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25th European Meeting of Environmental and Engineering Geophysics
- Conference date: September 8-12, 2019
- Location: The Hague, Netherlands
- Published: 08 September 2019
1 - 50 of 141 results
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Monitoring of Subrosion Induced Mass Changes by Time-Lapse Gravity Surveys — Two Case Studies from Germany
Authors G. Gabriel, M. Kobe, A. Weise and L. TimmenSummarySubrosion is a geological process, which occurs worldwide. In extreme cases, it leads to the development of collapse sinkholes, which can pose a severe hazard, especially in urban areas. Hence, concepts are required to monitor the processes, which are related to subrosion. In two case studies we show the feasibility and the success of an approach that combines repeated levelling and time-lapse gravity surveys in the subrosion-prone urban areas of Bad Frankenhausen and Hamburg-Flottbek in Germany. Over several years of observation, in which quarterly measurement campaigns were carried out, we found both evidence of ongoing subsidence and mass redistribution in the subsurface. Especially for the known sinkhole areas, a decrease of gravity in the order of up to 2 µGal.a-1 suggests ongoing underground leaching and mass loss at both locations. In the context of data processing, we successfully applied a correction of the effects of varying soil water content on the adjusted gravity differences using the global model GLDAS Noah.
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3D Inversion of Gravity Data Using Graph Theory; Application of the Method on Mobrun Ore Body
Authors S. Soodmand Niri, S. Vatankhah, V. Ebrahimzadeh Ardestani and R. Anne RenautSummaryThe inversion of gravitational data due to the ill-posed nature of the problem is of importance. To cope with this problem and better estimation of the physical parameters, some kinds of regularization and optimization approaches have been proposed. However, providing an integrated method for inversion is demanding. In this study, we performed a 3D inversion methodology based on graph theory which delineates the skeleton of a subsurface body. Our approach is based on the equidistance function stabilizer with genetic algorithm optimization. Also, we introduced a weighting term in data misfit function using noise level estimation. The algorithm includes an effective method for selecting the regularization parameter. Finally, A MATLAB package for 3D inversion of gravity data using the graph theory approach is developed during this study.
We performed some numerical examples for both synthetic and real data. The results indicate applicability and high performance of the proposed approach.
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2D Inversion of Time Domain Induced Polarization Data Observed on the Black Shales Near Bramsche/Germany
By B. TezkanSummaryInduced polarization measurements in the time domain (TDIP) are carried out near Bramsche/ Germany. The study area is well known for a lateral IP and resistivity anomaly caused by a thermally metamorphosed layer of Jurassic clays which marks the almost vertical Lias/Dogger boundary. Two kilometers westward of the survey area, the anomaly is geophysically well explored using radiomagnetotelluric (RMT) and DC-resistivity. The possible east continuation of the anomaly should be investigated with the new IP measurements.
The IP measurements were realized with the ABEM Terrameter LS device. In total, six parallel profiles with a length of 202.5 m and an electrode spacing of 2.5 m were investigated. A gradient array with a separate cable spread was chosen to eliminate electromagnetic disturbances by capacitive coupling between the potential and current multielectrode cables.
The data were inverted by using a newly developed 2D DC/IP inversion algorithm (Adrian and Tezkan, 2017) The algorithm uses finite elements and unstructured meshes. A clear lateral limited low resistive (5 Qm) and high chargeable (180 mV/V) anomaly was detected at about 15 m depth beneath all six profiles as a result of the 2D inversion indicating a good corrrelation with the geology.
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Non-Standard Responses in Time-Domain Induced Polarization Measurements
Authors G. Fiandaca, P. Olsson, P.K. Maurya, A. Kühl, T.S. Bording, T. Dahlin and E. AukenSummaryToday, negative induced polarization (IP) time-domain responses and responses with non-standard shapes are generally considered as measurement errors and removed in data processing before inversion. However, synthetic and field measurements show that these responses are in fact physically possible and that a simple theoretical explanation of the basic mechanism for their origin can be found by means of superposition of contributions from regions with different sensitivities. The basic mechanism for IP responses with non-standard shapes is investigated by considering the subsurface Cole-Cole parameter sensitivities and time varying IP potential for 2D synthetic models. The time-domain forward response and sensitivities are computed from the finite-element solutions of the frequency-domain differential equation, through a time transformation that takes the current waveform into account. This approach allows for quantitative unbiased estimates of the time-domain responses and sensitivities, which are different from the estimates that can be obtained when using multiple DC forward computations, as often done in the inversion of time-domain IP data. With the increased understanding, previously disregarded IP responses, which can contain valuable information of the subsurface, can be kept for the inversion process and thus contribute to the final parameter distribution.
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The Inversion of Vector Array Data Sets for 3-D Resistivity and I.P. Surveys
Authors M.H. Loke, J. Gance, C. Truffert and O. LeiteSummaryThree-dimensional resistivity and I.P surveys are widely used in mineral exploration to map the conductive minerals associated with base and precious metals deposits. Several field survey strategies have been designed to reduce the survey cost such as the offset pole-dipole array. Another method is the vector array where triplets of potential electrodes at placed at each station. Two measurements that are approximately at right angles are made of the electric field at each station. In some cases, one potential dipole is almost on an equipotential line. This results in an array configuration with a low measured potential value which is sensitive to noise, and in some cases a negative apparent resistivity value. The logarithm of the apparent resistivity is commonly used as the data parameter in inversion algorithms as the values in a data set can vary by several orders of magnitude. However, this is not possible if negative values are present. Instead of treating the two vector measurements as separate data points, they can be combined into amplitude and direction values. The amplitude is always positive and is less sensitive to noise compared to the individual components. This results in a more stable inversion algorithm.
