern submarine hydrothermal deposits, in order to provide constraints on the interpretation of electrical structures obtained from marine electromagnetic surveys. Measurements of resistivity and spectral induced polarization (IP) were made using drillcore samples taken from the Iheya North Knoll and the Iheya Minor Ridge in Okinawa Trough, Japan. These hydrothermal sediments are dominantly composed of disseminated sulfides, with minor amounts of massive sulfide rocks. The depth profiles of resistivity and spectral IP properties were successfully revealed to correspond well to layer-by-layer lithological features. Comparison with other physical properties and occurrence of constituent minerals showed that resistivity is essentially sensitive to the connectivity of interstitial fluids, rather than by sulfide and clay content. This suggests that, in active modern submarine hydrothermal systems, not only typical massive sulfide rocks but also high-temperature hydrothermal fluids could be imaged as low-resistivity anomalies in seabed surveys. The spectral IP signature was shown to be sensitive to the presence or absence of sulfide minerals, and total chargeability is positively correlated with sulfide mineral abundance. In addition, the massive sulfide rock exhibits the distinctive IP feature that the phase steadily increases with a decrease of frequency. These results show the effective usage of IP for developing and improving marine IP exploration techniques.Open image in new windowGraphical abstract.
... In a marine setting, electromagnetic systems have been applied successfully in exploring for oil and gas reserves (for a review, see Constable, 2010), gas hydrates (Yuan and Edwards, 2000;Weitemeyer et al., 2006;Schwalenberg et al., 2010aSchwalenberg et al., , 2010bConstable et al., 2016), and submarine massive sulfide (SMS) deposits (Cairns et al., 1996;Kowalczyk, 2008;Lipton, 2008;Hölz et al., 2015;Kowalczyk et al., 2015;Swidinsky et al., 2015;Mueller et al., 2016). SMS deposits contain valuable resources of Cu, Pb, Zn, Au, and Ag (Herzig, 1999), and some of these deposits, such as the Solwara 1 deposit in the Bismark Sea (Lipton, 2008), may have the potential to be economically mined. ...
The transient electromagnetic (TEM) method has recently been proposed as a tool for mineral exploration on the seafloor. Similar to airborne TEM surveys conducted on land, marine TEM systems can use a concentric or coincident wire-loop transmitter and receiver towed behind a ship. Such towed-loop TEM surveys can be further augmented by placing additional stationary receivers on the seafloor throughout the survey area. We examine the electric fields measured by remote receivers from an inductive source transmitter within a 1D layered earth model. At sea, it is conceivable to deploy either a horizontal transmitter (such as the analogous standard airborne configuration) or a more exotic vertical transmitter. Therefore, we study and compare the sensitivity of the vertical and horizontal towedloop systems with a variety of seafloor conductivity structures. Our results indicate that the horizontal loop system is more sensitive to the thickness of a buried conductive layer and would be advantageous over the vertical loop system in characterizing the size of a shallowly buried mineralized zone. The vertical loop system is more sensitive to a resistive layer than the horizontal loop system. The vertical electric field produced by the vertical loop transmitter is sensitive to greater depths than the horizontal fields, and measuring the vertical field at the receivers would therefore be advantageous. We also conducted a novel test of a towed horizontal loop system with remote dipole receivers in amarine setting. The system was tested at the Palinuro volcanic complex in the Tyrrhenian Sea, a site of known massive sulfide mineralization. Preliminary results are consistent with shallowly buried material in the seafloor of conductivities >1 S/m.
... seismic methods to provide volume information for resource evaluation (e.g. Constable 2010, Kowalczyk, 2008. Here we present two case studies to investigate the resource potential of shallow seafloor targets. ...
Marine controlled source electromagnetic methods are useful to reveal the electrical properties of the shallow seafloor. The electrical resistivity derived from EM data is sensitive to resistive hydrocarbons and to conductive ore bodies. We present two academic case studies using unique instrumentation to investigate seafloor massive sulfide fields in the SW Indian Ocean and submarine gas hydrate targets in the western Black Sea.
... First SMS studies using EM methods revealed high-conductivity zones associated with the TAG field, but lacked in structural resolution (e.g., Cairns et al., 1996). However, investigation strategies established for the exploration of sulfidic ore deposits on land should also be applicable to their marine counterparts (e.g., Kowalczyk, 2008). To accomplish high-resolution EM imaging in deep sea environments, the EM profiler GOLDEN EYE ( Figure 1) has been developed at the University of Bremen on behalf of BGR (Müller and Schwalenberg, 2015). ...
