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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 - 100 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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An Optimal Combination of Geophysical Methods to Image Geological Structures
Authors B. Owoc, A. Marciniak, S. Kowalczyk, J. Dzierzek and M. MajdanskiSummaryNowadays seismic methods are commonly used in geological research because they give a clear image of the structure at various depths. The resolution of the seismic image depends on the amount of data. Therefore to maximize the use of recorded seismic data the combination of a few seismic methods is proposed. In this paper, an optimal combination of geophysical methods is suggested to study the near-surface geological problems. A standard reflection seismic imaging is supported by multi-channel analysis of surface waves, first breaks traveltime tomography and Ground Penetrating Radar. All these methods are cost-efficient and easy to perform. Using all recorded wavefield, high-quality images are obtained down to 200 m with small additional processing. The Mesozoic bedrock is clearly visible in the seismic imaging as a very strong reflector. Its depression is probably connected to erosion or structure (syncline). Under this horizon, it is possible to find one or two detachments related to the nearby fault. Moreover, the field work was an opportunity to test in-house modifications to seismic source and GPS based timing device, and both systems worked correctly.
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Enhancing Time-Lapse Geoelectrical Models of Soil Moisture Dynamics with a-priori X-Ray CT Derived Information
Authors M. Cimpoiasu, O. Kuras, T. Pridmore and S. MooneySummaryDeveloping a better understanding of soil hydraulic properties is of significant importance for such diverse fields as agriculture, soil and ecosystems management, civil engineering and geotechnics. Electrical Resistivity Tomography (ERT) and X-ray Computed Tomography (CT) are two state-of-the-art methodologies with great potential for applications in soil science. ERT allows time-lapse monitoring of solute transport. X-ray CT is sensitive to bulk density changes at high resolution.
We monitored the infiltration of a KCl solution through a partly deionized water saturated undisturbed cylindrical soil column using ERT. Prior the infiltration, we scanned our sample and segmented the pore architecture out of the resulting X-ray radiograms. Based on pore characteristics such as size and connectivity, we split the pore volume into two distinct zones, percolating and non-percolating pores. Afterwards, we reconstructed non-percolating pore features within the ERT model mesh. By comparing the ERT inversion result, with or without the pore architectural information, we noticed a sensible improvement in how the electrical model is able to represent the fluid flow path. This result sets the scene for a new joint methodology, which constrains the geoelectrical interpretation of the subsoil moisture with soil structural information and its contribution to solution infiltration.
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Co-Operative Inversion of Multi-Channel Analysis of Surface Waves and Electrical Resistivity Tomography Using Fuzzy Clustering Approach
More LessSummarySeismic and electrical resistivity tomography methods are recognized as powerful tools to investigate landslides. The effectiveness of the investigation significantly increases if we can exploit the strength of the each method and complement this information in a combinative model. In this study, we performed co-operative inversion of two datasets (multi-channel analysis of surface waves and electrical resistivity tomography) via a fuzzy clustering technique. Our results demonstrate the robustness of our routine. In our single data inversion process, this method can construct models that include zones relating similar parameters. A conventional inversion process uses smoothing criteria to find a final solution, but this approach cannot image a subsurface with highly variable physical properties due to faults and groundwater seeps present in our survey area. The co-operative inversion of the seismic and geo-electrical data exploits the advantages of the both methods, to build a better model than models inverted from the single datasets. Our inversion model provides a more reasonable subsurface model than conventional inversion results. Finally, our process can provide the clustering image, namely pseudo-lithology, from the co-operative inversion models of the two methods. This makes interpretation much easier than using the inverted models.
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Identifying Sedimentary Facies in Mediterranean Ephemeral Channels (MEC) by Using of Electrical Resistivity Tomography
SummaryThe effects of Climate Change and the intensification of land use on the territory show the importance of Mediterranean ephemeral channels. The morphological adjustments of this type of system are mainly controlled by the magnitude and frequency of hydrological events.
The primary objective of this case of study is to analyse the morphological and systemic adjustments produced in the MECs, as a consequence of Climate Change and new environmental scenarios. To analyse these effects, a small semi-arid Mediterranean basin located in the Southeastern Iberian Peninsula, has been chosen. The analysis of geomorphological evolution carried out in several narrowing of this basin has been made from the identification of the structure and sedimentary facies associated with the geoformations of the fluvial bed. For each cross-section of the bed chosen as the control section, geoelectric profiles will be made to be correlated with stratigraphic data of shallow depth mechanical boreholes, from which the tomographies can be calibrated. The use of both techniques will allow knowing the texture and sedimentary structure of the bed, which will help to detect changes in the processes of erosion, transport and sedimentation produced in recent periods, as well as determine the depth of the rocky substrate.