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Resistivity and Induced Polarization (ERT/IP) Survey for Bedrock Mapping in Permafrost, Svalbard
Authors S. Bazin, A. Lysdahl, A. Olaus Harstad and R. FrauenfelderSummaryPermafrost demands for special requirements when designing infrastructure and for this, it often needs to be characterized in details. Ground-based Electrical Resistivity Tomography (ERT) provides an indirect near-surface indication of permafrost down to a few tens of meters depth, when carefully calibrated by boreholes. While ERT is very successful for mapping the active layer (i.e., the layer which thaws and refreezes annually), further interpretation of resistivity profiles is impeded by the lack of resistivity contrast within the permafrost. Indeed, the lithological structures are hidden by the strong resistivity of the frozen layer. We present an example combining ERT with Induced Polarization (IP) surveying which allows to uncover the interface between the sediments and the bedrock. Although IP is rarely acquired on permafrost, the present case study shows the benefit of measuring it simultaneous to the ERT survey, at no extra costs or time consumption. The study is located near the cities of Longyearbyen and Barentsburg, on the island of Spitsbergen, Svalbard. Eight profiles were carried out in September 2018, when expected active layer thicknesses were at their maxima. Two-dimensional inversion was performed with the commercial software RES2DINV for the resistivity data and Ahrusinv for the chargeability data.
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Vs Data Space Retrieval and Inversion for Surface NMR
Authors L. Liu, D. Grombacher, G. Osterman, E. Auken and J.J. LarsenSummaryEnvelope detection is an integral step of the surface NMR data processing workflow to estimate these parameters. We present a new method for retrieving NMR signal envelopes using spectral analysis and the subsequent inversion scheme with the new data space. By exploiting the fact that the spectral magnitude at the Larmor frequency is proportional to the product of the initial amplitude and relaxation time, a high-SNR complex envelope can be extracted by Fourier transform for a number of sliding windows. However, SA estimated envelopes are weighted by the NMR relaxation time during processing and the envelopes have units of volt-seconds. We propose to modify the surface NMR forward model such that it predicts data directly in the voltage-time data space. Field data inversions are presented to demonstrate advantages of pairing the SA envelope detection scheme with a forward model that works in the voltage-time data space.
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Apsu - A New Surface NMR Instrument for Hydrogeophysics
Authors J.J. Larsen, L. Liu, D. Grombacher, G. Osterman and E. AukenSummarySurface nuclear magnetic resonance (NMR) holds a special place among the tools used for hydrogeophysical measurements as it provides a direct depth-resolved measurement of the water volume in the shallow surface. Further, the NMR data also provides information on the pore-space properties of the subsurface, which informs about hydraulic properties. The method has been successfully applied in many scenarios, but it is still challenged by e.g. low production rates and low signal-to-noise ratios in many places of interest. In an effort to address these challenges, we present a new surface NMR instrument named Apsu. The instrument is designed modularly, and it consists of a transmitter system, a wirelessly connected receiver system using small coils, and a control system. The transmitter is powered by a generator and drives an un-tuned transmit coil with a peak current of 105 A. The frequency and amplitude of each half-oscillation of the transmit pulse are independently controlled giving full flexibility in NMR pulse design. The system has a quasi-zero dead time and a 30 m depth of investigation. The system performance is verified with field example.
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Surface Nuclear Magnetic Resonance Measurements in Berlin - Proof-Of-Concept for Applying the Prepolarisation Technique in Urban Areas
Authors S. Costabel, T. Hiller, T. Radic, R. Dlugosch and M. Müller-PetkeSummaryMeasurements of surface nuclear magnetic resonance (NMR) combined with a prepolarisation previous to the conventional spin excitation are expected to enable non-invasive soil moisture investigations even in urban areas with high environmental noise. To test this expectation, we conducted prepolarised surface NMR using a very small coil (figure-of-eight coil with a diameter of 0.5 m) on a water-filled basin in the city area of Berlin. Using an additional prepolarisation coil with a diameter of 2 m, the NMR signal amplitudes could be amplified by factors up to ten. These NMR signals from depths up to 1 m could be measured and quantified despite the high urban noise level in contrast to those without prepolarisation. Significant Earth’s magnetic field heterogeneities due to the reinforcement of a near-by building yielded a shortened relaxation time for the bulk water (about 40 ms) and correspondingly a broad distribution of Larmor frequencies. We conclude that for the general applicability of surface NMR in urban areas, field heterogeneity and corresponding off-resonance effects must be considered in the forward operator. Our future research will combine prepolarisation with adiabatic spin excitation and will include the exact spin dynamics in the forward calculation.
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The Influence of Non-Adiabatic Switch-Off on the Prepolarization of Surface Nuclear Magnetic Resonance Measurements
Authors T. Hiller, R. Dlugosch, M. Müller-Petke and S. CostabelSummarySince several years, surface nuclear magnetic resonance (SNMR) is a well-established method for the hydrogeological characterization of the subsurface up to depths of 150m. When used with small surface loops of only a few square meters, SNMR suffers from low signal-to-noise ratios. This limits the methods applicability in urban areas or areas with low water content (vadose zone). Recently, the SNMR method has been extended by the application of prepolarization (PP) pulses prior to the classical spin excitation, to amplify the measured response signal. Depending on the applied PP-current, the amplification can reach up to two orders of magnitude very close to the PP-loop. This theoretically achievable amplification crucially depends on the assumption of perfect adiabatic switch-off of the corresponding PP-pulse. To study the effect of non-adiabatic switch-off, which is most likely always the case in practical applications, we implement the full spin dynamics simulation of the PP switch-off into the SNMR forward modelling. We show, that depending on the shape and duration of the PP-switch-off ramp, the resulting sounding curves can have decreased signal amplitudes of up to 45%. Neglecting this effect would yield an underestimation of the corresponding subsurface water content of similar magnitude.
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Seismic Risk Assessment Through Spatial Integration of Seismic Hazard and Social Vulnerability in South Korea
More LessSummaryIn Korea, where the frequency of earthquakes was low, mid - magnitude earthquake recently occurred and damaged. Accordingly, in this study, seismic risk assessment study was performed to find vulnerable areas of earthquake in South Korea. We defined a seismic risk model that means integration of seismic hazard and social vulnerability. The basis of seismic risk assessment model was constructed by examining spatial analysis and statistical methods. Characteristics of the study area were also considered. This was applied to assess the vulnerable areas of earthquake damage in South Korea qualitatively. The results of the study was suggested as a seismic risk map that can identify vulnerable areas in South Korea at a glance.