A novel frequency domain electromagnetic (EM) sounding system has been developed for mapping active and relict hydrothermal vent sites in the deep ocean. We present the concept and development state of this deep-sea electromagnetic profiler GOLDEN EYE, and first results of an interdisciplinary survey from the Central Indian Ridge. High-resolution electromagnetic conductivity data of the horizontal in-loop system show clear evidence for the presence of seafloor massive sulfide deposits related to active and inactive vent sites. In addition several buried structures were recognized that were not observable by acoustic and optical methods. We show that EM seabed characterization provides clear insights into the structure and spatial distribution of submarine hydrothermal deposits.
... The replacement of saline pore fluids in sediments by hydrocarbons and the resulting formation of resistive layers in the subseafloor make controlled-source EM a prime choice in oil and gas exploration and gas hydrate research (Constable, 2010) and the investigation of mud volcanoes (Hölz et al., 2015). Recently, controlled-source EM methods have shown their potential to identify layers with higher conductivity in the subseafloor (Swidinsky et al., 2012) extending marine mineral exploration of seafloor massive sulfides using CSEM (Kowalczyk, 2008) to the subseafloor. ...
Definition
Technology within this chapter defines the instrumentation and technologies used mainly to observe and sample the seafloor and subseafloor for geoscientific purposes. Equipment used specifically to observe or sample for other scientific disciplines may not be listed.
... Consequently, to properly map these small scale magnetic structures, ship based surveys are inadequate, and mapping near the seafloor with subsurface vehicles is required. In conjunction with standard high-resolution AUV multibeam, sidescan, and subbottom surveys over these areas, the collection of magnetic data adds considerable value to mineral exploration programs with little extra cost [5]. ...
Magnetic surveying is well established in land based mineral exploration. Magnetic data is routinely used to map geology in covered terrains, to identify altered zones, mineralization, bedding attitudes, and fault networks. However, other than during specialized commercial, military, and academic surveys, magnetic data is not normally collected on autonomous underwater vehicles (AUVs) and remotely operated vehicles (ROVs) conducting geological mapping and hydrographic survey operations. One reason for this is the magnetic field produced by the local geology is often overwhelmed by the heading and attitude dependent magnetic fields of the vehicle when the magnetometer is mounted close to or inside the AUV. Magnetometers can be mounted away from the AUV with specialized mounting apparatus e.g. a towed body or pole-mounts, but at the cost of increased complication to operations and risk to vehicle safety. To produce useful data from a magnetometer mounted inside the body of an AUV, it is necessary to compensate not only for the attitude of the AUV in the earth's field, but also for secondary effects related to the strength of the electric currents flowing in the vehicle propulsion and vehicle control circuits. To calculate the compensation terms, both a physical calibration routine and a mathematical treatment of the data are required. Prior to each survey, flying a short calibration maneuver enables the calculation of correction terms to the raw magnetic data. The calibration maneuver consists of two sequential, coincidental squares of four calibration legs per square. These squares are flown in opposite directions with the sides of the squares aligned parallel with the primary survey and tie lines. The AUV is typically flown in terrain following (constant altitude) mode at the nominal altitude of the survey, but is changed during each leg to induce pitch into the flight of the AUV. Correction terms are then calculated from the calibration magnetic field data, AUV attitude and heading and vehicle control data. Applying these correction terms to the raw magnetic data collected during the survey produces very useful magnetic maps for interpreting regional, subsurface geology in the survey area. As shown in this paper, OFG has successfully demonstrated this process in commercial surveys.
... Recently, theoretical and experimental attempts have been conducted to survey SMS using electromagnetic method. Kowalczyk (2008) surveyed SMS using electromagnetic method with magnetic source, although the sounding depth was possibly limited within several meters. Mills et al. (1994) shows SMS exist in less than few hundreds of meters in depth. ...
