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The Effect of Shallow Quaternary Deposits on the Shape of the H/V Spectral Ratio
Abstract
In the last two decades, the horizontal-to-vertical (H/V) spectral ratio of seismic noise technique has been widely used for site-effect estimation and geophysical exploration through the soil fundamental frequency. Usually, only one peak is observed in the H/V spectral ratio, but in some cases, a second peak can also be obtained. Nevertheless, to date, the peaks at higher frequencies are rarely studied in detail. Geological and geophysical data are especially needed to better explain the presence of this second peak, which normally is neglected. An extensive survey of H/V measurements was conducted in the Llobregat river delta, located to the south of Barcelona. At most sites, two clear peaks were identified: one at low frequencies (1 Hz). To understand this behaviour, a seismic noise array and active surface wave measurements have been conducted to obtain a shear-wave velocity profile (V
s) up to the bedrock. Two impedance contrasts have been detected: the first one at a shallow depth and the second one between the soft sedimentary cover and the bedrock. During the modelling process, the theoretical H/V computed from the obtained V
s models fits well with the experimental H/V peaks. The results from this study show that the structure of shallow quaternary layers can clearly change the shape of the H/V ratio, producing two clear peaks in some situations. In this case, the contact between the low-velocity clay layer and the gravels with a high seismic wave velocity produces a shallow impedance contrast related to the second peak observed in the H/V ratio. Comprehension of these secondary peaks could avoid a misreading of the soil fundamental frequency that could produce errors in a site-effect evaluation or in the calculation of the bedrock depth. Finally, we show that passive seismic techniques provide the quaternary overburden and bedrock geometry in urban areas and allow for the limitations of other geophysical techniques in these environments to be overcome.
... Even though the largest amplitude H/V peak is considered critical [15], recent developments in this field show that other peaks must also be considered for seismic vulnerability assessment, especially the one at a frequency between the natural frequency of the building and its half [19,37,56,57]. It has been proved that one HVSR peak corresponds to the major impedance contrast [49], whereas two HVSR peaks signify the presence of two impedance contrasts [58]. The lower frequency peak corresponds to impedance contrast at deeper depth whereas a higher frequency peak means a shallow depth strong impedance contrast is present [37]. ...
... The lower frequency peak represents the contact between the deep sedimentary cover and bedrock, whereas the higher frequency peak represents the contact between two layers at the surface. If the frequency content of an earthquake matches any of the frequencies, resonance can occur in the subsoil, however, at the higher frequency, the superficial layer can vibrate independently regardless of the entire soil column [58]. The shallow quaternary layer structure can introduce secondary peaks and thus modify the shape of HVSR curves significantly. ...
... Zaslavsky et al. [112] reported a peak at low frequency corresponding to presence of limestone bedrock, and a second peak at higher frequency (4-17 Hz) due to presence of shallow soft alluvial sediments in Zevulun plain, Israel. Macau et al. [58] attributed the first peak in HVSR curve to impedance contrast between soft sediment and bedrock, while the second higher frequency peak to the interface between the shallow quaternary layer of soft clay over gravel in Llobregat river delta, Barcelona. Double peaks were also observed in HVSR results conducted by Wotherspoon et al. [113] in the Canterbury plains, New Zealand due to the presence of soft sand layer over stiff gravel layer followed by the bedrock at deeper depth. ...
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Extensive geophysical surveys (MHVSR and MASW) conducted for the microzonation of the Kashmir Valley, Himalayas, revealed unexpected dynamic characteristics at certain sites pointing out the weak relationship between fundamental frequency (f0) and time-averaged shear wave velocity over 30 m depth (VS,30). Unusual low-frequency amplification at stiff soil sites and high-frequency amplification at weathered rock sites was obtained. On the contrary, high-frequency amplification was attained at a soft soil site over shallow bedrock. Instances of topographic amplification on slopes, hills, and valleys were encountered. Consequently, the commonly used VS,30-based single-proxy site classification failed to explain these atypical site effects, thus underscoring the caution to be exercised for site classification in geologically complex regions. These findings motivated the documentation of the anomalies and the search for the most suitable site characterisation scheme for the geological deposits of the Kashmir Valley. The coupled MHVSR-VS,30 proxy approach accomplished the best results for the study region.
... In addition, 3D geological modelling in recent years also has concentrated on the development of methods and techniques, as for example: • Programing CAD applications for the 3D analysis of geological traces, outcrop data, and borehole logs to facilitate the reconstruction of geological surfaces and cross sections. • Determination of soil-rock boundaries and some Quaternary sedimentary horizons from the analysis of passive seismic data (e.g., Macau et al. 2015). ...
... Some of the more common methods are: 1) Comparing digital terrain models. The comparison of topographical documentation of different periods highlights the impact of human activities on the ground through time (Vilà et al. 2015). Thus, from a 3D geological modelling perspective, it is possible to define the geometry of certain artificial deposits by comparing detailed pre-and posturbanisation digital terrain models, such as infilled river channels. ...
... The reconstruction of 3D subsurface structures that represent the diverse types of environments of Catalonia can be obtained by applying and utilizing the above methods. These methods were used for the development of the 1:5,000-scale Urban Geological map of Catalonia (Vilà et al. 2015). However, other methods of reconstruction have been used and, of-ten the most effective way to build a particular surface is applying a hybrid procedure by combining different methods. ...
... As a starting point, we compiled all the information on Barcelona's soils that had been obtained by experimental methods. This information was integrated into a geo-referenced, homogeneous database including information on the fundamental soil period obtained by H/V measurements [13,14,19,20]. The available data, together with the corresponding references, are shown in Table 2. ...
... The ambient noise measures were obtained with a Tromino seismometer. Macau et al. [20] H/V measurements were estimated along the Llobregat Delta, Barcelona. The seismic ambient noise vibrations were recorded using a six-channel Cityshark datalogger connected to one Lennartz LE-3D 0.2 Hz triaxial sensor. ...
... Another interesting aspect to consider in the evaluation of the seismic response of soils and structures is the presented by Macau et al. [20]. These authors show how H/V measures in shallow quaternary layers in the Llobregat Delta exhibit a second peak corresponding to another period of the soil. ...
In this study, previous microzonation studies in Barcelona (Spain) were revisited, and available data on the predominant periods of soils in the city were compiled to develop an updated microzonation map of the city. In addition, the building database was updated and used to create a map of building fundamental periods. The crossing of soil predominant period and building fundamental period maps led to the detection of areas in which resonance phenomena and, indeed, increased amplification of the structural response are expected. Thus, zones of Barcelona were identified in which the seismic hazard is probably greater due to resonance effects. The improved microzonation maps and the detection of soil-building resonance areas contribute significantly to enhanced precision and awareness of seismic hazard and risk in Barcelona.
... After obtaining the H/V spectra for each window, the average of all windows was obtained as the H/V spectrum for a particular site. The peak frequency of the H/V spectrum plot shows the fundamental frequency of the site (Ullah and Prado 2017;Macau et al. 2015). ...
... Meanwhile, some stations (e.g., B5, B30, B40) show H/V spectral ratio with two peaks probably attributed to the fundamental frequency of the entire soil deposit at the lowest frequency and the shallower soil layers at higher frequencies (Macau et al. 2015;Pastén et al. 2016). ...
... Hence, considering the present conditions, the best thing to do is to compare the H/V results with the available geotechnical and geological data and try to correlate peaks with interfaces producing the resonance (i.e., characterized by an impedance contrast). This method has been used by many researches (Gosar 2007;Mahajan et al. 2007;Mundepi et al. 2010;Kockar and Akgun 2012;Kolat et al. 2012;Vella et al. 2013;Paolucci et al. 2015;Macau et al. 2015). ...
Every year, numerous casualties and a large deal of financial losses are incurred due to earthquake events. The losses incurred by an earthquake vary depending on local site effect. Therefore, in order to conquer drastic effects of an earthquake, one should evaluate urban districts in terms of the local site effect. One of the methods for evaluating the local site effect is microtremor measurement and analysis. Aiming at evaluation of local site effect across the city of Babol, the study area was gridded and microtremor measurements were performed with an appropriate distribution. The acquired data was analyzed through the horizontal-to-vertical noise ratio (HVNR) method, and fundamental frequency and associated amplitude of the H/V peak were obtained. The results indicate that fundamental frequency of the study area is generally lower than 1.25 Hz, which is acceptably in agreement with the findings of previous studies. Also, in order to constrain and validate the seismostratigraphic model obtained with this method, the results were compared with geotechnical, geological, and seismic data. Comparing the results of different methods, it was observed that the presented geophysical method can successfully determine the values of fundamental frequency across the study area as well as local site effect. Using the data obtained from the analysis of microtremor, a microzonation map of fundamental frequency across the city of Babol was prepared. This map has numerous applications in designing high-rise building and urban development plans.
... As a starting point, we compiled all the information on Barcelona's soils that had been obtained by experimental methods. This information was integrated into a geo-referenced, homogeneous database including information on the fundamental soil period obtained by H/V measurements (Alfaro et al., 2001;Cadet et al., 2011;Santos-Assuncao et al., 2016;Macau et al., 2015). The available data, together with the corresponding references, are shown in Table 21. ...
... • Macau et al. (2015) found that H/V measures in shallow quaternary layers in the delta of the Llobregat river at the south-eastern part of Barcelona, exhibit a second peak at which significant amplification may occur. Additional research on this finding and on its influence on buildings' response is suggested. ...
Earthquakes are defined as a "violent shaking of the Earth’s crust and mantle, caused by forces acting inside the Earth". In most cases, these forces are caused by an energy release process generated from the contact of the Earth’s tectonic plates. Other less common causes are the human-induced earthquakes or those generated through volcanic activity. In either case, the energy is released in the form of multi-directional waves, which reach the surface, causing different effects. However, the intensity of an earthquake is not uniform in all its propagating directions. Many times, the motion is polarized due to the type of fault and/or the proximity to it, causing higher intensities in specific directions, depending on the dynamics and geometry of the rupture. This is what is known as the directivity effect. Furthermore, both the intensity and the shape of the wave vary depending on the propagation medium. Ground motion prediction models deal with the spread of the released energy from source to site. Local site effects, both soil effects and topographical effects, are also important. Rigid media, such as rocky and stiff soils, do not tend to amplify the seismic motion, while soft soils amplify specific frequencies depending on local sub-soil geology and on the motion characteristics. Directionality effects refer to the strong motion in a specific site. This thesis deals with two important issues related to directionality. The first one refers to the orientation of the sensors recording the seismic actions; the second one refers to the expected damage in buildings depending on the directions of their strong and weak main axes. It is worth to mention that nowadays, directionality effects are not considered in most structural regulations. In this thesis, special attention is paid to the directionality and soil effects. Since 2008, around 360,000 earthquake fatalities have been reported. This evidence demonstrates the need to develop more and better ways to assess and to prevent seismic risk. Therefore, the main objective of this thesis is to identify and evaluate the strong-motion directionality and the soils’ effects on the seismic hazard and risk, with applications to strong-motion data sets and soils’ in urban environments. This thesis is divided into three principal blocks: I) directionality effects, II) Soils effects, site classification and other seismic risk-related issues and, III) relevant case studies related to the previous two blocks. In the first block, directionality effects have been considered in the expected strong seismic actions, through the estimation of intensity measures using databases from Italy and Costa Rica. Also, in the assessment of the expected damage of buildings through non-linear dynamic analyses, a simplified approach has been proposed to consider directionality effects. In the second block, microzonation and soil-building resonance effects in the city of Barcelona are studied. In addition, a seismic site classification is defined for the Spanish strong-motion network. The dynamic soil-structure interaction, considering directionality effects and, the proposal of a new drift-correlated intensity measure, appeared as supplementary subjects in this block. Finally, in the third block, other relevant contributions were included to complement this dissertation. The results demonstrate 1) that directionality effects in expected seismic actions are significant and should be considered in Probabilistic Seismic Hazard Analysis (PSHA) and in seismic risk assessments; and 2) they confirm the relevance that site effects (soil effects), has both in seismic hazard studies and in the assessment of the expected damage. This PhD thesis wants to be an additional step towards the assessment, prevention, and reduction of the risk due to earthquakes.