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Reflection Polarity Estimation of GPR Data for Under-Road Cavity Detection
More LessSummaryWhen we locate subsurface cavities using the ground penetrating radar (GPR) method, the estimation of reflection polarity is required to distinguish cavity-induced reflections from the other reflections. In general, we investigate the polarity of reflections through the visual inspection of GPR profile. However, reflections are often overlapped with other reflections and noise, which makes this visual inspection unreliable. As a method to quantitatively estimate the reflection polarity, the power spectrum difference method was proposed, but it sometimes fails to estimate the polarity of reflections for shallow reflectors. In this study, we analyze this limitation of the conventional polarity estimation method and propose a modified approach that can reliably estimate the polarity of reflections even for shallow reflectors. Through the synthetic and field examples, we demonstrate that our approach effectively estimates the polarity of reflections, which eventually enables us to select potential cavity-induced reflections among many reflections on GPR profile.
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Attributes of 2D GPR Data to Characterize Eolian-Fluvial Interaction Deposits
Authors P. Zabala Medina, N. Bonomo, A.M. Osella, S. Salvo Bernárdez and O. LimarinoSummaryGPR is one of the geophysical methods most used to explore and characterize the shallow surface, in particular, to study eolian and fluvial deposits in sandy environments. A usual prospecting strategy is to acquire longitudinal profiles and transects, with the goal of determining the geometry of the structures along and through a determined vertical plane, which is often parallel to the predominant wind direction of a given period. Normally, the data are acquired by using the reflection mode and the constant offset configuration, and then processed through standard procedures. With this methodology, detailed images of the reflectors in the soil can be obtained, from which the interpretation is performed. A complementary practice, which has been little used in the area of GPR, is to calculate attributes of the data. The main objectives of using attributes are to reveal and quantify different properties of the reflection patterns that improve its interpretation. In this work, we analyze different attributes of the GPR data sections, to investigate present eolian-fluvial interaction deposits. In particular, we show that attributes as the rms frequency, apparent dip, curvature and parallelism produce information that is useful to differentiate similar sedimentary units and characterize them in detail.
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Searching for the Roman Port of Emporiae Using Frequency Domain Electromagnetic Induction Method
Authors A. Casas, M. Himi, R. Lovera, L. Rivero, A. Urruela, A. Sendrós and R. GarcíaSummaryIn this paper we present the results of a geophysical survey conducted using frequency domain electromagnetics (FDEM). The geophysical survey is part of a wider archaeological research project designed to obtain conclusive evidences about the location the Roman harbour expected to be buried under alluvial sediments in the bay close to the remains from Greek and Roman times.
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Multimethod 3D Geophysical Survey of a Monument - The Bell Tower of Batalha Abbey
Authors M.J. Senos Matias, F. Almeida, R. Moura and N. BarracaSummaryBatalha Abbey is a 14th century UNESCO world heritage site that shows signs of decay. During the last years, high resolution geophysical methods have been used to contribute to the knowledge of its construction characteristics and to an informed maintenance and rehabilitation project. Here in it is presented a multimethod high-resolution geophysical investigation of its main tower. A 3D resistivity survey was carried out on the surface around the tower to investigate the ground beneath it. A GPR survey was used on the tower walls surface to investigate its interior. Three frequencies, 250MHz, 500MHz and 800MHz, were used. Finally, a seismic tomography study was done around the tower with both geophones and sources on the tower walls to provide a 3D velocity image of the tower interior.
3D resistivity results give a clear image of the walls foundations and of the ground beneath the tower. GPR 250MHz data provide a complete GPR image across the tower, although of low resolution. Higher resolution GPR results provided clearer information on the constructive elements of the tower. Finally, the seismic tomography results gave, for the first time, a complete image of the tower interior and proved it a compact construction with no voids.
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Application of GPR for the Reconstruction of the Ancient Copper Quarry in the Trans-Urals, Russia
Authors V. Noskevich, N. Fedorova and A. YuminovSummaryGeophysical studies were conducted on the ancient quarry of the Bronze Age “Novonikolaevsky” in theTrans-Urals, Russia. The purpose of these studies was to investigate the possibility of determining the relief of bedrock rocks buried under a thick layer of sedimentary rocks and processed soils according to GPR data, to build a model of an ancient quarry using these data, as well as to estimate the amount of ore and copper mined. The survey was conducted by SIR-3000 with a 400 MHz antenna in 16 parallel profiles. A description of the method of survey and interpretation of the results was given. A ditch was dug in the quarry and geological sections along the western and eastern walls were made. The obtained GPR sections were compared with geological data. The results of the excavation at the quarry correspond well to the boundaries of the reflections of electromagnetic waves from the bottom of the ancient quarry. A three-dimensional model of the quarry was built during its operation. Based on the model obtained, an estimate was made of the amount of ore mined. The results allowed establishing that in the Bronze Age, 2.5 tons rough copper could be smelted from mined ore.
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GPR Data Background Removal Using DWT
Authors S. Ilkhanipour and A. GoudarziSummaryGPR is one of the most regular applications for shallow explorations. In this study based on UDWT and f-x AR modeling, we tried to reduce the noise of real and synthetic sections. Several studies have been presented in this literature; Nuzzo et al, (2004) studied the attenuation of noise in the DWT domain that is compared with the results of the Radon transform and FT; They concluded that the discrete wavelet transform that provides high-quality results compared to Radon’s and the procedure is fast, in this domain the results are more accurate compared to the FT method.
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Preliminary Results of an Integrated Passive Seismic Survey at the Gerolekas Bauxite Mining Site — Central Greece
Authors K. Polychronopoulou, C. Orfanos, N. Martakis and D. DraganovSummaryAs the quest for mineral resources becomes more and more challenging and exploration geophysics is actively evolving, the mining industry is in a constant pursuit of more efficient and cost-effective exploration methodologies that would provide a means of overcoming exploration limitations of the past. In this context, an integrated passive seismic survey was designed and launched at the Gerolekas bauxite mining site, in Central Greece, in the scope of checking the potential of passive seismic methodologies as a mining exploration alternative.