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Geometrical Characterisation of a Marine Intrusion by Utilising Electrical Resistivity Tomography
Authors M.D. Vásconez-Maza, M.A. Martínez-Segura and M.C. Garcîa-NietoSummaryIn semiarid zones, agricultural activity exerts much pressure on the aquifers. Every year much water is utilised in crops; wells are overexploiting the aquifers which do not have enough recuperation time. In this study, the “Minor Sea” baths all the coast of the overexploited aquifer what is promoting the intrusion of the seawater in the continent. The principal aim of the paper is to characterise the underground layers of an agricultural terrain located near to the cost and determine the geometry of the marine intrusion by using Electrical Resistivity Tomography and boreholes.
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Electrical Resistivity Tomography (ERT) Applied to the Characterization of Fractured and Weathered Zones in Subsurface Granites
Authors R. Garcia, M. Himi, L. Rivero, L. Camarero, A. Urruela, R. Lovera, A. Sendrós and A. CasasSummaryTwo ERT campaigns were carried out to characterize the subsurface geology and the possible infiltration and flow pathways of the water through the granites (fractured and weathered zones) from the Barranc de Contraix area (Aiguestortes i Estany de Sant Maurici National Park, Catalonia, NE Iberian Peninsula). Three types of geoelectric levels were defined: high resistivity levels (>15.000 Ω-m), medium resistivity levels (between 6.000 and 15.000 Q-m) and low resistivity levels (<6.000 Ω-m). The ERT profiles have been shown highly resolutive to identify the existence of these fractured and/or weathered zones on the granites, represented by relatively low resistivities in comparison with the high resistivities associated to fresh granites. This fractured or weathered zones on the granites may allow the flow of water from the high mountain lakes Estanys Gelats (>2700 m.a.s.l.) to the actual fluvial system (1900 m.a.s.l.).
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Numerical Estimation of Geoelectrical Data Errors -Comparison with the Method of Reciprocal Measurements
Authors P. Martinelli, V. Grunhut, P. Bordon, F.E. Robledo, M.E. Franzoni and A.M. OsellaSummaryIt is well-known that considering appropriate estimations of geoelectrical data errors improves inversion results, especially in the presence of data with atypical values. In these cases, other option is to apply some procedure for removing the outliers from the datasets, and then perform inversions without considering data errors. However, these data are not always easily identifiable.
In this work, we present four numerical methods for estimating data errors, alternative to reciprocal measurements. They are based on the analysis of the differences among the data corresponding to neighboring measurement points. They are computationally cheap, and present two important advantages: they do not increase survey times because they do not require any additional measurements, and they can be used with any electrode configuration. Using a representative experimental example, we demonstrate their effectiveness for reducing the effects of outlier data on the inversion results. The subsoil models obtained considering the proposed error estimations are, in general, similar to that obtained considering the errors estimated from direct and reciprocal measurements. These models are also in agreement with the results obtained without considering data errors, after removing the outlier data.
The formulations presented are 2D, but can easily be generalized to three dimensions.
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Modular Resistivity Device for Physical Model Studies
Authors M. De la Vega, V. Bongiovanni and A.M. OsellaSummaryA programmable automated resistivity device was designed and constructed using an open-source electronic platform. In this work we present a low power version of the device that was implemented for physical model laboratory studies.The low power version of the device for physical model measurements use a 24V battery system to generate the A-B current flow. A resistance of 560Ω limits the current flow to less than 40mA. We present results obtained in a simple physical model to verify the performance of the automated resistivity device developed. We program the electrodes positions to make in line dipole-dipole, Schlumberger-Wenner, and Y11n configuration surveys. The tomography obtained from the D-D and S-W data show an excellent agreement with the physical model, the convergence is of 1% for these configurations. Y11n configuration give a convergence of 12%.