... The ambient noise measures were obtained with a Tromino seismometer. Macau et al. (2015) H/V, array 57 H/V measurements were estimated along the Llobregat delta, Barcelona. The seismic ambient noise vibrations were recorded using a sixchannel Cityshark datalogger connected to one Lennartz LE-3D 0.2 Hz triaxial sensor. ...
... Another interesting aspect to consider in the evaluation of soils and structures is the presented by Macau et al. (Macau et al. 2015). They present results from H/V measures of shallow quaternary layers in the Llobregat delta, obtaining a second peak corresponding to another dominant period of the soil. ...
In this work, different zones of increased hazard due to resonance effects in Barcelona are identified. For this, all the available data from the predominant periods of soils was compiled to develop an updated microzonation map of the city. In addition, the building's database of the city was used to create a map of the building's fundamental periods. The cross of both maps leads to detect areas where the resonance phenomena and the amplification of the structural response are expected. The results obtained are significant in the evaluation of seismic risk in Barcelona when considering local effects in regional seismic hazard studies.
... On the other hand, at the hard surface layer, both the horizontal and vertical motions are similar in terms of maximum value and waveform (Nakamura, 2008). The H/V spectral ratio curves show the predominant period (Macau et al., 2014;Perron et al., 2018) for a site irrespective of the time, place, and season (Nakamura, 1989). In this study, H/V response spectral ratio curves are not considered the expression of the site amplification. ...
... In this method, all steps given in the H/V spectral ratio curve procedure are followed. Instead of FFT, 5% damped response spectra are used at step 4, the predominant period from the H/V response spectral ratio curve is estimated (Macau et al., 2014;Perron et al., 2018). Site class definition, given in Table 3, categorizes sites as per the Japanese earthquake-resistant design code (JRA, 1980) and site classification scheme given in NEHRP site classes (BSSC, 2004). ...
Site classification is vital to carry out seismic hazard analysis of a region and get the damage patterns caused by earthquakes. In the present study, the ground motion of earthquakes recorded from 2015 to 2019 at 84 sites of the seismic network array of Earthquake Early Warning System for Uttarakhand are analyzed for site classification purposes. The predominant period from the mean horizontal to vertical spectral ratio curves was estimated. The classification schemes devised by Japan Road Association and National Earthquake Hazards Reduction Program are applied to classify the sites. Along with this, two other site classification indexes schemes are also applied for classification purposes. Data winnowing techniques are used over the ground motion records to pick out desirable quality records. Conclusively, the site class with the highest recurrence rate amongst the used methods is selected as the final class for that particular site. The effect of magnitude, distance and depth on horizontal to vertical spectral ratio are described and concluded that these factors do not significantly affect the ratio curves. The average horizontal to vertical spectral ratio curves obtained for all the sites matches well with the existing literature. The classification of a few sites are verified from the classification done by other methods in recent studies. It is evident from the results that the classification done in this study matches well with them.
... • Identifying the lateral heterogeneities in the subsoil when the H/V curves present some peculiar shapes [62,63,64,65]. ...
... Therefore, it is not wise to estimate a preliminary fundamental frequency just by applying the H/V technique. The application of complementary active or passive geophysical methods such as the multichannel analysis of surface waves (MASW), helps to avoid misinterpretations of the soil fundamental frequency [65,61] and better evaluate the site effects. ...
It is well known that seismic waves are affected by the geological nature of the different layers in which they propagate close to the surface. The various structural and topographic elements cause the incident signal to be reflected, refracted, or diffracted. As a result, the combination of those different waves modifies the spatial, spectral, and temporal characteristics of the wavefield. These phenomena are called site effects. Especially amplifications of the ground motion acceleration is observed, which may induce failure of the structures located on the surface , as well as slope instabilities. The spatial variability of the ground motion can be very significant over small distances, because of the changes in the geometry and the soil conditions. During the last years, many research works have been conducted through numerical simulations on simplified slope geometries in order to identify the effect of geological, topographic and input signal parameters on the amplification of the amplitude of seismic waves. However, the predictions resulting from the numerical simulations are often different from the field observations. So, this paper firstly presents an in-depth literature review about available methods to model site effects and their implications. Secondly, it also focused on results of existing works that benefit from experimental data in the analytical modeling; more especially works considering geophysical data obtained through H/V and MASW experiments are examined.
... Traditionally, the frequency at the maximum in the horizontal-to-vertical spectral ratio has been linked to the soil fundamental frequency (soil/bedrock contact). In another delta environment, Macau et al. (2015) highlighted the presence of a secondary maximum at higher frequencies than the fundamental one related to the contrast between the deltaic sediments and the Pleistocene gravels. In the near-surface study, we will focus on this secondary maximum which corresponding frequency will be transformed into depth of the gravel top. ...
... This would be coincident with the presence of the maximum in the electrical resistivity model. Although P-wave velocity profile is characterized by smooth changes in contrast with the layered modelled of resistivity or S-wave velocity profiles, changes on velocity gradient and velocity ranges can help to constrain the lithological interpretation in this profile ( We use the frequency associated to the second maximum as an indicator of the Pleistocene gravel top according to Macau et al. (2015). This maximum has been identified in practically all the stations. ...
The Ebro Delta (Spain) faces anthropogenic and climate changes that can cause a significant decrease on its subaerial surface. One of the causes of this reduction is subsidence that take place due to the compacting of the sediments. In order to identify zones vulnerable to subsidence, we have defined several targets such as: characterization of Holocene sediments (sands versus clays/silts and location of organics) and delineation of the bottom of deltaic sediments. Regarding these objectives, we have acquired geophysical data sets at different environments of the Ebro Delta (from delta plain to marshes among others). The methodology includes combination of seismic methods (surface waves, refraction and reflection methods) and electrical resistivity tomography for sediment characterization as well as the use of the H/V spectral ratio of microtremor measurements to delineate the base of the Holocene sediments. We show the results for one line located on the delta plain and close to alluvial fans. Geophysical models are characterized by a high-resistivity and high-velocity layer at around 25 m depth interpreted as the base of the deltaic sediments. H/V results provide evidence of maximum thickness of deltaic sediments located at the right side of the river mouth.
... The CMMT station has a flat HVSR curve with no amplification, consistent with it being a hard rock site suitable as a reference site, while the amplification of the HVSR curve of the CMCA station from 2 earthquake events shows ground shaking amplification up to 4 -4.5 times at a resonance frequency of about 0.4 -0.5 Hz, which corresponds to the HVSR fundamental resonance frequency and ground motion amplification results we obtained from microtremor measurement at the Chiang Mai airport area and shown in Figures 6 and 7. The second peak (high frequency peak of ~5 -~6 Hz) was found to be significantly related to the shallow impedance contrast between soft clay and gravel [36]. ...
The seismic site effect which is controlled by local geological conditions is a key parameter of seismic hazards analysis. This paper presents an observational study of microtremor data to investigate the dynamic characteristics of soil in the Chiang Mai basin (Chiang Mai and Lumphun province), Northern Thailand. The Chiang Mai basin was formed on terrace sediments and alluvium sediments. The horizontal vertical spectral ratio (HVSR) analyses of ambient noise data at 101 sites in the basin were analyzed for an average smoothed HVSR curve to estimate the amplification factor and fundamental resonance frequency of each observation point. We also evaluated the shear wave velocity using the HVSR inversion technique based on the diffuse field assumption. Tests undertaken include a comparison of the HVSR of a significant earthquake that was detected at stations in the basin and a nearby bedrock site. The results indicate that the resonance frequency ranges between 0.15 - 0.4 Hz at sites having large thicknesses of soft sediments. The lowest resonance frequencies occur in the center of the basin, whereas higher resonance frequencies were observed in the areas of the shallowest bedrock in the west and east of the basin. It can be inferred that the western margin of the Chiang Mai basin is a steep slope, while the eastern margin of the basin is a low-angle west-dipping basement. The amplification factor ranges between 3 - 5 times, in the middle of the basin. Most of the Chiang Mai basin area is classified as site D soil (stiff soil) relative to alluvium sediments, and a region of class C soil (very dense soil) conforms to the terrace sediments located on the eastern edge of the basin. The soil classification is based on shear wave velocity (Vs30) determined by HVSR inversion.
... Many studies have used geotechnical and geological data to validate and correlate HVSR results with in-situ conditions (e.g., Vella et al., 2013;Macau et al., 2015;Paolucci et al., 2015;Rezaei and Choobbasti, 2018). In this aim, geological investigations were conducted in several missions to investigate the area and draw a lithological and geomorphological map that serves as support to interpret the seismic data, since no detailed geological map is available in the area. ...
Ghabt Admin, located in western Morocco, is prone to seismic activity and site effects due to its position within an alluvial basin situated between two faulted mountain belts. The region has experienced several devastating earthquakes, including the 1960 Agadir earthquake that resulted in the loss of 12,000 lives and the destruction of over 75% of the city. Moreover, the area has recently undergone rapid urbanization, necessitating the implementation of seismic studies to make informed decisions regarding land use. This study aims to conduct the first seismic microzonation and site characterization studies by using HVSR, 1D-MASW, and geotechnical investigations. The datasets consist of 80 microtremors, 15 1D-MASW profiles, detailed geological observations, boreholes, and soil/rock identification tests. Due to challenges related to anthropogenic sources and soil heterogeneities, only 31 H/V measurements met the SESAME criteria. The HVSR results indicate the following: (i) the predominant peak frequency (f0) ranges from 1.4 to 15.7 Hz, with lower values (1.4–3 Hz) observed in the Oued Issen alluvial fans and the sand dune area, while moderate to high values (4–15.7 Hz) are found in the lacustrine limestone and Tagragra’s dome. (ii) The amplitude peak frequency (A0) ranges from 2 to 7.7, with higher values observed in the southwestern zone and certain parts of the glacis-fans zone, particularly in the Oued Issen zone. (iii) The seismic vulnerability index (Kg) ranges from 0.28 to 39.5. The Oued Issen area, which is composed of recent unconsolidated deposits and affected by the Oued Issen fault, exhibits high Kg values. Analysis of HVSR curve typology reveals complex geology. The VS30 map shows VS values ranging from 179 m/s to 830 m/s, classifying the study area into soil classes A, B, and C according to Eurocode 8. Furthermore, the 1D-MASW results are compatible with the HVSR results and the distribution of the geological units.
... For this purpose, a joint fit between HVSR curves and dispersion curves from MASW survey is carried out using the information of shear wave velocity of the subsoil as a constraint for the analysis. Multiple frequency peaks are possible in HVSR curves suggesting the presence of more than one impedance contrasts at a site [10]. The lower frequency peak corresponds to impedance contrast at deeper depth whereas higher frequency peak means a shallow depth strong impedance contrast. ...