A 3D full waveform inversion is proposed as a processing method in controlled-source electromagnetic (CSEM) exploration. Using synthetic data simulated for a model, we demonstrate that conductive anomalies beneath the surface could be estimated with the proposed method. We discuss the resolution of our CSEM inversion method, considering the orientation of the dipoles of a transmitter and plural receivers. The synthetic inversion results show that the horizontal location of conductivity anomalies would be imaged using horizontal dipoles with high resolution, while the vertical location using vertical dipoles. On the other hand, the degradation of the resolution is observed for the vertical and horizontal locations of conductivity anomalies using horizontal and vertical dipoles, respectively. We then explain that these differences in the resolution of the inversion results are originating from those of secondary electric current. From our numerical results, we conclude that it is efficient to employ dipoles of plural orientations both for transmitter and for receivers to apply our inversion method with the high resolution.
... These land-based volcanic-hosted sulfide deposits often have originated in marine settings and have been obducted in collisional orogens to their nowadays terrestrial setting. The same electromagnetic principles should hold for the marine case (Kowalczyk, 2008, Goto et al., 2011. However, the detectability of small scale conductive targets in the topmost 5-30 m below seafloor with EM methods is hampered by the attenuation of EM fields in the conductive seawater, complexity of near-seafloor operations in rough deep-water environments, and limited viability of EM instrumentation developed for the hydrocarbon industry to detect SMS deposits. ...
We show first results from the German licence areas in the Central and SE Indian Ridge obtained with GOLDEN EYE, a new deep-sea electromagnetic system, for seafloor massive sulfide exploration. Golden Eye has been developed by the Marine Geophysics Group of University of Bremen by order of and in cooperation with BGR. The system consists of a non-magnetic and non-conductive frame made from glasfibre inforced plastic, and holds the CSEM Loop system, a current transmitter and electrical source and receiveing dipoles for DC and IP measurements, a three-axial broadband magnetometer, a CTD sensor, and video cameras, LEDs and altimeter for secure navigation. During the INDEX 2015 cruise the system was deployed over previously known active and inactive fields. First analysis of data show a clear correlation between anomalous conductivities and chargebilities, and SMS sites.
... For example, pioneer surveys around SMS were done in 1990's (e.g., Von Herzen et al., 1996;Cairns et al., 1996), but a number of observation points were limited and detailed structures are still unknown. Recently, Kowalczyk (2008) tried the resistivity mapping around a SMS using their own electromagnetic sounding system, however, only the observed responses are published and no information about seafloor resistivity distributions are reported. ...
The recent growth of world-wide requirement of metals demands advanced explorations for finding metal mine and deposits, such as seafloor massive sulfide (SMS). We developed instruments for the marine controlled-source electromagnetic (CSEM) survey with autonomous underwater vehicle (AUV), on which a transmitter was attached. For the real field test, R/V Yokosuka and AUV Urashima were used. The target region is a real deep-sea mine in a caldera structure called Bayonnaise, located in the Izu-Bonin island arc, south of Japan. We succeeded in the test experiment along four survey lines with current shooting from AUV. Six ocean-bottom receivers (OBEM) simultaneously recorded those signals. For evaluating the anomalous attenuation or amplification of received electric field at OBEMs, the three-dimensional forward modeling including the real bathymetry and a simple subsurface structure having an uniform resistivity (1 Ohm-m) was employed. Comparison between the observed and synthesized received field gives us a three-dimensional pseudo-section of anomalous received field, which can visualize heterogeneity of sub seafloor structure qualitatively. On the basis of the preliminary result of our AUV-CSEM survey around the SMS, high conductive features are observed around the SMS exposed area, and also in the caldera basin. They would reflect both the mineral deposits in and around the SMS and/or highly conductive pore water below the surface due to warm temperature by hydrothermal activities below the SMS. We conclude that our new technology imaging the near sub-seafloor structures will be useful for discussion about the geological background of SMS, and also be a powerful tool for the SMS detection and developments.
... Many explorations were made to survey the submarine massive sulphides near Japan or around the world (e.g., Kowalczyk, 2008), but we cannot get the detail cross section of submarine massive sulphides yet. We assume the submarine massive sulphides to be a simple rectangular block. ...