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A Litho-Constrain Approach for Gravity Data Analysis at Gerolekas Bauxite Mining Area
Authors C. Orfanos, K. Leontarakis and G. ApostolopoulosSummaryDelphi-Distomon S.A., one of the largest bauxite producers in Greece and in Europe, is interested to explore for new bauxite deposits in a promising unexploited area. Although, getting a direct geophysical response from bauxite deposits in great depths is almost impossible, indirect ore indications may occur through geophysical observations associated to the bauxite host rocks or particular geological structures. In this study, a litho-constrained approach is proposed for the analysis of gravity data in the Gerolekas mining area. The specific strategy is starting from building a coarse 3D lithology model by exploiting all available geo-information and its conversion to an equivalent density one. The emerged density model can assist in the design phase of a gravity survey as well as in the processing and interpretation stages. Finally, a 3D litho-costrained inversion can be applied to real gravity measurements, in order to build a geologically realistic density model that contributes significant to the better understanding of the survey area’s geotectonical regime. The litho-constrained density model can also assist in the reprocessing of existing 2D legacy reflection seismic data and in the integration with other methods such as magnetotellurics and passive seismic that are in progress in the survey area.
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Seismic Exploration of Mineral Resources in Western Australia with Distribute Acoustic Sensing
Authors M. Urosevic, A. Bona, S. Ziramov, R. Pevzner, K. Tertyshnikov, R. Martin, J. Dwyer, D. Felding, C. Guarin and A. FoleySummaryDistributed Acoustic Sensing (DAS) technology is undergoing rapid developments in different areas of science and industry including exploration of natural resources. The primary reason is that DAS could bring a step change in the current application of seismic reflection method in the mineral sector. While the reflection seismic, in its current form of application is useful for mineral exploration it is often considered prohibitively expensive to use over the mining lease or to use as a primary exploration tool, alongside magnetic and gravity surveys.
Here we present recent developments in the application of DAS in the surface reflection mode for imaging in hard rock environments. One clear case is the use of DAS for reflection imaging over numerous salt lakes, particularly in Western Australia (WA), where vast mineral resources are yet to be characterised and produced. The use of conventional equipment in hypersaline environments is limited and prone to hardware issues, equipment damage and significant material losses. In contrast, DAS technology is resistant to the affects of a hypersaline environment. We show two very encouraging results obtained over two different salt lakes of WA. Subsequently this methodology was extended to the hard ground with the overwhelming initial results.
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Mine Bench-Tunnel Seismic Data Acquisition for Characterizing Shear Zones in the Siilinjärvi Phosphate Mine, Finland
Authors A. Donczew, A. Malehmir, E. Koivisto, M. Savolainen and B. BrodicSummarySiilinjärvi in central Finland, is currently the only phosphate mine with significant phosphorus production in western Europe. Phosphate rock and phosphorus are critical for the EU because of their supply risks and economic importance. The phosphate bearing rocks in Siilinjärvi are within a major Archean alkaline and carbonatite complex deformed and intruded by several shear and dyke systems. By understanding their spatial and temporal relationships an improved exploration and extraction of this critical ore would be possible. The objective of the study was to employ a novel seismic survey using the existing mine infrastructure for better planning and geological understanding. An innovative in-tunnel seismic survey was conducted in the Siilinjärvi open-pit mine in October 2018. A water-drainage tunnel nearly in the bottom of the pit intersecting two major shear zones was used to enable bench-tunnel seismic data acquisition. High-quality data were acquired using 144 receivers inside the tunnel, with the sources located both inside the tunnel (Bobcat-mounted vertical drophammer) and on the surface (combined explosives and Bobcat). Two reflections interpreted to originate from subvertical shear zones intersecting the tunnel were observed illustrating the importance of such surveys for shear-zone imaging and site characterization.
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A Geophysical Approach to the Phenomenon of Ground Fissures at the East Thessaly Basin (Greece)
Authors J. Alexopoulos, S. Dilalos, G.S. Mitsika, E. Vassilakis and S.E. PoulosSummaryThe disastrous phenomenon of aseismic ground fissures along the eastern Thessaly basin has deteriorated since 1989. The main reason for these fissures is the over-pumping, which leads to differential vertical compaction of the aquifer system and subsidence on the land surface. In this paper, we present the results of a combined geophysical survey applied in the area (VES and TEM soundings), in order to investigate the subsurface geological conditions. The geological regime of the area is comprised of the alpine basement, transgression formations and finally a package of post-alpine deposits with respectful thickness. A peudo-3D representation of resistivity maps for several depths of investigation was produced. Additionally, a dataset of deep boreholes was used for the calibration of the geophysical data. All the borehole and sounding interpreted data were grouped into three categories in order to produce the Lithology Model of the area. The alpine bedrock was adumbrated only at the southeast and central part of the basin, where we do not observe ground fissures. The absence of alpine bedrock for depths up to 300 meters, the thick and coarse-grained deposits and finally the over-pumping seem to contribute to the creation of the ground fissures.
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Sensitivity and Uncertainty of Petrophysical Models to Predict Storage Properties in Weathered/Fractured Hard-Rock Aquifers
Authors J.A. Mézquita González and J. ComteSummaryWeathered/fractured hardrock aquifers are complex, heterogeneous groundwater systems with overall low storage. This work develops a methodology using computer coding to estimate hardrock aquifers’ storage properties applying Archie (AR) and Waxman & Smits (WS) petrophysical models, and further assess their sensitivity and uncertainty. The data consists in both surface and borehole hydrogeophysical measurements in a micaschist aquifer in Ireland. In this case study, WS appears as most suitable because it allows to account for the significant clay content present in the subsurface as a result bedrock weathering. This property is not accounted for in AR, which therefore largely overestimates the pore space. An important finding is the high sensitivity of both models to the cementation factor. WS models are in addition highly sensitive to the clay properties, namely the cation exchange capacity. Uncertainty analyses determined the higher uncertainty in the deep, high resistivity, massive bedrock, with WS being the most affected. Availability and accuracy of spatial data on the cementation factor and clay properties are key to achieve realistic storage models with high confidence.