Srinagar metro rail is set to be constructed as part of the Mass Rapid Transport System planned for the cities of Srinagar and Jammu in Jammu and Kashmir. In this paper, we present results of single station microtremor (MHVSR) tests conducted at 27 locations along the three corridors proposed in the project. Microtremor recordings were analysed using Nakamura’s horizontal-to-vertical spectral ratio (HVSR) method. The results reveal that the fundamental frequency (f0) at the sites ranges between 0.21 and 1.16 Hz with peak H/V amplitude 2.01–5.89. At many locations, more than one peak was observed in HVSR curves signifying the presence of multiple impedance contrasts within the subsurface. Low frequency peak indicates an impedance contrast at deep depths, whereas peak at higher frequencies indicates shallow impedance contrast. Further, the results are compared with Vs30 (average shear wave velocity over 30 m depth) obtained at the same sites in a previous study by our group. It was found that f0 and Vs30 do not follow a direct correlation at majority of the sites. Locations like Batpora, Soura, Hazratbal, and Osmanabad are found to have low f0, despite having high Vs30 and characterized as site classes C and D according to NEHRP guidelines. The study concludes that these apparent anomalies essentially indicate that Vs30 might not be an appropriate proxy for site amplification, as was also pointed out by Castellaro et al. (2008) and several other authors. Consequently, a new site characterization procedure must be adopted which incorporates other parameters in addition to Vs30.
... Many studies have pointed out that the correlation between secondary MHVSR peaks and the depths to shallow interfaces with a strong impedance contrast is in fact of geologic origin (e.g., Refs. [22,[50][51][52][53][54][55][56][57]). Fewer studies have found good agreement between secondary MHVSR peaks and higher modes of f 0 (e.g., Refs. ...
Seismic site characteristics, specifically depth to stiff glaciated Pleistocene sediments (zgl) and shear wave velocity (Vs) profiles, of the Fraser River Delta (FRD), located in southern Metropolitan (Metro) Vancouver, British Columbia, Canada are determined using the most comprehensive geodatabase to date. The relationships between zgl and theoretical fundamental frequency determined from known thickness of post-glacial sediments and experimental peak frequency from microtremor horizontal-to-vertical ratios (f1,HVSR) are investigated. A zgl predictive model based on f1,HVSR is developed for shallow FRD sites (zgl ≤ 56 m) and is compared to existing microtremor horizontal-to-vertical spectral ratio (MHVSR) peak frequency-sediment thickness models developed in other regions in the world. At 16 FRD sites, a combination of active- and passive-source surface wave array methods are performed to obtain the fundamental-mode Rayleigh wave dispersion curve which is jointly inverted with the MHVSR fundamental peak frequency (f0,HVSR) using three parametrization models with varied layering. The jointly inverted Vs profiles with their uncertainties are presented for the 16 sites and compared with existing proximal Vs profiles measured by other in situ Vs profiling methods. The comparison shows a notable match (9.1% depth-averaged absolute relative difference) to a significant depth of 220 m from the two datasets. In addition, 8 of the developed Vs profiles are co-located with strong-motion accelerograph stations, which allows for correlation between recorded earthquake ground motions and seismic site parameters. The proposed predictive zgl model and validation of evolving non-invasive in situ Vs profiling methods are important for 1D and 2D seismic site effects quantification and mapping in the FRD.
... (Analysts were not given the geological data until phase 4.) The frequency of the peak is clearly sensitive to basement depth on any modeling study of Rayleigh wave ellipticity. Multiple case histories have likewise used strong observed mHVSR peaks to guide the choice of a basement or other strong V S contrast (Asten et al. 2014;Macau et al. 2015). Inversion of mHVSR data combined with SPAC data was first described by Arai and Tokimatsu (2005) Full 3C array processing of microtremor data using SPAC methods has been described by Kohler et al. (2007) and Puglia et al. (2011) and FK methods used by Fäh et al. (2008) and Poggi et al. (2017). ...
Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 (VS10), 30 (VS30), 100 (VS100), and 300 (VS300) meters and show its usefulness in quantitative comparisons of VS profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.
... The obtained MHVSR curves carry information about the main characteristics of the subsurface, such as lateral heterogeneity of sediments composition and velocity reversals (Bonnefoy-Claudet et al., 2008;Castellaro and Mulargia, 2009;Uebayashi et al., 2012;Macau et al., 2015). ...
... Having continuous records for several months will make it possible to verify whether temporal variations in background noise can affect the MHVSR measurements. Salinas et al. (2014) have already pointed that f 0 resonance peaks retrieved from MHVSR studies can vary strongly between very close sites and Macau et al. (2015) have also observed that some locations in the Barcelona conurbation present two HVSR peaks, which makes it difficult to determine what the representative value for engineering studies is. ...
The high degree of human activities in urban environments produces large background vibrations that makes it difficult to use data acquired in these areas for classical seismology. Seismometers installed within cities have been typically been used for the study of seismic hazard or for monitoring civil engineering problems. However, with the development of monitoring techniques based on the interpretation of the so-called seismic ambient noise, these data have gained scientific interest. Our objective is to discuss an additional utility of seismometers deployed within a city; its use as a tool to connect society with Earth sciences. Many citizen activities, from traffic to music concerts, produce vibrations that can be recorded seismically, and our experience shows that these records attract the attention of the media and social networks. With the emergence of low-cost and easy-to-use instruments in recent years, more citizens can now record ground motion and become interested in the interpretation of the recorded seismograms. The installation of permanent seismic networks in educational centers has proven to be a good approach to introduce students to Earth sciences at the national level and can also be developed at the urban scale using this new instrumentation. In this contribution we will first review the previous results related to the identification of the sources of vibration in urban areas and then present a new ongoing project based on the deployment of a seismic network in educational centers located in the city of Barcelona.
... A double-peak HVSR curve exhibits two clear distinct peaks (see Fig. 8b), which typically correspond to two significant impedance contrasts beneath the station at two different scales: one for a thick structure and another for a shallow structure (e.g., Refs. [8,68,98]). A broad/multiple-peak HVSR curve exhibits a broad peak or multiple peaks (see Fig. 8c), which are related to the presence of an underground sloping interface between softer and harder layers (e.g., Refs. ...
Despite the significant upgrade and expansion of the seismic monitoring network in Chile over the past several years, precise site characterization of a large number of seismological stations is still lacking, thus hindering a robust development of tools that could improve prediction of regional seismic hazard. To enhance our understanding of site effects in Chilean seismological data, this study classifies 136 stations using the horizontal-to-vertical spectral ratio (HVSR) method based on a database of 283 triaxial ground-motion records from 22 subduction interface earthquakes with moment magnitudes ranging from 6.8 to 8.8 that occurred between 1985 and 2015. This classification, which is based on the SESAME guidelines, divides recording stations into four types according to the shape of the station-average HVSR curve (single-peak, double-peak, broad/multiple-peak, and flat). Furthermore, the variability of the mean HVSRs with different source, path and site parameters is investigated. The derived HVSR results are also compared with those presented in previous HVSR studies for Chile. Finally, representative shear-wave velocity profiles along with parametric uncertainties are estimated for stations with a single-peak HVSR curve using a Bayesian statistical inversion framework.
... As demonstrated by this case study, this is clearly erroneous and only a joint analysis with quantitative constrains from the analysis of surface wave propagation can provide a reliable subsurface V S model without any apRioRi assumption and free from major ambiguities (see also Macau et al. 2015;Asten and Hayashi 2018). ...
An efficient system for the joint acquisition and analysis of multi-component active and passive seismic data is presented. It is shown how, in spite of the limited field equipment (the system requires just a 4-channel seismograph, one 3-component and four vertical-component geophones), it is nevertheless possible to define up to seven different (but mutually related and complementary) objects used to constrain a multi-objective joint inversion capable of providing a robust subsurface shear-wave velocity (Vs) profile for both geotechnical and seismic-hazard studies. The presented approach relies on acquisition techniques that require simple and straightforward field procedures useful in particular, but not solely, in the characterization of urban and remote areas where, due to logistical problems, standard acquisition procedures cannot be easily applied. Active data recorded by a single 3-component geophone are processed so to define up to five objective functions: the group-velocity spectra of the three components, the radial-to-vertical spectral ratio and the Rayleigh-wave particle motion frequency curve. Passive data are used to compute two further objects: the horizontal-to-vertical spectral ratio and the phase-velocity dispersion curve obtained via miniature array analysis of microtremors. These seven objects are jointly inverted by means of a multi-objective inversion procedure based on the Pareto criterion. Performances are assessed through a comprehensive field dataset acquired in an urban area of NW-Italy. The consistency of the overall procedure is assessed by comparing the results with the analyses accomplished by considering classical multi-channel active and passive data and methodologies (multi-component MASW, multichannel analysis of surface waves and ESAC, extended spatial auto-correlation).
... Moreover, in some areas, a thin soft soil layer covering the volcanic rocks produces a second peak in the H/V spectral ratio (case VII, purple dia-(v) Two clear peaks in the H/V spectral ratio curves are observed (Figure 7e), one at low frequencies (<1 Hz) and the other at high ones (>1 Hz). The presence of this second clear peak in the H/V curve implies a large acoustic contrast between the two shallow layers (Macau et al. 2015). (vi) The amplitude of the H/V curve remains below 1 for a large frequency interval (Figure 7f). ...
Amplitude level, duration, and spectral content of earthquake ground motions are strongly influenced by local soil conditions. Reliable estimation of site effects is, therefore, crucial in order to avoid damage to infrastructures and mitigate other losses. Traditional geophysical exploration techniques are restricted in urban environments due to the presence of anthropogenic noises causing low seismic signal-to-noise ratio and other logistical issues. These problems are even more critical when the maximum investigation depth extends to hundreds of metres. The city of Girona, located in northeastern Spain, has seismic hazard represented by a peak ground acceleration value of 0.08 g for a return period of 500 years. The city was built at the confluence of four rivers, generating a complex surface geology with bedrock outcrops and the presence of stiff soils, soft soils, and also a volcanic basalt layer. This paper presents the results of the geophysical work, which was carried out in order to characterise the soil layers present in the urban area of Girona. All information obtained in this research will be useful in computing the amplification of ground motion and to perform microzonation studies. We have obtained the shear-wave velocity profile in the study area using a combination of seismic noise array and multichannel analysis of surface waves techniques. Using the horizontal-to-vertical spectral ratio method, we have obtained the soil fundamental frequency. The combination of shear-wave velocity and values of soil fundamental frequency provides a complete map of the bedrock topography. The expected velocity inversion due to the presence of shallow basalt flow has been targeted. Electrical resistivity tomography is found to be suitable to define the volcanic basalt thickness. This thickness value is used to constrain the inversion of surface wave dispersion curves and reduce shear-wave velocity uncertainty. The new methodology overcomes the limitations that are typical to urban conditions and other geological complexities.
... The shape of the observed HVSRN curve can constrain the thickness and impedance contrast of geological layers overlying the regional basement (Ibs-von Seht and Wohlenberg 1999; Arai and Tokimatsu 2000;Bonnefoy-Claudet et al. 2006;Macau et al. 2014), as well as the structure of the sediment-bedrock interface (Woolery and Street 2002;Bonnefoy-Claudet et al. 2008;Uebayashi, Kawabe and Kamae 2012). Castellaro and Mulargia (2009b) have explained the case where the HVSRN amplitude is less than 1 for frequencies greater than 1 Hz by the existence of a negative velocity gradient due to the decrease of the H component below the V component. ...