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Simultaneous Inversion of Multi-Channel IP Data with Structural Simliarities
More LessSummaryTime-domain spectral induced polarization data can be inverted by using the concept of four dimensional (4-D) inversion method. In this approach, the entire multi-channel data set is simultaneously inverted, where the apparent chargeability and model chargeability at entire time-channels are composed into a single data and model vectors in the space and channel domains. Regularizations not only in the space domain but also in the channel domain is applied to reflect the characteristic of chargeability curves. Furthermore, we introduce the concept of structural similarity between resistivity and chargeability models by using minimal cross-gradient constraints. Developed inversion algorithm was successfully applied to several simple subsurface models to obtain reasonable chargeability distribution. Through the structural simlarity constraint, more reasonable IP data distribution can be obtained since noisy IP data can resort to resistivity distribution which more robust and stable due to higher S/N ratio.
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Theoretical Fundaments to Obtain Complex Permittivity and Resistivity in CR Method
Authors A. Lucuy, M.V. Bongiovanni, V. Grünhut and A.M. OsellaSummaryThe resistive capacitive contact (CR) method is a non-intrusive geophysical technique that allows to characterize the subsoil, by means of electrical images. There are several models that interpret how the electromagnetic signal propagates in terms of the conductivity and polarization of the charges under the action of an electric field. In general, most of the works focus on the phenomenon of polarization. When a time-varying electric field is applied on a surface with a sufficiently high frequency for there to be a considerable relaxation time, not only a complex susceptibility but also a complex conductivity are generated and therefore, a complex resistivity. The purpose of this work is to analyze the behavior of the electromagnetic signal on a soil, with relatively high frequencies, and to link the complex conductivity and resistivity through the tangent of losses, as a function of a scale factor and a phase shift between the emitted and received signal. Through an adequate description of the physical principles involved, a theoretical formulation will be developed. It could be obtain through experimentally measuring the amplitudes and the phase shift, both the usual information of the apparent resistivity of the subsoil, and also information about its apparent permittivity.
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Combined GPR and ERT Survey at the Marathon (Plasi) Archaeological Excavation Site
Authors J.D. Alexopoulos, N. Voulgaris, S. Dilalos, N. Souglakos, G.S. Mitsika and Y. PapadatosSummaryThe excavation trenches by the Department of History and Archaeology of the NKUA at the archaeological site of Plasi Marathon (Attica, Greece) have already revealed several architectural remains of the Classical period. A combined GPR and ERT survey was initiated in order to determine the existence and geometry of additional buried structures in the unexcavated parts of the site. The results of this geophysical survey will not only assist the archaeologist's planning for the next excavation steps, but will also provide valuable information for the density of habitation in the area, a significant factor for the understanding of the importance of ancient Marathon. Fifty-two GPR and eighteen ERT profiles were carried out in a selected area. After data processing, the obtained radargrams and resistivity tomograms are presented in the form of fence diagrams. Moreover, volume/depth slices have been extracted for specified depths from GPR and ERT in order to compare their results. Several identified geophysical anomalies can be interpreted as archaeological domestic remains, supporting a rather dense pattern of habitation, hence the archaeological significance of the site.
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High Resolution Electrical Resistivity Tomography Imaging of a Coastal Mine Tailings Deposit, Portman Bay, SE Spain
Authors A. Urruela, M. Canals, L. Rivero, M. Himi, A. Casas, R. Lovera, R. García, A. Sendrós, J. Frigola and D. AmblàsSummaryThe Portman bay (SE of Spain) was infilled with mineral tailing for 33 years by the mine industry that operated in the area. What was a clean crescent-shaped bay in the year 1957 had disappeared by 1990 under 60 Mt of dumped waste. In this study we propose the use of electrical resistivity tomography (ERT) for the characterization of the mineral tailings that infilled the bay.
Two field data acquisitions were performed obtaining regional and detailed ERT data. The combination of this acquired data with boreholes and historical aerial photography has allowed to generate a 3D integrated model of the mineral tailing infill. This detailed model suggests a 44% increment of the volume of the infill from previous studies. Furthermore, a dense ERT grid was acquired and yielded a meter-scale resolution model of the internal heterogeneities.
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Near Surface Seismic Reflection Survey - A Comparison of P- and S-waves Vibratory Seismic Sources
Authors F. Gallais, K. Becker, M. Le Feuvre, M. Verdurme and P. CôteSummaryThis paper describes the results of five seismic reflection surveys planned to evaluate the capacity of different vibratory sources to image a target interface in 40 m depth. Between 2015 and 2017, we acquired a 48-channels seismic profile that serves as a reference. During the different phases of acquisition, we used a sledgehammer and four different light vibratory sources, able to emit P- and/or -S-waves. Then, we compared the performance of the sources used in term of resolution, S/N ratio, optimum stacking number and repeatability. One of the vibratory source and the sledgehammer in S-wave mode show good performance and are particularly successful sources to give high-resolution seismic profile of the near surface.