Recently, controlled-source electromagnetic (EM) method is widely used
for shallow sub-seafloor explorations. In conventional marine CSEM
methods, we need to connect a survey vehicle and an EM transmitter using
a long cable, and also connect the EM transmitter and towed receiver
using a cable. However, in practice, we must tow cables far from
seafloor because of rough topography (e.g, chimneys) around submarine
massive sulphides (SMS). Therefore, it is difficult to get information
about shallow sub-seafloor structure. In this paper, we propose a new
controlled-source electromagnetic method using two autonomous underwater
vehicles (AUVs) for the exploration of SMS. We set an EM transmitter to
one AUV, and also set an EM receiver to another AUV. Using this method,
it is possible to keep a low height of diving AUVs from the seafloor, so
we can carry out the exploration of SMS effectively. A numerical
simulation code for 2.5 dimensional (2.5-D) electromagnetic fields in
the frequency domain is developed in order to estimate electromagnetic
responses on possible conductivity structures. In this research, we
compared the behavior of electric fields as various functions such as
the distance between source and receiver, and discussed the possibility
of our CSEM method to be applied for the exploration of SMS. From the
simulation results, we found that it is possible to detect the electric
field for about 150~200m offsets even under the contamination of noise.
Among various combination of source and receivers, we also found that
the anomalous amplitude rate becomes greatest, in particular around the
edge of SMS, when polarizing both the source and receiver in the
horizontal direction. We next considered the sensitivity of
electromagnetic field to the location of SMS using two model
calculations. We found that the received electric field becomes steeply
weaker as setting the receiver apart from the transmitter when the
source was placed near SMS. Above all numerical calculations, we propose
the high efficiency of new CSEM method using AUVs. We conclude that it
is possible to observe the electric field within 200m from the source
with two AUVs. When the source-AUV is arranged near the anomaly, the
received electric field by the other AUV would strongly be attenuated
even with far offset. When the receiver AUV approaches near the boundary
of SMS, the received electric field (horizontal-component) would show
abrupt change near the horizontal boundary of the anomaly. These
predicted features conclude that the new controlled-source
electromagnetic method is available for exploration of SMS.
Seafloor massive sulphides (SMSs) are regarded as a potential future resource to satisfy the growing global demand of metals including copper, zinc and gold. Aside from mining and retrieving profitable amounts of massive sulphides from the seafloor, the present challenge is to detect and delineate significant SMS accumulations, which are generally located near mid-ocean ridges and along submarine volcanic arc and backarc spreading centres. Currently, several geophysical technologies are being developed to detect and quantify SMS occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock. Here, we use a short, fixed-offset controlled source electromagnetic (CSEM) system and a coincident-loop transient electromagnetic (TEM) system, which in theory allow the detection of SMS in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings.
In 2016, CSEM and TEM experiments were carried out at several locations near the Trans-Atlantic Geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor. Measurements were conducted in an area that contains distinct SMS sites located several kilometres off-axis from the Mid-Atlantic ridge, some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased. Based on the quality of the acquired data, both experiments were operationally successful. However, the data analysis indicates bias caused by three-dimensional (3D) effects of the rough bathymetry in the study area and, thus, data interpretation remains challenging. Therefore, we study the influence of 3D bathymetry for marine CSEM and TEM experiments, focusing on shallow 3D conductors located beneath mound-like structures. We analyse synthetic inversion models for attributes associated with 3D distortions of CSEM and TEM data that are not sufficiently accounted for in conventional 1D (TEM) and 2D (CSEM) interpretation schemes. Before an adequate quantification of SMS in the region is feasible, these 3D effects need to be studied to avoid over/underestimation of SMS using the acquired EM data. The sensitivity of CSEM and TEM to bathymetry is investigated by means of 3D forward modelling, followed by 1D (TEM) and 2D (CSEM) inversion of the synthetic data using realistic error conditions. Subsequently, inversion models of the synthetic 3D data are analysed and compared to models derived from the measured data to illustrate that 3D distortions are evident in the recorded data sets.
Seafloor massive sulfide (SMS) deposits are generated by high-temperature hydrothermal systems. Their precious resources have attracted global interest. A number of investigations with controlled-source electromagnetic (CSEM) methods have been implemented in recent years. There are three major problems with SMS surveying using EM methods. First, SMS imaging techniques for hydrothermal systems have a limited range. Simulations and applications have validated only simple layered models. Second, their inversion efficiencies must be improved further. Laterally constrained inversions and spatially constrained inversions are usually used to map geological structures. However, choosing their suitable weighting parameters is inefficient. Third, the effects of induced polarization (IP) on ore deposits are not considered in such inversions. A non-polarizable model is unable to accurately depict a polarizable model. To resolve these problems, an advanced strategy is used to improve the efficiency of the pseudo-3D inversion process. The proposed imaging method has the ability to map complex 3D geoelectrical structures, and therefore, it can both obtain information regarding surface ore deposits and distinguish between active and inactive hydrothermal systems. However, this method can also be used to depict the distributions of alteration zones and buried deposits. Furthermore, the influences of IP on the inversion are discussed with respect to the Cole- Cole model, and it is shown that the effects of IP on polarizable deposits cannot be ignored during the inversion.