Chlef City, Algeria, which is located in the Lower Cheliff Basin, is vulnerable to seismic hazards. Since there is no constrained velocity model for the Lower Cheliff Basin, particularly at greater depths, we conducted an ambient vibration investigation to map the shear-wave velocity structure beneath the city, with the primary goal of supplementing the existing microzonation studies. Here, we inverted the Rayleigh wave ellipticity measurement curve of ambient vibrations measurements from 120 sites in Chlef City to estimate the shear-wave velocity structure. The study area was subdivided into six zones based on similarities between the observed horizontal-to-vertical spectral ratio of the ambient vibrations curves. Our resultant shear-wave velocity models show that the observed fundamental frequencies (0.3–1.6 Hz) are related to a thick layer (~800 m) of upper Miocene deposits, where Vs = 1000 m/s on average. Generally, the Mesozoic basement shows Vs > 2000 m/s. Moreover, the clear peaks observed at higher frequencies and only in the northwest part of the city are related to a thin layer of Quaternary deposits (Vs = ~250 m/s).
... An HVSR peak at low frequencies is especially useful due to a common practical limitation with temporary arrays, whereby inter-station coherencies at low frequencies (e.g., below 2 Hz) are reduced by the presence of wind or cultural noise with consequent degradation of array processing methods, but HVSR spectra retain sufficient shape to provide useful information on propagating Rayleigh waves. Macau et al. (2015) also demonstrate how secondary peaks in HVSR curves at high frequencies may be diagnostic of significant velocity contrasts within unconsolidated overburden (clay overlying gravels). Thus, combined use of SPAC and HVSR in routine surveys is highly desirable. ...
Ambient seismic noise or microtremor observations used in spatial auto-correlation (SPAC) array methods consist of a wide frequency range of surface waves from the frequency of about 0.1 Hz to several tens of Hz. The wavelengths (and hence depth sensitivity of such surface waves) allow determination of the site S-wave velocity model from a depth of 1 or 2 m down to a maximum of several kilometres; it is a passive seismic method using only ambient noise as the energy source. Application usually uses a 2D seismic array with a small number of seismometers (generally between 2 and 15) to estimate the phase velocity dispersion curve and hence the S-wave velocity depth profile for the site. A large number of methods have been proposed and used to estimate the dispersion curve; SPAC is the one of the oldest and the most commonly used methods due to its versatility and minimal instrumentation requirements. We show that direct fitting of observed and model SPAC spectra generally gives a superior bandwidth of useable data than does the more common approach of inversion after the intermediate step of constructing an observed dispersion curve. Current case histories demonstrate the method with a range of array types including two-station arrays, L-shaped multi-station arrays, triangular and circular arrays. Array sizes from a few metres to several-km in diameter have been successfully deployed in sites ranging from downtown urban settings to rural and remote desert sites. A fundamental requirement of the method is the ability to average wave propagation over a range of azimuths; this can be achieved with either or both of the wave sources being widely distributed in azimuth, and the use of a 2D array sampling the wave field over a range of azimuths. Several variants of the method extend its applicability to under-sampled data from sparse arrays, the complexity of multiple-mode propagation of energy, and the problem of precise estimation where array geometry departs from an ideal regular array. We find that sparse nested triangular arrays are generally sufficient, and the use of high-density circular arrays is unlikely to be cost-effective in routine applications. We recommend that passive seismic arrays should be the method of first choice when characterizing average S-wave velocity to a depth of 30 m (Vs30) and deeper, with active seismic methods such as multichannel analysis of surface waves (MASW) being a complementary method for use if and when conditions so require. The use of computer inversion methodology allows estimation of not only the S-wave velocity profile but also parameter uncertainties in terms of layer thickness and velocity. The coupling of SPAC methods with horizontal/vertical particle motion spectral ratio analysis generally allows use of lower frequency data, with consequent resolution of deeper layers than is possible with SPAC alone. Considering its non-invasive methodology, logistical flexibility, simplicity, applicability, and stability, the SPAC method and its various modified extensions will play an increasingly important role in site effect evaluation. The paper summarizes the fundamental theory of the SPAC method, reviews recent developments, and offers recommendations for future blind studies.
... Besides these regional-scale applications, several studies have demonstrated the applicability of seismic interferometry on the high-frequency local traffic noise (Nakata et al. 2011;Behm and Snieder 2013;Behm et al. 2014;Nakata 2016). Highfrequency passive surface wave method has also attracted much attention from geophysical and civil engineering communities (Foti et al. 2011;Hayashi et al. 2015Hayashi et al. , 2016Le Feuvre et al. 2015;Macau et al. 2015;Imposa et al. 2016;Gabàs et al. 2016;Cheng et al. 2016Cheng et al. , 2018. Due to the elastic properties of near-surface materials, their effects on seismic-wave propagation are of fundamental interest in groundwater, engineering, environmental studies, and petroleum exploration. ...
Passive surface wave methods have gained much attention from geophysical and civil engineering communities because of the limited application of traditional seismic surveys in highly populated urban areas. Considering that they can provide high-frequency phase velocity information up to several tens of Hz, the active surface wave survey would be omitted and the amount of field work could be dramatically reduced. However, the measured dispersion energy image in the passive surface wave survey would usually be polluted by a type of “crossed” artifacts at high frequencies. It is common in the bidirectional noise distribution case with a linear receiver array deployed along roads or railways. We review several frequently used passive surface wave methods and derive the underlying physics for the existence of the “crossed” artifacts. We prove that the “crossed” artifacts would cross the true surface wave energy at fixed points in the f–v domain and propose a FK-based data selection technique to attenuate the artifacts in order to retrieve the high-frequency information. Numerical tests further demonstrate the existence of the “crossed” artifacts and indicate that the well-known wave field separation method, FK filter, does not work for the selection of directional noise data. Real-world applications manifest the feasibility of the proposed FK-based technique to improve passive surface wave methods by a priori data selection. Finally, we discuss the applicability of our approach.
... 1), depth of soil thickness and/or resonance (e.g., Ibs-von Seht and Wohlenberg 1999;Hinzen et al. 2004;Gosar and Lenart 2010;Castellaro 2016). Second, their variation in space or peculiar shape usually indicates lateral heterogeneities in the subsoil (e.g., Uebayashi 2003;Roten et al. 2006;Guéguen et al. 2007;Bonnefoy-Claudet et al. 2008b;Özalaybey et al. (1) f n = n(V S ∕4h), 1 3 2011; Uebayashi et al. 2012;Le Roux et al. 2012;Matsushima et al. 2014;Macau et al. 2015) and the interpretation of peak frequency and amplitude should be carefully considered for such sites. Third, they can show velocity reversals (Castellaro and Mulargia 2009). ...
Nakamura (Q Rep Railway Tech Res Inst 30:25–33, 1989) popularized the application of the horizontal-to-vertical spectral ratio (HVSR) analysis of microtremor (seismic noise or ambient vibration) recordings to estimate the predominant frequency and amplification factor of earthquake shaking. During the following quarter century, popularity in the microtremor HVSR (MHVSR) method grew; studies have verified the stability of a site’s MHVSR response over time and validated the MHVSR response with that of earthquake HVSR response. Today, MHVSR analysis is a popular reconnaissance tool used worldwide for seismic microzonation and earthquake site characterization in numerous regions, specifically, in the mapping of site period or fundamental frequency and inverted for shear-wave velocity depth profiles, respectively. However, the ubiquity of MHVSR analysis is predominantly a consequence of its ease in application rather than our full understanding of its theory. We present the state of the art in MHVSR analyses in terms of the development of its theoretical basis, current state of practice, and we comment on its future for applications in earthquake site characterization.
... The one corresponding to frequency f o is related to the contact sediments/bedrock. According to Macau et al. (2015), f 1 associated to the second maximum is an indicator of the base of Holocene sediments (Pleistocene gravel top). These frequency values can be transformed into depth values using and average Vs velocity value. ...
The Earthquake Early Warning System Laboratory, Centre of Excellence in Disaster Mitigation & Management, Indian Institute of Technology Roorkee has developed India's first Earthquake Early Warning System (EEWS). For this, Uttarakhand, a Himalayan state of the Republic of India, was selected for setting up the regional EEWS. Seismic sensors have been installed in the central seismic gap region, an area where strong and higher magnitude earthquakes are predicted and likely to occur in the future. A total of 169 sensors have been installed in this seismically active region. The control room has been set up in the Earthquake Early Warning System Laboratory. The data is streamed to the laboratory through dedicated VPNoBB network of BSNL and SWAN. The server does the processing of the data in real time. Warning of the earthquakes that occurred in the instrumented region is issued through two modes. Under first mode, sirens have been installed in the Government-owned buildings of Dehradun, Haldwani, and District Emergency Operation Center in all 13 districts of Uttarakhand. In the second mode, earthquake warning are issued through the mobile app ‘Uttarakhand Bhookamp Alert’, which has now been renamed now as ‘Bhukamp Disaster Early Vigilante (BhuDEV)’. This app was launched to the public on 4th August, 2021 by the honorable Chief Minister of Uttarakhand. In this article, site classification for the instrumented field sites is performed. It is vital for analyzing the seismic hazard of an area and understanding the damage patterns caused by earthquakes. In the present study, 169 sites of the seismic network array of EEWS for Uttarakhand are classified on the basis of geological settlement at the locations of the stations.KeywordsEarthquake early warning systemSite classificationGeological conditionsUttarakhandSeismic gap
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A combined set of multichannel analysis of surface waves (MASW) and single-station microtremor horizontal-to-vertical spectral ratio (MHVSR) testing was performed at over 190 sites within the complete Kashmir Basin, mainly concentrated within the Greater Srinagar region. The aim was to establish the dynamic parameters of the main geomorphological units, especially the sedimentary deposits (Alluvial and Karewa) in the valley through widespread geophysical testing. Shear wave velocity (VS) and frequency parameters were estimated for each of the sites. Multiple impedance contrasts in the HVSR curves were found within the sedimentary deposits indicating a number of stratigraphic layers which were then correlated with the geology of the region. Further, forward modelling routine was utilised to extend the VS profile to deeper depths, to determine the first-order estimate bedrock depth and establish the basin structure which remains largely unexplored till now. Lastly, geological cross-sections were constructed using these results from the geophysical tests.
Failure Mechanisms of Pile-Supported Port Building in Liquefiable Sloping Ground During Earthquake
The empirical power law relation (PR) between resonance frequency (f0), obtained from H/V spectral ratio analysis of ambient noise, and sediment thickness (h), obtained from boreholes, is frequently used in microzonation studies to predict bedrock depth. In this study, we demonstrate (i) how to optimally construct a PR by including the error on the picked f0 in the regression, and (ii) how to evaluate a regression quality by identifying the under- or overestimation of the sediment thickness prediction. We apply this methodology on f0 data derived from 74 ambient noise recordings acquired above boreholes that reach the Brabant Massif bedrock below Brussels (Belgium). Separating the f0 data into different subset based on the cover geology does not significantly improve the bedrock depth prediction because the cover geology in Brussels has common base layers. In Brussels, the PR relation h = 88.631.f0−1.683 is the best candidate to convert f0 to depth, with a prediction error of 10%. The Brussels PR was subsequently applied on a local survey (404 measurements; 25 km2) in southern Brussels with the aim to study Brussels’ Brabant Massif bedrock paleorelief. By linking the obtained paleorelief, Bouguer gravity data and aeromagnetic data, a NNW-SSE oriented, 20 m-high subsurface ridge could be identified. This ridge stands out because of differential erosion between less-resistant and hard quartzitic rock formations of the Brabant Massif. This subsurface ridge deflects the local radiation of seismic energy resulting in an anomaly in the otherwise regional consistent azimuthal dependency of the resonance frequency. We conclude that adding a polarisation analysis to a microzonation survey analysis allows detecting anomalous features in the paleorelief.