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Seismic Imaging of the First 30-Meter Weathered Layer in a Karst Environment (H+, Poitiers)
Authors M. Dangeard and J.L. MariSummaryNear-surface seismic methods are mainly used to determine the geometrical characteristics of the hydrosystems. Recent methodological advances suggest the high potential of these methods to investigate the mechanical properties subsurface by exploiting the full wealth of seismic records. Indeed, with vertical component seismic data recorded we can extract: (i) P-wave first arrival times to provide P-wave velocity section; (ii) surface-wave dispersion to estimate local 1D VS model, and (iii) a reflectivity section starting from the surface. All this information can be combined in order to better characterize the subsurface providing hybrid seismic sections.
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Effects of Physical and Acquisition Parameters on Fundamental Dispersion Curve in MASW Method
Authors H. Karsli and M. ŞenkayaSummaryIn this study, the effect of physical and acquisition parameters on the continuity and resolution of fundamental dispersion curve (frequency-phase velocity) has been examined by modeling the synthetic seismograms in Multichannel Analysis of Surface Waves (MASW). The influences of decreasing S-wave velocity or increase in the thickness of the layer on the dispersion curve are observed rapidly as drop in lower frequency zone, while the effect of the increase in the velocity or decreasing of the thickness caused fundamental dispersion curve to shift to higher modes. For the effects of acquisition parameters (offset-XO, geophone interval-dx, number of geophones-N, spread length-L=(N-1)*dx), the fundamental dispersion curve was re-picked from seismograms synthetically modeled by using the simple harmonic summing technique. The tests show that the largest spreading length and offset must be used to obtain the dispersion curve in a widest frequency range and high quality. However, geophone interval for larger spreading should be selected by without wavenumber aliasing. Therefore, synthetic tests showed that if one wants to obtain continuity and resolution of fundamental dispersion curve when analyzing of field data, acquisition parameters may be selected as 1m≤dx≤2m, X0≥Zmax or X0=L for N<24 and 0.5m≤dx≤2m, X0≥L/5 or at least X0 4*dx.
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A Comparison of Multiple Attenuation Strategies for Shallow Water Ultra-High-Resolution (UHR) Seismic Reflection Data
More LessSummaryThe processing and interpretation of Ultra High Resolution (UHR, > 2k Hz) marine seismic data can be very challenging, especially in shallow waters (< 50m depth), where multiple reflections can overprint primaries in areas of interest, potentially creating ambiguities in the interpretation. In this paper, we compared the performance of five different multiple attenuation techniques (τ-p deconvolution, f-k demultiple, Radon demultiple, Surface Related Multiple Elimination (SRME), and τ-p deconv + SRME) on a very-shallow water (<15m depth) multichannel UHR seismic dataset. In this case, techniques independent of the moveout differences between primaries and multiples (τ-p deconvolution and SRME) attenuated multiples effectively, producing minor changes on primaries and without significant addition of noise. In contrast, f-k demultiple, Radon demultiple and the combined application of τ-p deconvolution and SRME were inefficient at removing multiples. SRME was identified as the most efficient technique and allowed for a significant improvement in the interpretability of the final seismic image.
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Wave-Equation Datuming Applied to Very High-Resolution Deep-Towed Multichannel Seismic Data
Authors F. Colin, S. Ker and B. MarssetSummaryUnderstanding geohazards in great water depths requires the use of remotely deployed technologies in order to obtain the vertical and lateral resolutions required to model the medium. Deep-towed multichannel seismic is a powerful tool that can provide such resolutions, however data it produces requires careful, dedicated, processing. During acquisition of deep-towed seismic data, the vertical movements of the tow-fish cause vertical changes in the 120 m long streamer's shape. These variations have to be calculated to recover the acquisition geometry. This was solved by using travel times to invert for key parameters of the streamer's shape. Furthermore, unlike conventional surface-towed seismic, the sources and the receivers do not share the same depth datum which prevents the application of most conventional marine seismic processing algorithms. Wave-equation datuming (WED) is a method that allows moving the positions of sources and receivers to a common depth datum. This contribution presents how the WED method has been adapted to the characteristics of deep-towed multichannel seismic acquisitions. The application of this processing step to deep-towed seismic data allows the application of “conventional” algorithms, such as a NMO-stack-migration approach, thus facilitating the processing sequence of these unconventional seismic data.