Geophysical exploration for Submarine Massive Sulfide (SMS) deposits is evolving rapidly from an ad hoc process dependent upon the visual mapping of deposits using Remotely Operated Vehicles (ROVs) to a systematic process of precision bathymetric mapping, magnetic mapping, and electromagnetic mapping using Autonomous Underwater Vehicles (AUVs) and ROVs. This change is driven by a desire for efficiency in a commercial environment. The exploration for SMS deposits has moved from the academic realm to that of commercial exploration and exploitation. Exploration for SMS deposits is becoming routine, with exploration companies refining their methods as they gather experience. The necessary geophysical equipment and processing methods have been proven to be both efficient and reliable during multiple exploration campaigns.
In 1977, the first black smoker was discovered on the East Pacific Rise. Since this discovery, many more hydrothermal vent occurrences have been discovered in the deep ocean. These vents are associated with compact high grade copper, gold, and zinc deposits that are being actively explored for by national and private organizations.
Exploration for these deposits usually begins with ship borne sonar mapping and towed water chemistry samplers. From bathymetric maps, potential targets for more detailed mapping with underwater vehicles are defined. Remotely operated vehicle (ROV) and autonomous underwater vehicle (AUV) mounted turbidity, pH, and oxidation reduction potential (ORP) sensors, magnetometers, electromagnetic (EM) systems, high-resolution multibeam echosounder (MBES) bathymetry, sidescan sonar and subbottom profiler surveys are used to delineate the extent and nature of the submarine massive sulfide (SMS) deposits.
EM surveying can be used to determine the resistivity of near-surface and subsurface structure. SMS deposits fall into two categories; zinc rich non-conductive and conductive copper-gold deposits. One system is an ROV-mounted EM system that operates near the seafloor (Kowlaczyk, 2008). It comprises a transmitter coil wrapped around the ROV and an electric field sensor mounted close to the ROV. This system has successfully mapped high-conductivity zones corresponding to high grades of copper and gold. This system has detected blind mineralization at a depth of several meters, but does not penetrate beyond five or ten meters into the ocean floor. It does outline the limits of the system accurately, and has been used to direct resource-definition drilling of SMS deposits.
Modelling shows that buried SMS deposits can be mapped using a controlled source electromagnetic (CSEM) system. A CSEM system consists of an electrical transmitter and one or more electric field receivers that are either on the seafloor or towed behind the transmitter. SMS deposits are conductive and will channel electrical current. Since the conductivity of the ocean is constant, electrical fields are a proxy for current density. Modelling shows that SMS targets produce a readily detectable electrical field anomaly. Using 3D inversion of the CSEM electrical field data, subsurface SMS deposits can be mapped.
SMS deposits can also be mapped using seismic methods. The depth of water and the deposit geometry make it difficult to deploy standard systems. A new system using a vertical cable array and a surface source has successfully mapped SMS deposits in 3D.
This paper will review the use of EM and seismic methods successfully used to map SMS deposits.
Shallow water applications of electrical and electromagnetic geophysical methods have grown in recent years with recognition of the information these methods can provide regarding groundwatersurface water interaction, geotechnical engineering, exploration, marine geology and other fields. In many applications, spatial variations in resistivity are useful as a proxy for variations in another bulk material property such as pore water salinity, clay content, porosity, or temperature. Applications of galvanic resistivity methods have been buoyed by the development of marine configurations that are now commercially available. In contrast, with two notable exceptions, most applications of EM induction methods have involved experimental adaptations of instruments originally designed for use on land. Methods for shallow water resistivity and EM induction surveys are at an exciting stage of development where several promising applications have been demonstrated but the suite of tools and components commercially available and widely tested remains relatively small.