The Horizontal-to-Vertical Spectral Ratio (HVSR) obtained from microtremor data recorded at three test sites are analyzed in order to highlight some issues related to the computation of the SESAME criteria that define the statistical robustness of possible peaks. In case of multiple-peak HVSR curves, it is shown that to properly assess the statistical properties of a peak and avoid the problem of multimodal data distribution, it is crucial to isolate each peak by reducing the frequency range around it.
It is also shown that, while the standard approach used to obtain a homogeneous data set is represented by the removal of large-amplitude transient events before the computation of the HVSR, the removal of outlier HVSR curves can be a more effective way to obtain such a goal, being outlier HVSR curves not necessarily associated to large-amplitude transient events. It is eventually briefly discussed the problem of the proper management of multi-peak HVSR curves in microzonation studies while defining the so-called soil frequency maps. It is argued that to focus these maps just on the lowest-frequency (f0) value is questionable because higher-frequency peaks can be much more important when the local urban landscape is characterized by low-rise buildings.
To explore the usefulness of the ambient seismic noise tomography method for characterizing the subsoil surface structure, in this study, we apply this method to contribute to geotechnical decision-making in the construction of a school building. We used a rectangular array (36 x 56 m) of 48-4.5 Hz vertical geophones and produce surface wave tomographies from the travel times of Rayleigh waves extracted by cross-correlation of seismic noise. We determined a final 3D Vs model using 1D models derived from the inversion of dispersion curves obtained from the tomography maps for different frequencies. The 3D model shows an excellent resolution (vertical and lateral); we observe critical velocity contrasts in the range of 2 to 15 m deep. At depths higher than 15 m, the velocity has values close to 900 m/s; however, we observe a low-velocity anomaly associated with a lava tube or crack that seems to continue under an adjacent building.
A “natural field” seismic technique is possible to attain by observing microseisms with a suitably designed array and by digitally processing the data to obtain estimates of the phase velocities of Rayleigh waves. Wavelengths of interest in detecting depth to the basement of sedimentary basins are in the range 2 to 20 km, and correspond to wave periods from 1 to 7 s. An array of five or seven seismometers deployed as an expanding cross configuration simplifies field procedures and is adequate for phase velocity measurements of Rayleigh waves in the required wavelength range, provided high‐resolution frequency‐wavenumber spectral analysis is used. This analysis can be implemented on a minicomputer in the field. Results obtained from observation in a sedimentary basin of known structure show predominantly fundamental‐mode Rayleigh wave propagation. The scatter of velocity estimates is small enough to allow inversion by curve matching, and depth to the basement can be computed to an accuracy of ±30 percent without requiring restrictive assumptions of a seismic velocity structure.
We have investigated the potential of 2D electrical imaging for the characterization of seawater intrusion using field data from a site in Almeria, SE Spain. Numerical simulations have been run for several scenarios, with a hydrogeological model reflecting the local site conditions. The simulations showed that only the lower salt concentrations of the seawater-freshwater transition zone could be recovered, due to the loss of resolution with depth. We quantified this capability in terms of the cumulative sensitivity associated with the measurement setup and showed that the mismatch between the targeted and imaged parameter values occurs from a certain sensitivity threshold. Similarly, heterogeneity may only be determined accurately if located in an adequately sensitive area. At the field site, we identified seawater intrusion at the scale of a few kilometres down to a hundred metres. Borehole logs show a remarkable correlation with the image obtained from surface data but indicate that the electrically derived mass fraction of pure seawater could not be recovered due to the discrepancy between the in-situ and laboratory-derived petrophysical relationships. Surface-to-hole inversion results suggest that the laterally varying resolution pattern associated with such a setup dominates the image characteristics compared to the laterally more homogeneous resolution pattern of surface only inversion results and hence, surface-to-hole images are not easily interpretable in terms of larger-scale features. Our results indicate that electrical imaging can be used to constrain seawater intrusion models if image appraisal tools are appropriately used to quantify the spatial variation of sensitivity and resolution. The most crucial limitation is probably the apparent non-stationarity of the petrophysical relationship during the imaging process.
The objective of this work is to perform a purely empirical assessment of the actual capabilities of the horizontal-to-vertical
(H/V) spectral ratio technique to provide reliable and relevant information concerning site conditions and/or site amplification.
This objective has been tackled through the homogeneous (re)processing of a large volume of earthquakes and ambient noise
data recorded by different research teams in more than 200 sites located mainly in Europe, but also in the Caribbean and in
Tehran. The original recordings were first gathered in a specific database with information on both the sites and recorded
events. Then, for all sites close to an instrumented reference, average site-to-reference spectral ratios (“spectral ratio
method” (SSR)) were derived in a homogeneous way (window selection, smoothing, signal-to-noise ratio threshold, averaging),
as well as H/V ratios (“HVSRE–RF”) on earthquake recordings. H/V ratios were also obtained from noise recordings at each site
(either specific measurements, or extracted from pre- or post-event noise windows). The spectral curves resulting from these
three techniques were estimated reliable for a subset of 104 sites, and were thus compared in terms of fundamental frequency,
amplitude and amplification bandwidth, exhibiting agreements and disagreements, for which interpretations are looked for in
relation with characteristics of site conditions. The first important result consists in the very good agreement between fundamental
frequencies obtained with either technique, observed for 81% of the analyzed sites. A significant part of the disagreements
correspond to thick, low frequency, continental sites where natural noise level is often very low and H/V noise ratios do
not exhibit any clear peak. The second important result is the absence of correlation between H/V peak amplitude and the actual
site amplification measured on site-to-reference spectral ratios. There are, however, two statistically significant results
about the amplitude of the H/V curve: the peak amplitude may be considered as a lower bound estimate of the actual amplification
indicated by SSR (it is smaller for 79% of the 104 investigated sites), and, from another point of view, the difference in
amplitude exhibits a questioning correlation with the geometrical characteristics of the sediment/basement interface: large
SSR/HV differences might thus help to detect the existence of significant 2D or 3D effects.
The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface, Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear-wave velocity profile is dependent on frequency as well as distance from the source. Multichannel recording permits effective identification and isolation of noise according to distinctive trace-to-trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real-time maximization of the SM ratio during acquisition and subsequent processing steps. Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component. Breaks in coherent surface-wave arrivals, observable on the decomposed record, can be compensated fur during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording. A multichannel shot gather decomposed into a swept-frequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear-wave velocity (v(s)) profile with a downhole v(s) profile.
- In order to evaluate soil effects in the urban area of Barcelona, the Nakamura's technique has been used to estimate the predominant periods of soils. Noise measurements for 195 sites were performed using a strong motion accelerograph and a velocimeter. In this work, the resulting preliminary map of predominant periods is presented. The obtained predominant periods are coherent with the geological and geotechnical features of the area. The analysis of the information has allowed the distinctions among several types of soil and underlying materials. A predominant period of about 0.06s is evaluated for sites located over outcrop Paleozoic rock in the Tibidabo-Collserola Mountains. For sites consisting of material named tricycle, that is the most extensive and also the most heterogeneous zone, predominant period range from 0.10s up to 2.0s depending on the thickness of the surface materials and the kind and thickness of the underlying materials. In the Besós river two zones are observed: the riverside with periods between 0.50s and 0.83s and a second area with periods between 1.0 and 2.1s. In the Llobregat river delta the obtained periods are quite homogeneous with values around 0.72s. Other predominant periods are found in some tertiary rock outcrop.
Focusing on a comparison of the accuracy of the extended spatial auto-correlation (ESAC) method and the frequency-wavenumber spectrum (FK) method, we carried out short-period microtremor measurements of arbitrarily shaped array configurations at a site with a well-known velocity structure at shallow depths. Using both techniques, we measured phase velocities for frequencies between 2.5 Hz and 13.5 Hz and compared the results with the theoretical Rayleigh-wave-dispersion characteristics of both the fundamental and the first higher modes calculated from the PS logging data. Next, we tried to estimate the S-wave velocity structure to a depth of 43 in by fitting the theoretically calculated phase velocities to the experimental data, taking into consideration both the higher mode contributions as well as the fundamental mode. The theoretical dispersion characteristics were successfully fitted to the results analyzed by the ESAC method but not by the FK method. The estimated S-wave velocity structure from the ESAC method results was in good agreement with the PS logging profile. We also confirmed, using the model, that the S-wave transfer function due to vertical incidence coincided with that from the PS logging data. As a result of the study, we conclude that the ESAC method gives more accurate results than the FK method in determining the Rayleigh-wave phase velocity from records of short-period microtremors using arbitrarily shaped array configurations. The ESAC method can also provide a better estimate of the S-wave velocity structure and site amplification than the FK method.
A 'natural field' seismic technique is possible to attain by observing microseisms with a suitably designed array and by digitally processing the data to obtain estimates of the phase velocities of Rayleigh waves. Wavelengths of interest in detecting depth to the basement of sedimentary basins are in the range 2 to 20 km, and correspond to wave periods from 1 to 7 s. An array of five or seven seismometers deployed as an expanding cross configuration simplifies field procedures and is adequate for phase velocity measurements of Rayleigh waves in the required wavelength range, provided high-resolution frequency-wavenumber spectral analysis is used. This analysis can be implemented on a minicomputer in the field. Results obtained from observation in a sedimentary basin of known structure show predominantly fundamental-mode Rayleigh wave propagation. The scatter of velocity estimates is small enough to allow inversion by curve match- ing, and depth to the basement can be computed to an accuracy of ±30 per cent without requiring restrictive assumptions of a seismic velocity structure.
Single station seismic noise measurements were carried out at 192 sites in the western part of Istanbul, Turkey. This extensive survey allowed the fundamental resonance frequency of the sedimentary cover to be mapped, and identify areas prone to site amplification. The results are in good agreement with the geological distribution of sedimentary units, indicating a progressive decrease of the fundamental resonance frequencies from the northeastern part, where the bedrock outcrops, towards the southwestern side, where a thickness of some hundreds meters for the sedimentary cover is estimated. The particular distribution of fundamental resonance frequencies indicates that local amplification of the ground motion might play a significative role in explaining the anomalous damage distribution after the 17 August 1999 Kocaeli Earthquake.Furthermore, 2D array measurements of seismic noise were performed in the metropolitan area with the aim of obtaining a preliminary geophysical characterization of the different sedimentary covers. These measurements allow the estimation of the shear-wave velocity profile for some representative areas and the identification of the presence of strong impedance contrast responsible of seismic ground motion amplification. Comparison of a theoretical site response from an estimated S-wave velocity profile with an empirical one based on earthquake recordings strongly encourages the use of the low cost seismic noise techniques for the study of seismic site effects.