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A Multipurpose Platform Towed TEM-System Imagining the Top 100m of the Subsurface; Surface, Water and Snow
More LessSummaryWe present new developments of the versatile towed transient electromagnetic system (tTEM). The system now meets the demand of near surface mapping down to 100m and is suitable for application not only ground-based but also in aquatic (rivers, lakes etc.) and snow environments. The receiver technology of the system has been upgraded and it has a 4 times lower noise level compared to previous system, increasing the depth of investigation down to 100 m. The latter two system are called FloaTEM and SnowTEM, respectively. FloaTEM surveys were conducted in several parts of United States including the Mississippi delta region for mapping and characterizing the alluvial aquifer system below the river beds. The results show that the FloaTEM system produces detailed subsurface resistivity images in quick time compared to other geophysical methods. We conducted over 300 km of survey along several rivers within a week in the Mississippi delta region. The SnowTEM system uses a 4 turn, 4 m x 4 m transmitter coil and a specially designed low noise receiver coil. It has been put on skies and is towed behind a snow scooter or a snow robot.
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Early-Time and Late-Time Limitations on the Performance of Near-Surface TEM Measuring Systems
Authors N. Kozhevnikov and M. SharlovSummaryReducing the exploration depth of the TEM sounding method by measurement at earlier times necessitates decreasing size of the transmitter loop. This causes problems in measuring TEM response at both early and late times. The early time problem is associated with fundamental constraint caused predominantly by the transmitter loop inertia. As the transmitter loop decreases, its intrinsic response decreases much slower compared to that of the earth. At late times, instead of measuring the earth’s transient response one measures the receiving unit’s intrinsic response. Its initial amplitude is small, but it decreases so slowly that, eventually, its contribution to the total transient response becomes predominant. This problem is technical. It can be solved by eliminating or reducing the impact of the voltage pulse induced in the receiver loop during the current turn-off in the transmitter loop.
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Application of Time-Domain Electromagnetic Survey for Seafloor Polymetallic Sulphides in the Okinawa Trough
Authors K. Nakayama, M. Moroori and A. SaitoSummaryJOGMEC (Japan Oil, Gas and Metals National Corporation) vigorously promotes bathymetric survey, geochemical exploration, geophysical exploration and drilling to evaluate the amount of the resources of the seafloor polymetallic sulphides in Japanese EEZ. Until today several promising ore deposits have been discovered in the Okinawa Trough and Izu-Ogasawara area, and electromagnetic surveys were conducted in several areas. In this presentation, we will introduce some of the WISTEM (Waseda Integrated Seafloor TEM) survey results in the Okinawa Trough in 2018.
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A 3D Inversion Algorithm Using a Triple Mesh Approach and Domain-Decomposition for Fast Computations
Authors K.W. Engebretsen, B. Zhang, G. Fiandaca and E. AukenSummaryWith the computer power available today full 3D inversions of transient electromagnetic data (TEM) is no longer a dream of the past. Although it is possible to perform these inversions, the problems still scale in three dimensions making large datasets slow to invert. We here propose a new triple mesh method for inverting TEM datasets with multiple transmitters and multiple receivers per transmitter. The code is relative fast and with a manageable memory consumption. In this new approach we show that by using a decoupled regular structured model mesh and two finite element forward meshes, one with a coarse discretization and one with a fine discretization, we get a substantial speed up in calculations times without sacrificing much in terms of how well we fit the data. We show that we can invert large datasets by decomposing our domain and applying this triple mesh method on each domain separately.
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Geophysical Monitoring of Redox Influenced Degradation Near the Water Table
Authors P.M. Fernandez, E. Bloem, H.K. French and A. BinleySummarySoil contamination of organic origin is a major environmental challenge. One of the most common reactions in nature is the electron exchange between chemical species, also known as a reduction-oxidation (redox) reaction. Redox reactions drive organic matter degradation in the ground. Monitoring these reactions is therefore needed to protect soil and groundwater. This work aims to provide non-invasive monitoring methods to study redox state changes that occur in the near-surface interface, at the boundary between aerobic and anaerobic conditions, as well as a potential remediation solution to attenuate the negative impact of anaerobic condition.
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Geophysical Monitoring of Initiated In-Situ Bioremediation of Chlorinated Solvent Contamination
Authors A. Nivorlis, T. Dahlin and M. RossiSummaryContaminated ground is a severe problem which is present in many countries, and it is of great importance to find efficient solutions to deal with it. In Sweden, there is a trend in the industry to move towards alternate remediation methods and on that scope in-situ bioremediation has received a lot of attention. The main challenge with in-situ bioremediation is to verify that the treatment has the intended effect, and it is hence important to understand the changes that happen in the subsurface and monitor them over time. The scope of our work is to use geophysics to extrapolate the punctual information from groundwater and soil samples and acquire a better understanding of the changes in the subsurface during in-situ bioremediation of the contaminated soil. In this work we have made a permanent installation in one of our field sites, in Alingsås, where a pilot bioremediation test is taking place to treat the contaminated soil. The autonomous system can measure the resistivity and chargeability distribution in the subsurface. The results indicate promising correlations with the geophysical signature and the contaminant. Currently we are investigating the correlations between the groundwater samples and the time-lapse imaging acquired while working towards real-time monitoring.
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Mapping Sediment Contamination with a Gamma-Ray Spectrometer, Underwater and on Land
Authors R. Koomans, Van der Veeke, P. Jacobs and H. LimburgSummaryEnvironmental contamination due to human activity is recognized as one of the modern dangers of our industrial advancements. The current approach to the management and monitoring of contaminated aquatic and non-aquatic sediments involves invasive, often labor-intensive methods. The method is time-consuming and costly and the resulting contours of the contaminant distribution are subject to a number of potential errors, with the potential for ‘hot spots’ to remain unmapped. At present, there are no means for the passive, in-situ mapping of contaminants in aquatic and non-aquatic sediments.
Our method addresses the main problems associated with the current approach. We propose a method that provide rapid, spatially detailed data on contaminant distributions and concentrations in sediments by using a gamma-ray spectrometer. A gamma-ray spectrometer can be used for mapping aquatic sediments (by using a vessel as platform) and non-aquatic sediments as floodplains by using a quad-bike or drone as platform for mapping.
The study shows how these gamma-ray spectrometers have been used to map the contamination of a river basin of the Elbe tributary in Germany. The combination of all platforms result in information on contaminant levels of a river system in aquatic and non-aquatic sediments.