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Rock Masses Characterisation Using High Frequency Piezoelectric Systems
Authors A. Butcher, A. Ridsdale and J. KendallSummarySeismic methods provide important in-situ measurements of the rock mass strength, which can be assessed through variations in seismic velocities. The frequency range in which seismic instruments operate dictates numerous key elements of a survey (e.g.detectability limits and resolution), and for engineering applications it is often advantageous to operate in kHz frequency range.
Piezoelectric sensors are rarely deployed in seismic surveys, but offer the potential for higher frequency content and more repeatable measurements compared with conventional instruments. This results in increased spatial resolution, and improvements in the stability of spectral measurements (e.g., spectral ratio methods).
To better understand some of the main characteristics of the piezoelectric system, we carry out a series of laboratory measurements on a range of different lithologies. Measurements were made to initially determine the frequency content on the system, followed by the response of the rock samples to these high frequency signal. We then present two crosshole datasets from ground excavations at Hinkley Point C, acquired using a piezoelectric seismic system and a conventional sparker system.
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Global Inversion of Pressure Dependent Acoustic Velocity Data Based on a New Double Relaxation Model
Authors J. Somogyine Molnar, T.E. Dobroka, T. Ormos and M. DobrokaSummaryIn this paper with the further development of our previously published single relaxation model (SRM), a new petrophysical model (the double relaxation model - DRM) describing the hysteretic pressure dependence of acoustic P wave velocity is presented. Since it is likely that several relaxation mechanism occur in the rock at the same time, the new model considers two or more physical mechanisms (e.g. the closure of pore volume or microcracks or friction on grain boundaries etc.) responsible for the pressure dependence of propagation velocity. Using the model equation as forward modeling formula, a global optimization algorithm (Simulated Annealing) was applied to solve the inversion problem and to determine the petrophysical parameters of the model. Inversion results proved that the calculated data matched accurately with measured data, so the new model was tested with success on laboratory measured data. On the other hand one can see that the new DRM model resulted in a more accurate fit compared to the SRM model.
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Landslide Investigation and Monitoring Using Self-Potential Methods
Authors C. Sujitapan, M. Kendall, J.S. Whiteley, J.E. Chambers and S. UhlemannSummaryLandslides are complex and heterogeneous phenomena involving properties of the subsurface. To study landslide processes, we use the self-potential (SP) method to investigate physical characteristics of the subsurface and monitor the accumulation and movement of groundwater associated with reactivated landslides at Hollin Hill, UK. The SP measurements consist of SP mapping in 2D and monitoring SP profiling. Six SP mappings are acquired in different climatic conditions. They show positive charges carried in the direction of the fluid flow, producing positive anomalies on the surface located at water discharge area and negative anomalies on infiltration area. The anomaly pattern reveals strong relationship between landslide morphology and flow regions particularly in the back scarp and secondary scarp. Moreover, the mappings are compared to precipitation data. They show the significant seasonal variations that SP signature possibly links to rainfall. However, it is still a bit unclear. Consequently, an analysis of the continuous SP monitoring are applied for high temporal resolution. The monitoring result shows that most of SP variations with time are linked to rainfall events and soil moisture contents. Consequently, the SP could be a possible method to monitor groundwater conditions and provides precursory information as an early warning of landslides.
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Detection of Capacitive Couplings in Ground-Based TEM Data with a 1D Convolutional Neural Network
Authors T. Frederiksen and J.J. LarsenSummaryTransient electromagnetics (TEM) is a well-established method for imaging of sub-surface resistivity structures. The method is often used from a moving platform providing densely sampled measurements of the sub-surface structures in 2D or 3D. When measurements are performed close to man-made conductors e.g. buried cables and pipes, the data will be contaminated by coupling artefacts, which must be culled from data sets prior to inversion. This can be a labor-intensive task for large data sets and several methods for automation of this process has been investigated. Here we explore the use of a convolutional neural network (CNN) for detection of couplings in data from a towed, ground-based TEM system. The CNN consists of two 1D convolutional layers followed by three fully connected layers. The proposed method is evaluated by comparing with a neural network based approach that have previously been used to detect couplings in airborne TEM data and is found to have better performance. The performance is presently limited by the size of the training data set.