We analyse the Cenozoic topographic evolution of the Catalan Coastal Ranges (NE Spain) and the role of fault activity, erosion, sedimentation, and isostasy in uplift and subsidence. A forward numerical model constrained by an extensive geological and geophysical data set is used to examine the temporal and spatial record of the Cenozoic vertical motions. We show that the effect of isostasy, erosion and sedimentation is as important as the contribution of fault deformation on the topography of the Catalan Coastal Ranges. The model predicts that Paleogene topography generation by thrusting was compensated by erosion (up to 1.3 km) and isostatic subsidence (up to 1.2 km) resulting in a 1.2-1.9 km high mountain range by the end of compression (29 Ma). During Neogene, strong tectonic subsidence related to normal faulting and the consequent flexural uplift (of 0.7-1.2 km) and surface erosion (as much as 1.6-2.3 km) and sedimentation (up to 4.5 km) led to the present landscape configuration. Extension rates along the Barcelona fault controlled flexural uplift and, in combination with erosion and sedimentation processes, led to the migration of the topographic maximum of the Prelitoral Range towards the easternmost Ebro Basin.
- Seismic responses of different sites of Barcelona have been investigated through numerical modelling. Geological maps and geotechnical data available from drillings for buildings and infrastructures have been used to determine the dynamical properties of the soils through different correlations between standard geotechnical data and dynamical parameters obtained in other regions. An estimation of the depth of the Palaeozoic basement has been obtained through an inversion of a detailed gravity survey. A 1-D equivalent linear method has been used to compute complete transfer functions and other spectral responses, such as PSA and PSV for various damping values, with the purpose of classifying zones with similar behaviour. Given the uncertainties associated with the input data, a Montecarlo's simulation process has been carried out. Four zones, characterized by their corresponding transfer function and by PGA amplifications, are proposed. The numerical results are compared with those previously obtained through microtremor measurements, showing that predominant periods derived from Nakamura's technique should be taken carefully.
Three methods are used to determine the site effect in the town of Grenoble, located in the Western Alps. First we use the classical spectral ratio method in 14 sites to calculate the transfer function of the basin. We find an amplification of 10 in the frequency range of 0.25 to 10 Hz. Second, we compare these results with the H over V spectral ratio method, and propose a map of resonance frequency of the basin. We find a lower resonance frequency in the center of the basin than on the edge, that is consistent with the structure deduced from a gravity Bouguer anomaly map. Finally we use the empirical Green’s function method to simulate a M
w
5.5 earthquake at a distance of 20 km from the town. The simulated acceleration reaches the level of 2 m/s2 in the center of the basin compared to 0.2 m/s2 on the edges. The simulated ground motion we compute is smaller than the French seismic codes on the edge of the valley but significantly larger in the center.
Numerical simulation of noise is used to investigate the characteristics of the spectral ratio between horizontal and vertical components (H/V ratio) and its sensitivity to various parameters in order to better appreciate the reliability of the technique proposed by Nakamura (1989) to estimate site amplification effects from single station noise recordings. Noise is simulated as the signal produced at a single site by a set of superficial sources (unidirectional forces or dipoles) disposed all around with random amplitude and time delay. Individual signals from a single source are computed by the discrete wave number technique. Synthetic calculations for 15 soil profiles show that this ratio exhibits a single, clear peak, the location of which is independent of the source excitation function, but strongly correlated with the local geological structure: its frequency is very close to the S wave resonance frequency. This peak appears to be mainly controlled by the polarization curve of the fundamental Rayleigh waves, which in turn exhibits a sharp peak around the fundamental resonance mode of the sedimentary structure. A similar result is found for the H/V ratio obtained for incident plane SV waves. In contrast, the amplitude of this peak exhibits a poor correlation with the ground motion amplification of S waves at resonance frequency. It is shown to be related with a high sensitivity on the value of the Poisson's ratio in the uppermost layer presumed to be the noise source layer, and, though to a much lesser extent, on the mean distance between site and noise sources. It is concluded that Nakamura's method can clearly allow the resonance frequency of a given sedimentary site to be measured very efficiently and very cheaply, but that its use for deriving the amplification at resonance frequency seems still premature from a theoretical point of view.
Ambient vibration techniques such as the H/V method may have the potential to significantly contribute to site effect evaluation, particularly in urban areas. Previous studies interpret the so-called Nakamura's technique in relation to the ellipticity ratio of Rayleigh waves, which, for a high enough impedance contrast, exhibits a pronounced peak close to the fundamental S-wave resonance frequency. Within the European SESAME project (Site EffectS assessment using AMbient Excitations) this interpretation has been tested through noise numerical simulation under well-controlled conditions in terms of source type and distribution and propagation structure. We will present simulations for a simple realistic site (one sedimentary layer over bedrock) characterized by a rather high impedance contrast and low quality factor. Careful H/V and array analysis on these noise synthetics allow an in-depth investigation of the link between H/V ratio peaks and the noise wavefield composition for the soil model considered here: (1) when sources are near (4 to 50 times the layer thickness) and surficial, H/V curves exhibit one single peak, while the array analysis shows that the wavefield is dominated by Rayleigh waves; (2) when sources are distant (more than 50 times the layer thickness) and located inside the sedimentary layer, two peaks show up on the H/V curve, while the array analysis indicates both Rayleigh waves and strong S head waves; the first peak is due to both fundamental Rayleigh waves and resonance of head S waves, the second is only due to the resonance of head S waves; (3) when sources are deep (located inside the bedrock), whatever their distance, H/V ratio exhibit peaks at the fundamental and harmonic resonance frequencies, while array analyses indicate only non-dispersive body waves; the H/V is thus simply due to multiple reflections of S waves within the layer. Therefore, considering that experimental H/V ratio (i.e. derived from actual noise measured in the field) exhibit in most cases only one peak, we conclude that H/V ratio is (1) mainly controlled by local surface sources, (2) mainly due to the ellipticity of the fundamental Rayleigh waves. Then the amplitude of H/V peak is not able to give a good estimate of site amplification factor.
Dispersive Rayleigh waves, contained in conventional P‐wave refraction records, can be used to determine the shear‐wave velocity profile. In the present paper, the synergies of a cross interpretation of refraction and surface wave data are exploited using data collected at sites where other geotechnical and geophysical information was available.
Selected examples are presented to emphasize the relative advantages and limitations of the two techniques in the cases of hidden layer, velocity inversions and shallow water table.
Surface wave analysis was performed to estimate the experimental dispersion curves in the f–k domain and the results of the dispersion curves inversion were compared with refraction results in terms of velocity profiles.
The experimental results prove that many advantages, in terms of resolution and reliability, can be obtained with joint acquisition and cross interpretation of P‐wave refraction and surface wave data, without a significant increase in testing time.
High levels of ambient noise and safety factors often limit the use of "active-source" seismic methods for geotechnical investigations in urban environments. As an alternative, shear-wave velocity–depth profiles can be obtained by treating the background microtremor wave field as a stochastic process, rather than adopting the traditional approach of calculating velocity based on ray path geometry from a known source. A recent field test in Melbourne demonstrates the ability of the microtremor method, using only Rayleigh waves, to resolve a velocity inversion resulting from the presence of a hard, 12 m thick basalt flow overlying 25 m of softer alluvial sediments and weathered mudstone. Normally the presence of the weaker underlying sediments would lead to an ambiguous or incorrect interpretation with conventional seismic refraction methods. However, this layer of sediments is resolved by the microtremor method, and its inclusion is required in one-dimensional layered-earth modelling in order to reproduce the Rayleigh-wave coherency spectra computed from observed seismic noise records. Nearby borehole data provided both a guide for interpretation and a confirmation of the usefulness of the passive Rayleigh-wave microtremor method. Sensitivity analyses of resolvable modelling parameters demonstrate that estimates of shear velocities and layer thicknesses are accurate to within approximately 10% to 20% using the spatial autocorrelation (SPAC) technique. Improved accuracy can be obtained by constraining shear velocities and/or layer thicknesses using independent site knowledge. Although there exists potential for ambiguity due to velocity–thickness equivalence, the microtremor method has significant potential as a site investigation tool in situations where the use of traditional seismic methods is limited.
Modal calculations of wave propagation in layered, elastic media exhibit numerical difficulties at high frequencies. This report shows that these difficulties are caused by the computation of squares of exponential terms which, although they cancel numerically, cause a loss of significant figures in the secular function and the components of certain matrix products. A matrix formulation is presented that avoids these difficulties and gives a natural decomposition of the modal solutions into source and receiver parts. The formulation is applied to layered systems containing both fluid and solid layers as well as to systems composed of solid layers alone.
A matrix formalism developed by W. T. Thomson is used to obtain the phase velocity dispersion equations for elastic surface waves of Rayleigh and Love type on multilayered solid media. The method is used to compute phase and group velocities of Rayleigh waves for two assumed three-layer models and one two-layer model of the earth's crust in the continents. The computed group velocity curves are compared with published values of the group velocities at various frequencies of Rayleigh waves over continental paths. The scatter of the observed values is larger than the difference between the three computed curves. It is believed that not all of this scatter is due to observational errors, but probably represents a real horizontal heterogeneity of the continental crusts.
The amplitude characteristics as the fundamental natures of microtremors are discussed at present paper. The experimental field is Hakodate City in Hokkaido.
The power spectral density functions of microtremors were compared with amplitude functions of Rayleigh wave (M11) computed from underground structure. Furthermore, theoretical ratio of the horizontal component to the vertical component of Rayleigh wave (M11) was compared with that of the observed microtremors. From these comparisons, it was shown that the amplitude characteristics of microtremor was mostly composed of Rayleigh wave (M11).
The solution to problems of elastic wave propagation in multilayered media, in which each layer is homogeneous and where the ensemble of layers has physical properties that vary only with one coordinate, may be given as the quotient of products of matrices. In the case of SH waves, the matrices are of order two; in the case of P-SV waves the matrices are of order four. The individual matrix elements are themselves determinants of order two or four in the two cases. The solution is obtained by means of Laplace's development by minors.
Fundamental natures of microtremors have already been discussed by many investigators. The grouth mechanism of microtremor is not yet known. In this paper, the propagation characteristics of microtremor was investigated. The experimental field is Hakodate City in Hokkaido.
The propagation of microtremor was confirmed from the tripartite observation, and by the orbital motions, directions and velocities of microtremors, it was found that these waves were Rayleigh type wave, Love type wave and a combination of both. Phase velocity observed at the place where confirmed the propagation of microtremors agreed partly with the dispersion curves of fundamental mode of Rayleigh and Love wave. A ditch with a depth of 1m was made at the place confirmed the propagation of microtremor. However, it didn't affect the vertical component of microtremor. Nextly, microtremors were observed on the moat with a depth of 3m at Goryokaku in Hakodate City, by means of tripartite and 3-component observation.
From results of observation, it was found that the attenuation of the frequency characteristics of microtremors with vertical component between two places across the moat were more rapid with higher frequency and microtremors on the isolated island around the moat were only composed of Love type waves. The attenuation of energy ratio for k(wave number)×d(depth of the moat or slit) in case of the moat was well in accordance with results of model experiments which made in order to show the attenuation of Rayleigh wave for a slit. From a point of view of the propagation characteristics of microtremors, the vertical component of microtremors may be almost composed of Rayleigh wave.
The most important aquifers of the Llobregat river basin are located in its lower part, namely the Low Llobregat Aquifer System (LLAS). Intensive groundwater development since the late nineteenth century and mostly after the 1950s has dramatically changed its hydrological functioning. Groundwater quality has suffered from salinity increase due to the potash mining area in the centre of the basin and seawater intrusion, as well as agriculture, waste disposal, sand and gravel pit backfilling and leakages. A stricter management is reversing and improving the situation and thus preserving the role of the LLAS as a crucial factor for the water supply to the Metropolitan Area of Barcelona. Besides model-based groundwater quantity management and stricter rules to protect water quality, additional actions include artificial recharge and installing a barrier to halt seawater intrusion by injecting highly treated reclaimed waste water. A groundwater user community was created and plays an important role in moving towards a sustainable use.