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An Integrated Methodology for Hydrogeological Assessment around Industrial Installations
Authors M. Claprood, C. Couegnas, E. Gloaguen, M. Krimissa and D. ParadisSummaryWe present a nested hydrogeological characterization methodology to optimize the use of existing data and better plan the acquisition of new data around man-made installations. The workflow is presented at an industrial site where the construction of deep infrastructures has disturbed the local hydrogeology settings. The first step is to lever historical data coming from hydrogeological tests and civil engineering operations before and during the construction of the industrial installations to build the frame of hydrogeological model. Based on the review of this information, new geophysical data acquisition can be scheduled to refine the interfaces between geological units. This initial model serves has a training image to simulate multiple equiprobable scenarios of the site geology while preserving the well information and the location of the buildings as, obviously, deterministic. These geological scenarios are populated with anisotropic hydraulic conductivity fields using sequential Gaussian simulation. These heterogeneous hydraulic conductivity models are ran with a flow and transport simulation algorithm to constitute an ensemble of realizations that is used in an ensemble Kalman time series assimilation scheme.
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Processing 2D ERT Data in 3D Environment - A Case Study Inside a Karstic Cave in Greece
Authors I. Fikos, G. Vargemezis, C. Pennos, B. Lønøy, K. Jensen and J. TverangerSummaryThe present work deals with the application of ERT method inside a karstic cave in Macedonia, North Greece. The cave geometry is incorporated as a priory information to 3D inversion software and the results are evaluated with forward modelling of simple realistic 2 layer models. The effect of the resistive walls and ceiling of the cave is investigated and the artefacts are discussed. Finally, the results of the 3D inversion are compared with the results of a typical 2D inversion of the same data, ignoring the 3D environment of the cave.
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ERT and SP Measurements for the Characterization of Fault-Controlled Soil CO2 Degassing
Authors C. De Paola, R. Di Maio and E. PiegariSummaryDue to the significant impact that high atmospheric CO2 concentrations can have on human and ecosystem health, a growing interest is focused on the characterization of non-volcanic CO2 degassing. The natural release of CO2 is often controlled by faults and fracture systems that favor the formation of permeable channels allowing gas migration toward the surface. In this framework, geophysical methods represent very useful investigation tools as they not only are able to identify focused gas release, but also to provide an estimate of the extent of influence areas of CO2 degassing and their preferential ascent pathways. To detect fault-controlled soil CO2 degassing, electrical resistivity and self-potential measurements were performed in a survey area located to the south of Matese Ridge (southern Apennines, Italy), where very high gas emissions are observed. Due to the nature of the investigated soils, preferential pathways associated with CO2 flux are found as resistive channels and negative self-potential anomalies.
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A New Approach to Incorporate Prior Information in MGS Inversion of ERT/IP Data
Authors R. Thibaut, T. Kremer, A. Royen, F. Nguyen and T. HermansSummaryThe current paradigm in geophysical inversions is to compute the simplest solution according to Occam's principle. The implicit assumption is that the parameters of interest have a smooth spatial distribution, which is rarely geologically plausible. An alternative is the Minimum Gradient Support (MGS), a functional that allows to compute a regularized inversion favoring sharp contrasts. Its use is currently mostly restricted to research studies and the MGS solution is highly sensitive to the selection of a variable called the focusing parameter ß. There is still no consensus on its optimal choice. To propose a methodology for applying this functional to real applications, the MGS is first used on ERT/IP synthetic data mimicking a real case study. For complex geometries, a smooth solution is first computed and used as a starting model for sharp MGS inversions. Including prior information on resistivity from drillings further improves chargeability features. The developed methodology is then applied to ERT/IP data collected on a gold deposit. The method allows different interpretations about the mineralization key properties, and seems more indicated to compute plausible electrical resistivity spatial distributions regarding the extensive prior geological knowledge. The choice of ß is challenging and should be automated in future developments.
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A Homotopy Optimization Method for Non-Liner Inversion of Geoelectrical Sounding Data
Authors Z. Esmaeili, R. Ghanati and M.K. HafiziSummaryIn nonlinear inversion of geophysical data, bad initial approximation of the model parameters usually leads to local convergence of the normal Newton iteration methods, despite enforcing constraints on the physical properties. To mitigate this problem, we present a globally convergent Homotopy continuation algorithm to solve the nonlinear least squares problem through a path-tracking strategy in model space. The global convergence of the Homotopy algorithm is compared with a conventional iterative method through the synthetic and real 1-D resistivity data. Furthermore, a bootstrap-based uncertainty analysis is provided to quantify the error in the inverted models derived from the case study. The results of blocky inversion demonstrate that the proposed optimization method outperforms the Marquardt-type algorithm in the sense of the stability and the recovered models.
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Time-Lapse Target-Oriented Crosswell Full Waveform Inversion without Downhole Sources
More LessSummaryTime-lapse seismics has a wide range of application in different scales, from near-surface to resource exploration. Crosshole sesmics is used to characterize fluid reservoirs and to obtain highly resolved rock/soil-dynamic parameters e.g., elastic moduli and Poisson ratio. Developments in distributed acoustic sensing shows the potential of deploying permanent downhole receivers at low costs. In order to achieve an efficient and accurate time-lapse seismic measurement in such scenarios, we have developed a nonlinear waveform inversion to reconstruct velocity structure between boreholes using VSP data with source located only at the surface, and no downhole sources. The new approach formulates the forward modelling using wavefield representation theorem, which enables directly estimating the velocity structure by minimizing data residuals and calculating the gradient from the adjoint state problem. We test the approach using numerical modelling of time-lapse VSP data to detect layer-specific temporal changes. A heterogeneous shallow vadose zone represents a low-velocity layer. The results show that the new approach provides more stable and more accurate temporal velocity profiles than conventional full waveform inversion, when the initial velocity model does not include the shallow low-velocity layer. The new approach is robust and highly advantageous as it does not require downhole seismic sources.
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High-Resolution Imaging of the Shallow Subsurface with Elastic Full Waveform Inversion of Surface and Body Waves
Authors A. Adwani, M. Danilouchkine, A. Soni, R. Plessix, F. Ten Kroode, S. Abri, F. Ernst and F. AlshukailiSummaryIn this abstract, we discuss an elastic full waveform inversion approach to estimate high resolution shallow subsurface elastic parameters from both surface and body waves in onshore seismic data. We have used a spacetime domain weighting function to balance the amplitudes of the dominant surface waves with respect to other body waves. This allows us to simultaneous extract information from the surface waves and from the refracted and reflected/converted body waves. The different wave types are complimentary in terms of depth and wavenumber information. The concept and results are illustrated using a complex 2D synthetic example representing a typical Middle-east geology.