The recent growth of world-wide requirement of metals demands advanced explorations for finding metal mineral deposits. On the basis of the feasibility studies, we developed instruments for the marine DC resistivity survey system attached on a remotely operated vehicle(ROV). We conducted to image the shallow resistivity structures around the submarine massive sulfide (SMS) deposits located off Okinawa, southwest of Japan. ROV Hyper-Dolphine (HPD), JAMSTEC, was used for the marine DC resistivity survey. A transmitter, a voltmeter and a vertical electrode array with length of 10m were loaded on HPD during the dive to the seafloor. The sounding depth with this survey configuration was less than about 5m based on the numerical simulations. One-day operation around the SMS gave us the marine DC resistivity survey at about 20 sites The obtained apparent resistivity is moderately low at the most of sites, but is extremely low around the SMS area with hydrothermal chimneys. Also, the surface heterogeneity of resistivity is obvious around the vents area. On the basis of comparison between our results and geological features and heat flow measurements, we conclude that the conductive features around the chimney area could be explained by conductive SMS and/or hydrothermal fluid with high conductivity. Thus, the marine DC resistivity survey with ROV is effective to interpret the seafloor features around the SMS.
The recent growth of world-wide requirement of metals demands advanced explorations for finding metal mine and deposits. Especially, the submarine massive sulphides (SMS) have attracted mining companies because of its compactness with high grades. We propose a new marine controlled-source electromagnetic (CSEM) sounding using an Autonomous Underwater Vehicle (AUV) and DC resistivity sounding technique using Remotely Operated Vehicle (ROV) for resolving the distribution of mineral deposits near the seafloor. In this study, we introduce a seafloor experiment of electromagnetic sounding on a submarine massive sulphide (SMS) using ROV. We conducted a research cruise at a hydrothermal area in the Izu-Bonin islands area, off Japan, in February 2011. The research vessel “Kairei” and ROV “Kaiko 7000 II” was used in January, 2011. We attached the DC resistivity sounding system to the ROV, newly developed for SMS sounding. Our system stably obtained the apparent resistivity and the induced polarization (IP) on the seafloor. Although our sounding depth is limited (about five to several tens of meters), the hydrothermal area shows relatively conductive feature except for a localized resistive spot. A part of them inferred conductive sediments/rocks with almost same resistivity as the seawater. In addition the deep-sea mine and the surrounding area indicates high IP effect. These features suggest us that the ROV-DC resistivity survey (and possibly the AUVCSEM survey in future) gives us valuable information about the exposed and sub-seafloor distribution of SMS.
Measured electromagnetic fields in marine controlled source electromagnetic (CSEM) method always have some uncertain- ties to some extent. One is the structure in the subsurface and the other is the source signature that could have uncertainties due to the environmental conditions around the source. We hy- pothesized that the perturbation in the source waveform would be estimated using the backpropagation of anomalous electric fields and tested the hypothesis. We first developed an elec- tromagnetic full waveform inversion that could deal with both the conductivity of the subsurface and the source waveform si- multaneously. We compared the simultaneous inversion with a conventional inversion for a synthetic data. The synthetic example shows that even if the amplitude of an initial source waveform is erroneously underestimated with 5 % smaller than true source waveform, we could estimate the source waveform employing this simultaneous inversion algorithm. We also find that the obtained conductivity structure of the subsurface from the simultaneous inversion is appropriate than that from the conventional inversion. From numerical results, we conclude that it is realizable to estimate the unknown perturbation of the source parameters.
Geophysical exploration for Submarine Massive Sulfide (SMS) deposits is evolving rapidly from an ad hoc process dependent upon the visual mapping of deposits using Remotely Operated Vehicles (ROVs) to a systematic process of precision bathymetric mapping, magnetic mapping, and electromagnetic mapping using Autonomous Underwater Vehicles (AUVs) and ROVs. This change is driven by a desire for efficiency in a commercial environment. The exploration for SMS deposits has moved from the academic realm to that of commercial exploration and exploitation. Exploration for SMS deposits is becoming routine, with exploration companies refining their methods as they gather experience. The necessary geophysical equipment and processing methods have been proven to be both efficient and reliable during multiple exploration campaigns.
In conventional marine CSEM methods, we need a survey vessel that tows a long cable to which both an EM transmitter and receivers are attached. Therefore, it is difficult to survey shallow sub-seafloor structure below the seafloor of complex topography around submarine massive sulphides (SMS) because of the risk of cable-tangling. In this research, we propose a new marine CSEM method to solve this problem using two autonomous underwater vehicles (AUV). Using this method, it is possible to keep a low height of diving AUVs from the seafloor, so we can carry out the exploration of SMS effectively. We discussed the possibility of new CSEM method employing the 2.5-D FEM program. From numerical results, it is possible to detect the rough existent area of SMS and the rough thickness of SMS.
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