Engineering seismology now requires a convenient and easy survey method for S-wave-velocity structures which enables exploration down to the basement even in urbanized areas. We have attempted an application of long-period (0.5Hz to 3.0Hz) microtremors to answer this demand. The method consists of three steps: (1) microtremors are observed using an array of seismometers; (2) their phase velocities are determined by the frequency-wavenumber-spectral analysis of array data; and (3) the S-wave-velocity structure is determined from the obtained phase velocities by the generalized inversion method. As an exploration method, this procedure has several advantages: (1) microtremors can be observed at any time and location; (2) observation is much easier than with other exploration methods; (3) it causes no environmental problems; and (4) geological conditions down to a depth of more than 100m can be inverted, as far as microtremors of required frequency range are observed. The method was applied at two sites located in and near urban areas, and the whole S-velocity structure above the basement was determined. This method proves to be a useful and practical tool for determining S-wave-velocity structures especially in urbanized areas.
The spectral ratio technique is a common useful way to estimate empirical transfer function to evaluate site effects in regions of moderate to high seismicity. The purpose of this paper is to show that it is possible to estimate empirical transfer function using spectral ratios between horizontal and vertical components of motion without a reference station. The technique is presented briefly and it is discussed why it may be applicable to study the intense S-wave part in earthquake records. Results are presented for three different cities in Mexico: Oaxaca, Oax., Acapulco, Gro., and Mexico City. -from Authors
The observations about the behavior of microtremor spectra presented here show that noise measurements can be used as a powerful tool to determine the thickness of soft cover layers. The most suitable method for this determination is Nakamura's technique, which is the ratio of the horizontal-component noise spectrum and that of the vertical component (H/V spectrum). The frequency of the main peak in these spectral ratios correlates well with the sediment thickness at the site. Using an extensive database of microtremor measurements carried out in the western Lower Rhine Embayment (Germany), it was possible to show that this correlation is clearly valid for a wide range of thickness, namely, from tens of meters to more than 1000 m. A simple formula was derived that, for the sediments to be found in the area investigated, directly calculates the cover thickness from the frequency of the main peak in the H/V spectrum. A comparison with calculated resonant frequencies suggests the relation derived from the noise measurements depending on the velocity depth function of the shear wave. Classical spectral ratios are shown to be strongly influenced by the noise level and are therefore less reliable in determining the resonant frequency of the subsoil. The practical relevance of the investigation is illustrated by means of cross sections, constructed from results of the microtremor analyses, which provide a convincing image of the surficial structure of the areas investigated.
Mapping bedrock structure beneath overburden is crucial for understanding geological and hydrogeological processes. Acquiring this information is generally done using well drilling or geophysical surveys; but these studies are expensive and require large periods of acquisition and processing time. In addition, geophysical data acquisition can be logistically challenging in urban zones with limited available areas for instrumentation deployment.
Under favourable conditions (1D structure and high acoustic impedance contrast) the H/V microtremor technique can provide estimates of bedrock depth. This technique is used to obtain the soil resonance frequency in seismic microzonation studies. It is based on the computation of the horizontal to vertical spectral ratio of microtremor recordings acquired at a single station. The soil resonance frequency is related to soil shear‐wave velocity and thickness.
Here we investigate the capability of combining microtremor and traditional exploration geophysical techniques (electrical resistivity and seismic tomography) to obtain an empirical relationship relating soil resonance frequency and overburden thickness. Subsequently we propose to extend microtremor measurements to adjacent areas that have not been covered by geophysical surveys.
This methodology has been applied at a test site located in a granitic environment where alluvial/colluvial sediments cover the granite weathering profile. This area is characterized by urban development and sectors having rugged topography. A priori, this area has suitable conditions to apply the H/V microtremor technique. Overburden thickness has been estimated to range between 20–50 m. The proposed methodology has been validated at the test site, encouraging us to apply the H/V method as an exploration tool in similar geological environments.
Noise measurements were carried out in the Cologne area (Germany), and the resonance frequency of each site was estimated from the main peak in the spectral ratio between the horizontal and vertical component. For 32 of these sites, the thickness of the sedimentary cover was known from boreholes, and a clear cor-relation between resonance frequency and sediment thickness was observed. A for-mula that correlates cover thickness with frequency of the main peak in the horizontal-to-vertical spectral ratio was derived. In addition, a best-fitting shear-wave-velocity distribution with depth, v s (z), as well as a relationship between average shear-wave velocity V ¢ s and thickness of the sedimentary cover, was calculated. By using all of the noise measurements and applying the derived relationships, we ob-tained a subsoil classification for the Cologne area.
Urban geological mapping is a key to assist management of new developed areas, conversion of current urban areas or assessment of urban geological hazards. Geophysics can have a pivotal role to yield subsurface information in urban areas provided that geophysical methods are capable of dealing with challenges related to these scenarios (e.g., low signal-to-noise ratio or special logistical arrangements). With this principal aim, a specific methodology is developed to characterize lithological changes, to image fault zones and to delineate basin geometry in the urban areas. The process uses the combination of passive and active techniques as complementary data: controlled source audio-magnetotelluric method (CSAMT), magnetotelluric method (MT), microtremor H/V analysis and ambient noise array measurements to overcome the limitations of traditional geophysical methodology. This study is focused in Girona and Salt surrounding areas (NE of Spain) where some uncertainties in subsurface knowledge (maps of bedrock depth and the isopach maps of thickness of quaternary sediments) need to be resolved to carry out the 1:5000 urban geological mapping. These parameters can be estimated using this proposed methodology. (1) Acoustic impedance contrast between Neogene sediments and Paleogene or Paleozoic bedrock is detected with microtremor H/V analysis that provides the soil resonance frequency. The minimum value obtained is 0.4 Hz in Salt city, and the maximum value is the 9.5 Hz in Girona city. The result of this first method is a fast scanner of the geometry of basement. (2) Ambient noise array constrains the bedrock depth using the measurements of shear-wave velocity of soft soil. (3) Finally, the electrical resistivity models contribute with a good description of lithological changes and fault imaging. The conductive materials (1–100 Ωm) are associated with Neogene Basin composed by unconsolidated detrital sediments; medium resistive materials (100–400 Ωm) correspond to Paleogene, and resistive materials (600–1,000 Ωm) are related with complex basement, granite of Paleozoic. The Neogene basin-basement boundary is constrained between surface and 500 m depth, approximately. The new geophysical methodology presented is an optimized and fast tool to refine geological mapping by adding 2D information to traditional geological data and improving the knowledge of subsoil.
In this paper we present a site effects analysis carried out in Málaga city’s historical centre (Southern Spain). Two different methodologies have been used: an experimental technique using ambient noise measurements and a 1D numerical method. Soil fundamental frequencies have been obtained from the first technique, and soil transfer functions have been calculated from the numerical methodology. In order to use these results in vulnerability studies, intensity increments for each type of soil have also been estimated. From this information, a seismic microzonation has been proposed for the city centre, classified in six types of soils. Soil fundamental frequencies vary between above 5.0Hz at the hills of the city (where rock arises on the surface), and 1.0Hz near Guadalmedina river. The results show regions with high intensity increments (ΔI = +1.5) corresponding to areas which suffered heavy damage in the 1680 earthquake (Imax = VIII–IX). Moreover, most of the monuments and historical buildings in the city are located in these high risk areas. Results underline the importance of this kind of studies for seismic risk mitigation, historical preservation and emergency planning in the main cities’ historical centres.
Ancient fluvial valley systems are long recognized as important features in the stratigraphic record, but emerged as a specific focus of attention with publication of first-generation sequence-stratigraphic concepts. This paper reviews current understanding of paleovalley systems from the perspective of Quaternary analogs and experimental studies.Paleovalley systems can include distinct mixed bedrock–alluvial, coastal-plain, and cross-shelf segments. Mixed bedrock–alluvial segments are long-lived, cut across bedrock of significantly older age, and have an overall degradational architecture. By contrast, coastal-plain and cross-shelf segments are non-equilibrium responses to high-frequency cycles of relative sea-level change: most coastal-plain and cross-shelf segments form as a geometric response to relative sea-level fall, as river systems cut through coastal-plain and inner shelf clinothems, and extend basinward to track the shoreline. After incision and cross-shelf extension, lateral channel migration and contemporaneous channel-belt deposition creates a valley-scale feature. Coastal-plain and cross-shelf paleovalley widths are set by the number of channel-belt sandbodies deposited during this time.Paleovalley systems play a key role in source-to-sink sediment routing. Early views included the model of incision and complete sediment bypass in response to relative sea-level fall. However, this model does not stand up to empirical, theoretical, or experimental scrutiny. Instead, there is a complex dynamic between incision, deposition, and sediment export from an evolving valley: periods of incision correspond with sediment export minima, whereas periods of lateral migration and channel-belt construction result in increased flux to the river mouth. Sediment export from evolving valleys, and merging of drainages during cross-shelf transit, play key roles in sediment transfer to the shelf-margin and genetically-linked slope to basin-floor systems. Connection between the river mouth and the shelf margin likely occurs for different periods of time depending on gradient of the river and shelf, as well as amplitude of high-frequency sea-level changes.Late Quaternary analogs and experimental studies provide an alternative sequence-stratigraphic interpretation for paleovalley systems. In coastal-plain paleovalleys, basal valley-fill surfaces meet criteria for an unconformity and a classically-defined sequence boundary: however, this surface is mostly everywhere of the same age as overlying fluvial deposits, and does not correspond to a long period of incision and sediment bypass. In cross-shelf paleovalleys, the basal contact between fluvial and deltaic or shoreface deposits is commonly interpreted as a sequence boundary, but is not an unconformity characterized by incision and sediment bypass. Instead, this surface is a facies contact that separates genetically-related fluvial and deltaic strata: the surface that correlates to the basal valley-fill surface within the coastal-plain paleovalley dips below cross-shelf prograding deltaic and/or shoreface strata, which are fed by deposition within the evolving valley itself, and should be the downlap surface.Many issues deserve attention in the future. We have stressed understanding the inherent scales and physical processes that operate during the formation and evolution of paleovalley systems. We also suggest the relative roles of allogenic forcing vs. autogenic dynamics, and the potential significance of high-frequency isostatic adjustments should be topics for future discussion.
Passive recordings of seismic noise are increasingly used in earthquake engineering to measure in situ the shear‐wave velocity profile at a given site. Ambient vibrations, which are assumed to be mainly composed of surface waves, can be used to determine the Rayleigh‐wave dispersion curve, with the advantage of not requiring artificial sources. Due to the data uncertainties and the non‐linearity of the problem itself, the solution of the dispersion‐curve inversion is generally non‐unique. Stochastic search methods such as the neighbourhood algorithm allow searches for minima of the misfit function by investigating the whole parameter space. Due to the limited number of parameters in surface‐wave inversion, they constitute an attractive alternative to linearized methods. An efficient tool using the neighbourhood algorithm was developed to invert the one‐dimensional profile from passive or active source experiments. As the number of generated models is usually high in stochastic techniques, special attention was paid to the optimization of the forward computations. Also, the possibility of inserting a priori information into the parametrization was introduced in the code.