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Compact Full Waveform Inversion by Model Space Re-Parameterization
Authors L. Xu, E. Manukyan and H. MaurerSummaryFull waveform inversion is an ill-posed optimization problem. To address this issue, we propose a novel compact full waveform inversion (CFWI) scheme. For that purpose, we re-parameterize the problem in an alternative model space, which is the result of two transformations, namely a 2D Fourier transformation (Hartley transform), followed by a wavelet transformation (Haar transform). In this model space, the waveform inversion problem can be appropriately parameterized using only a small number of model parameters. As a result, we can improve the robustness of FWI. We demonstrate this with a simple crosshole example, where we obtain excellent results while reducing the number of model parameters by 98%. Besides this considerable reduction of model parameters, CFWI offers new opportunities to analyze the spatial resolution, which can be extremely valuable for optimized experimental design purposes.
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Shallow-Seismic Wavefield Scattering and Implications for Viscoelastic FWI
Authors N. Athanasopoulos and T. BohlenSummaryThe aim of this study is to investigate the seismic response from shallow structures using a 2D finite-difference method. Full-waveform inversion (FWI) suffers from so-called cross-talk between viscoelastic parameters, i.e. certain combinations of viscoelastic parameters have the same scattering signature. We evaluate the scattering response from individual perturbations in the viscoelastic material parameters. By subtracting a perturbed from a background medium and solving the viscoelastic wave equation we obtain the scattering wavefield. Additionally, we evaluate the instantaneous energy density of P- and S-waves to better understand the influence of scattering. This allows us to provide some fundamental insights into the composition of seismic recordings and ideas of how to mitigate cross-talk in shallow-seismic applications of FWI. Specifically, we find that the P-wave velocity structure is constrained similarly by Rayleigh, P- and S-waves. The S-wave velocity is the most reliable parameter to invert for from FWI of surface waves due to their large amplitudes. The scattering pattern of density differs fundamentally from those of velocities and attenuation. Finally, cross-talk between attenuation and velocity for both P- and S-waves is caused by the similarity of their corresponding scattering responses.
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A Workflow Allowing Multiple-Point Statistics Realizations to Reveal Where We Lack Information
Authors M.L. Gulbrandsen, N. Jensen and T.M. HansenSummaryIn recent years, the importance of representing the uncertainty of any earth model has met more understanding. One way of doing this is by generating a series of realizations of a statistical representation of the earth model instead of just one, in some way, 'optimal' model. Modelling the subsurface in a probabilistic way like this, allow for any kind of statistical questions to be answered. These questions are often very problem-specific, however, in this study we present a methodology combining multiple-point statistical modeling and information theory allowing to quantify the information content everywhere in an Earth model, which allow us to ask the question: Where do we lack information?
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Identification and Recognition of Landmine Internal Structure Scattering Contribution from GPR Data
Authors F. Lombardi, M. Lualdi, F. Picetti and P. BestaginiSummaryThe aim of the study was to quantify the potential increase in the information level produced by an increase in the data dimensionality, i.e. from analysing a 1D signature to the investigation of a 3D GPR volume. The experimental campaign was carried out employing two different neutralised landmines, characterised by a different internal structure and buried in controlled conditions. Obviously, the acquisition of a single monodimensional signature of the target has the advantage of being almost effortless, but shows significant limitations in achieving adequate performance, in particular for landmines showing an irregular internal structure. This is a consequence of the impossibility of effectively separating the different scattering contribution. As well, despite producing a clearer and more intuitive image of the target, a single 2D profile is not able to provide reliable performance, hence there is little benefit in acquiring a 2D profile as it still suffers from not producing unambiguous results. The analysis of a 3D volume, instead, allows for an accurate delineation of the internal structure of the target, providing a reliable solution to the complex target design critical issue.
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Tree Inspection by GPR Reflection and Transmission Measurements
Authors K.Takahashi Takahashi, K. Aoike, K.Kajino Kajino, Y. Ashiba, K.Kaneko Kaneko and N. IshizawaSummaryTree trunk inspection has been conducted to assess the risk of the collapse and maintain the safety of pedestrians. Conventional methods often involve invasive measurements that may cause further damages to trees. Radar can be used as a tool to non-invasively image the inside structure of tree trunk for the inspection. We developed a workflow of the inspection that combines reflection and transmission measurements. It makes inspection work efficient; the quick assessment by transmission measurement and detailed inspection by reflection imaging. In addition the permittivity of tree trunk, which is required for reflection imaging, can be estimated from the transmission measurement data. Consequently, we can improve the quality of reflection imaging by using the estimated permittivity. In the present paper, our workflow for the tree inspection by radar is illustrated and demonstrated with data collected for old trees in a shrine.
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Integrating Resistivity and GPR Data for Plant Root Study in Indoor Agricultural Environments
Authors A.G. Gerea, A.E. Mihai and P. AtkinsSummaryThe use of two geophysical methods is presented, the electrical resistivity tomography and GPR. Integrating these methods in agricultural environments posses significant advantages in water and fertilizer consumption representing a very fast and useful tool in the area of precision agriculture as well. The results are promising, achieving a level of precision by implementing various IoT (internet of things) devices and adaptations of the method and data processing.
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The Use of Resistivity to Complement GPR Surveys on Tree Roots in Urban Areas
More LessSummaryUrban areas are not usually favourable sites for geophysical surveys. However, GPR has shown promise in different environments, particularly in the mapping of coarse roots. Given its functionality on multiple urban surfaces, speed of data acquisition, and overall suitability for tree root detection, we find that in most scenarios, GPR is the most well-suited geophysical method for tree root detection. GPR is not a panacea, though. It has fundamental limitations when it comes to tree root detection , and given the variability of environmental parameters in urban areas, as well as the numerous sources of noise, the extent to which GPR can detect tree roots (and thinner roots in particular) remains site-specific and not fully clear.