This new numerical tool was successfully tested on synthetic data, with and without a priori information. We also present an application to real‐array data measured at a site in Brussels (Belgium), the geology of which consists of about 115 m of sand and clay layers overlying a Palaeozoic basement. On this site, active and passive source data proved to be complementary and the method allowed the retrieval of a profile consistent with borehole data available at the same location.
The usefulness of microtremors as a geophysical exploration tool has been analyzed. This application is possible due to the relationship between the main resonance frequency of a given soil, obtained from the H:V spectral ratios of microtremors, its thickness and average shear velocity. We first measured the ambient noise at 33 sites and determined their main resonance frequency. Detailed geotechnical information was available for 23 of the sites, thereby allowing us to establish a quantitative relationship between the resonance frequency and the thickness of the soil, and indirectly between either of them and the shear velocity of the soil. The practical application of this relationship has revealed its usefulness in determining the surface structure of a valley with excellent accuracy, with an error of only 15% in the depths calculated. These errors are due to the simplification of the problem that this method implies: it requires that the shear velocity vary constantly with depth throughout the study region, which is evidently not always so, and that the input data themselves intrinsically have a certain degree of uncertainty. This method is therefore not valid when there is no mechanical contrast between the study soil and the underlying layer or when the shear velocity varies irregularly with depth in the study area.
In media with strong vertical variations in elasto-dynamic properties, the characteristics of surface waves may deviate significantly from their typical appearance in media with small contrasts. Different numerical examples suggest that ignoring this behavior may lead to poor results in surface wave analysis. The cases analysed are: (1) Neighboring surface wave modes which exchange their major characteristics at osculation points of their dispersion curves; (2) Quasi-channel waves propagating inside a buried low velocity channel; (3) Surface waves in the presence of a low-velocity layer at the very surface; and (4) Rayleigh surface waves in the presence of a very high velocity contrast allowing normal quasi-compressional pseudo-modes. Amplitude-depth functions are used to analyze the physical cause of the unusual characteristics. One important result is a vanishing vertical displacement of the fundamental mode and dominance of the first higher Rayleigh mode at low frequency for cases with a large elastic contrast at shallow depth (increase in Vs approaching 300% at 5 m). If the displacement is treated as the fundamental mode, a systematic overestimate of deeper shear wave velocity will arise in the inversion, since higher modes have higher phase velocity. A second result is from a profile with a very thin, surficial low-velocity layer, where higher modes dominating the response might prevent fundamental mode detection and inversion. Finally, if shear wave velocity of the half-space exceeds the compressional wave velocity of an overlying layer, quasi-compressional waves (or guided P-waves) come into the velocity range of normal modes. This may lead to surface wave mode-misidentification.
Radiocarbon dating is the method most frequently used to date Holocene deltaic sequences, but less than one quarter of 14C dates are within +/-500 years of predicted age. Such dates tend to be unreliable, in other words, often too old and commonly inverted upsection, and core sample dates obtained near deltaic plain surfaces may be as old as mid- to late Holocene. Stratigraphic irregularities result primarily from downslope reworking of upland alluvial sediment, with displacement of "old carbon" in the sediment that accumulates in lower valleys and deltaic plains. Use of dates that are too old results in inaccurately calculated rates (most often too low) of relative sea-level rise and/or land subsidence. More reliable timing of deltaic sediment requires a multiple-method dating approach, including, where possible, identification of associated archaeological material. Developing an accurate dating strategy is a critical step for implementing reliable coastal protection measures needed for the rapidly increasing human populations in these low-lying, vulnerable nearshore settings.
An extensive S-wave velocity survey had been carried out in the frame of a recent seismic microzonation study of Basel and the border areas between Switzerland, France and Germany. The aim was to better constrain the seismic amplification potential of the surface layers. The survey included single station (H/V spectral ratios) and ambient vibration array measurements carried out by the Swiss team, as well as active S-wave velocity measurements performed by the German and French partners. This paper is focused on the application of the array technique, which consists in recording ambient vibrations with a number of seismological stations. Several practical aspects related to the field measurements are outlined. The signal processing aims to determine the dispersion curves of surface waves contained in the ambient vibrations. The inversion of the dispersion curve provides a 1-D S-wave velocity model for the investigated site down to a depth related to the size of the array. Since the size of arrays is theoretically not limited, arrays are known to be well adapted for investigations in deep sediment basins, such as the Upper Rhine Graben including the area of the city of Basel. In this region, 27 array measurements with varying station configurations have been carried out to determine the S-wave velocity properties of the geological layers down to a depth of 100-250 m. For eight sites, the outputs of the array measurements have been compared with the results of the other investigations using active sources, the spectral analysis of surface waves (SASW) and S-wave reflection seismics. Borehole information available for a few sites could be used to calibrate the geophysical measurements. By this comparison, the advantages and disadvantages of the array method and the other techniques are outlined with regard to the effectiveness of the methods and the required investigation depth. The dispersion curves measured with the arrays and the SASW technique were also combined and inverted simultaneously to use the advantages of both methods. Finally, the paper outlines and discusses the contribution of the S-wave velocity survey to the new seismic microzonation of the Basel region. In this regard one major outcome of the survey is the quantification of vertical and lateral changes of the S-wave velocity, due to changing lithology or changing compaction and degree of weathering of the layers.
ABSTRACT The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging,and character- izing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode,Rayleigh wave acquired,by stepping out a pair of receivers at inter- vals based on calculated ground,roll wavelengths. Inter- ference by coherent,source-generated,noise inhibits the reliability of shear-wave velocities determined,through inversion of the whole wave,field. Among,these nonpla- nar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates,the dis- persion curve and, ultimately, the inverted shear-wave velocity profile is dependent,on frequency,as well as dis- tance from the source. Multichannel,recording,permits,effective identifica- tion and isolation of noise according,to distinctive trace- to-trace coherency,in arrival time and,amplitude. An added,advantage,is the speed,and,redundancy,of the measurement,process. Decomposition,of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency,component,in a unique,and continuous,format. Coherent,noise contamination,can then be examined,and its effects appraised,in both fre- quency,and offset space. Separation of frequency,com- ponents,permits real-time maximization,of the S/N ratio during acquisition and subsequent,processing steps. Linear separation of each ground,roll frequency,com- ponent,allows calculation of phase velocities by simply
SUMMARY Near-surface seismic characterization (100 or 200 m) has an important role in hydrogeological and natural hazards studies among others. P-wave seismic methods can provide useful information in those cases where high-frequency signals (>100 Hz) can be generated, transmitted and registered. Favourable near-surface conditions in soft sediments areas are a combination of shallow water table and fine-grained sediments (clay, silt). This paper presents an update of seismic data acquired by the Unit of Geology of the Cartographic Institute of Catalonia. These data have been acquired in different fluvial-deltaic areas (Tordera, Llobregat). First, we will characterize the acquired seismic waves and link the seismic noise with parameters such as: water-table depth, sediment grain size and gas presence. Secondly, we will present some strategies to improve signal-to-noise ratio during the processing. Last, we will present how shear-wave acquisition can improve the subsurface knowledge in areas where P-wave soundings show poor resolution.
A matrix formalism developed by W. T. Thomson is used to obtain the phase velocity dispersion equations for elastic surface waves of Rayleigh and Love type on multilayered solid media. The method is used to compute phase and group velocities of Rayleigh waves for two assumed three-layer models and one two-layer model of the earth's crust in the continents. The computed group velocity curves are compared with published values of the group velocities at various frequencies of Rayleigh waves over continental paths. The scatter of the observed values is larger than the difference between the three computed curves. It is believed that not all of this scatter is due to observational errors, but probably represents a real horizontal heterogeneity of the continental crusts.
As methods for dynamic characteristics estimation of surface layers, investigation of boreholes and a method which employs microtremors are well known. Borehole investigation, one of the most accurate methods, is costly and time-consuming and is not available all the time. The method that employs microtremors is handy but has not produced satisfactory results to this day. This paper describes a new processing method that employs microtremor observations yet produces accurate estimates of the characteristics of the ground motion. The method uses a vertical component and horizontal components. As a result, the spectrum ratio of the horizontal components and the vertical component of the microtremors bears a resemblance to the transfer function for the horizontal motion of the surface layers.
As local site effects have a drastic influence on seismic hazard, it is a major issue to characterize them in vulnerable areas such as highly urbanized zones, like Barcelona city. The aim of this work is to improve the knowledge of geophysical characteristics of Barcelona in the perspective of a seismic microzonation that takes into account site effect. The first step was to gather the existing data from geological, geotechnical, geophysical, and seismological investigations, bringing us to keep the four zones proposed by previous work as a base of zonation. The second step was to characterize each zone by time-averaged shear-wave velocity and fundamental resonance frequency, with ambient noise techniques over 17 sites, providing new knowledge about the soil of Barcelona. The third step was to propose an amplification function between an average soil for each zone and a standard reference rock site, using empirically based propositions and to compare them to previous numerical approaches.
The Messinian Salinity Crisis (MSC) drastically affected the physiography, morphology and sedimentation all along the Mediterranean area at the end of the Miocene. This paper presents an analysis of the effects of the MSC on the Catalan continental margin, based on a broad database of seismic reflection profiles from the oil industry and academy. We study the sedimentary processes and their controlling factors and the correlation with the nearby and well-known Gulf of Lions. Because of the complexity of the MSC, we define the Margin Erosion Surface/Top Erosion Surface (MES/TES) as the surface marking the end of the Messinian event, which allows differentiating three domains along the Catalan margin: the Creus, Girona and Barcelona domains, separated by the Palamos and Blanes canyons. The Creus domain shows an important structural control on the MES/TES physiography, while the Girona and Barcelona domains are characterized by wide and flat MSC platforms below the present-day shelf. The entire margin developed a series of complex drainage systems as the result of the Messinian sea level drop. The location of the main valleys (Cap de Creus paleo-drainage system, Palmas and Blanes paleo-canyons and the downslope-trending valleys on the Barcelona domain) was directly controlled by the structural framework. In contrast, the Barcelona paleo-drainage system and the tributaries of the Blanes and Palamos systems were excavated by direct subaerial erosion by meteoric rain in the basins limited by basement blocks, and were afterwards totally filled by the Pliocene-Quaternary sedimentation. Regarding the MSC depositional features, the major part of the eroded sediment was deposited as detrital deposits on the foot of the slope. The distribution of detrital bodies and evaporites was also structurally controlled. A Zanclean sea level rise occurring in at least two phases is confirmed by this study, in correlation with interpretations from the Gulf of Lions.
Very high-resolution reflection seismic investigations off the Belgian coast have revealed an extensive area marked by poor seismic penetration that is probably caused by the presence of shallow gas. The gas is believed to be of biogenic origin, and its geographical distribution is bound to a wide band oriented more or less parallel to the coast. The main origin of the gas could to some extent be linked to the presence of a shallow, thin peat-rich layer of Late Pleistocene/ Early Holocene age. Local high sedimentation rates furthermore favoured gas formation in the shallow fine-grained Holocene sediments. The gas-related features observed on the seismic profiles include acoustic turbidity and blanking, strong multiple reflections, and to a lesser extent bright spots and phase reversal. The sea-floor morphology does not reveal any clear gas escape from the sea bed, although there are some indications of local seepage of small bubbles or dissolved gas into the water column. The top of the acoustically turbid layer is located between 0 and 7 m below the sea-bed surface. It generally forms a sharp boundary, often marked by a varying offset probably due to different levels of gas penetration which could be related to the lithology of the overlying sediments. Seismic characteristics and velocity data seem to suggest a low concentration of gas, most likely less than 1%.