In this case, resistivity surveys and ERT in particular can serve as a complementary or potentially even self-sufficient method of detecting tree roots.
Here, we use four different antennas covering a broad range of central frequencies (250 MHz, 750 MHz, 1.5 GHz, and 4 GHz) to showcase the potential and limitations of GPR in this scenario, as well as a hybrid between a commercial resistivity meter and lab-developed equipment to implement both conventional and non-conventional arrays.
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Recording Longer for Higher-Resolution Near-Surface Imaging - Shear-Wave Reflections from Vertical Sources and Receivers
By A. MalehmirSummaryMode-converted reflections have long been recognized in field seismic data and have been used for improved imaging and targeting. However, their use for near-surface imaging and characterization have only been limited to dedicated SH-SH data acquisition setups. In a quick-clay landslide prone area in southwest Sweden, collocated SH-SH using 1 m source and receiver spacing, and a conventional P-wave line using 2 m source and receiver spacing were separately acquired using different instruments and setups. Both datasets have so far been the subject of several studies, however, only recently after recovering longer records of the P-wave data, clear hyperbolic events with slow moveouts were observed at 0.5 second and later times. Thanks to the long record and available SH-SH dataset, these reflections are interpreted to have pure shear-wave origin. Confirmed by downhole logging data and core observations and to some extent other independent geophysical datasets, the shear-wave reflection imaging of the perceived vertical component data, allowed two sets of coarse-grained layer materials and extremely undulating bedrock to be resolved. The P-wave reflection imaging of the dataset did not provide such a high-resolution image as obtained by the shear-wave reflection imaging given the strong source-generated noise at the very early arrivals.
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High-Resolution Seismic Imaging Using a 2C Land Streamer System
Authors T. Szabþ, T. Toth, Z. Hamori, V. Németh and P. FilipszkiSummaryLand streamers are often used for seismic surveys in urban areas and paved surfaces, where conventional spike geophones can not be planted.
With development of shear-wave seismic methods, land streamers recording S-component also gained more importance. Due to their low velocity, shear-waves give us the opportunity to investigate the shallow (0-20 m) subsurface with high resolution, using the reflection seismic method. Worldwide published examples of land streamer surveys and a novel P- and S-wave vibroseis source for engineering applications initiated the development of a modular 2-C land streamer system. Our goal was to develop a seismic land streamer which enables fast data acquisition on varying surfaces for different types of data processing, such as multichannel analysis of surface waves, turning ray tomography, reflection and refraction processing.
Several surveys were carried out with the newly developed land streamer in noisy urban areas, on paved surfaces in order to investigate the applicability of the instrument. P and SH waves were generated simultaneously and the recorded data were processed with different methods Real data example will be shown demonstrating the data quality and productivity of the developed 2-C land streamer.
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Comparison of Site H/V Spectral Ratio and Predominant Frequency by Using Microtremor and Earthquake Data
Authors K.H. Coban and N.Sayil SayilSummaryThe determination of the predominant frequency and H/V ratio is an essential part of dynamic site characteristics. In the present study, we aim to evaluate the site effects of the Marmara region by calculating horizontal-vertical spectral (H/V) ratio and predominant frequency from microtremors and earthquakes data. We selected 5 earthquakes (M ≤ 4.5), which occurred in the Marmara region and recorded by 14 broadband seismometer stations. Also, we determined 1-hour microtremor records according to earthquakes date and time. The H/V ratio and predominant frequency were calculated by Nakamura method with Geopsy program and Seaseme criteria. Besides, the same parameter was estimated from S-waves and Coda waves of selected earthquakes. The results were compared with each other. According to the results, the H/V ratio and predominant frequency values vary according to the direction of earthquakes. In some stations, the microtremors and earthquakes results give close results. However, for reliable H/V ratios and predominant frequency values, we think that earthquake records and microtremors records should be compared. The study results will contribute to the Marmara region seismic risk assessment.
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Seismic Imaging Using Electromagnetic Vibrators - Storm versus Lightning
Authors B. Brodic, R. De Kunder, P. Ras, J. Van den Berg and A. MalehmirSummarySeismic imaging using two electromagnetic (EM) vibrators (E-vibes) based on linear synchronous motor principle was evaluated via a seismic survey in central Netherlands. The sources, Storm and Lightning, weigh 1650 kg and 90 kg, respectively. Storm is a vertical-type vibrator with full drive frequency 2-200 Hz and peak force of 7 kN. Depending on the orientation, Lightning can be used either as P- or S-wave vibrator with full drive frequency 8-400 Hz and peak forces of 1.7 kN (S-wave) and 1.2 kN (P-wave). Data were acquired using a 100-unit-long three-component microelectro-mechanical (MEMS-based) seismic landstreamer and 253 wireless seismic nodes. We analyze and compare data recorded using both sources and vertical components of different seismic receivers (MEMS versus geophones), along with different acquisition strategies in terms of signal quality, maximum penetration depth and offsets. Shot gathers and stacked sections indicate great potential of both sources for broadband (2-200 Hz) seismic imaging with excellent quality data from 50 ms to 1.6 s (~2000 m depth) using the Storm and comparable, but of lower quality and resolution, using the Lightning source. Bellow 1.6 s, coherent events are absent on the Lightning data while the Storm shows weaker events down to 2.2 s.
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Detection of Near-Surface Heterogeneities at Archaeological Sites Using Seismic Diffractions
Authors J. Liu, Q.Bourgeois Bourgeois, R. Ghose and D. DraganovSummaryWe develop a new approach to locate very shallow subsurface objects using seismic diffractions of low signal-to-noise ratio. In our approach we use the diffraction arrivals recorded from the subsurface objects. To image the objects, we apply spatial instantaneous-phase-coherency summation along diffraction hyperbolae. We demonstrate the performance of our method using synthetic and field data.
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Integrated Analysis of Geophysical Data Using a Data Mining Approach
Authors K. Sabor, D. Jougnot, R. Guerin, L. Apffel and B. SteckSummaryA data mining algorithm was tested in order to jointly interpret electrical tomography and seismic refraction profiles. As a first step, the geophysical profiles used are numerical ones before applying this approach to real measurements data in further studies.
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