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Airborne Electromagnetic and Radiometric Peat Mapping A Case Study from a Bog in Germany
Authors B. Siemon, M. Ibs-von Seht and S. FrankSummaryKnowledge on peat volumes of peatlands is essential to estimate carbon stocks accurately and to facilitate appropriate peatland management. This case study uses helicopter-borne electromagnetic and radiometric data to investigate a bog in Germany. Airborne methods provide an alternative to ground-based methods, which are labour intensive and unfeasible to capture large-scale spatial information.
One of the airborne surveys BGR conducted at the North Sea coast over the past two decades covers the Ahlen-Falkenberger Moor, an Atlantic peat bog (39 km2) investigated by the Geological Survey of Lower Saxony. This enables comparison of airborne and borehole results.
The lateral extent of the bog is derived from low radiometric and elevated surface data. The vertical extent results from smooth resistivity models in combination with a steepest gradient approach as well as from radiometric data. The latter requirs scaling of the relative depth values. Depths derived from electromagnetic data are usable for this scaling due to their similarity to borehole peat depths. The mean difference of the combination of electromagnetic and radiometric depths and the peat depths of about 100 boreholes is very small (-0.08 ± 1.09 m), but may differ significantly (>±2 m) at some points.
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Long Term Variation of Geomagnetic Curvature Recorded by Absolute Measurement in Different INTERMAGNET Observatories (Comprehensive Study)
By A. CsontosSummaryThe main task of the present day geomagnetic observatories is to record the elements of the geomagnetic field. This task is performed if the intensity and the direction of the geomagnetic field became known on one point. Standard observatory instrumentation gives further chances to measure the magnetic gradients and identify several changes of the magnetic gradient. These methods were tested before. In this paper we present the comprehensive study of three INTERMAGNET observatory’s long term dataset. In the focus of our data processing is the variation of a special magnetic gradient which is always observed during routine absolute magnetic measurements. As a cosequence we will study only the inclination readings of the complete set of absolute measurement. The difference between the two absolute values of inclination records will represents the curvature of the geomagnetic field measured in the magnetic meridian plane.
Long term high quality observatory dataset can provide a good basement to study the fluctuations when the observation is far away from strong conductivity anomalies.
It will be shown that we can find quiet periods when the studied difference shows high stability. On the other hand the disturbed periods also appear in every observatories.
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Analysis of Influence of the Data-Selection Procedure on 3-D MT Inversion Using Gyeongju Data in Korea
More LessSummary3-D MT inversion has been widely used to describe deep subsurface structures, but MT data are prone to be contaminated with noise. Although some pre-processing is done before inversion, the quality of some data can still be poor. To prevent noisy data from degrading inversion results, we may exclude noisy data in the inversion process. In this case, we need to consider the trade-off between the quality and quantity of data. We investigate the influence of the data-selection procedure on inversion results using AMT data acquired in Gyeongju, South Korea. Our inversion results show that the balance between the quality and quantity of data is important to obtain reasonable inversion results.
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On a New Method of Detecting Layer Boundaries at the Borehole Measurements by Gamma Logging
Authors S. Kataev, S. Kataeva and A. MillerSummaryA method for solving the problem of determining the boundaries of layers from logging measurements by gamma-ray logging, based on the method developed earlier by the authors to find the parameters of the incoming flow of events from observations of the moments of occurrence of flow events. The effectiveness of the method is demonstrated on the model laboratory experiment and on the real data. The applied algorithm allows to determine the initial option of splitting into layers in automatic mode. Clarification of the boundary layers is implemented by the interpreter in interactive mode
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Development of 3D Inversion Algorithm for Time-Domain Airborne Electromagnetic Data
Authors S. Jeong, M. Yi, A. Johmori and Y. SasakiSummaryRecently, airborne time-domain electromagnetic (ATEM) methods are gaining popularity for applications in mining, geotechnical, and environmental problems due to their efficiency in covering vast areas. To interpret ATEM data, 1D methods are commonly used because of their efficiency in handling large scale data. However, 1D methods yield an erroneous resistivity model when their basic assumptions are violated. In this study, we developed 3D inversion algorithm for ATEM data which is based on inverse Fourier transformation of frequency-domain responses and sensitivity. To efficient calculation of the multisource response and sensitivity, an edge-based finite element method with the direct solver is adopted. In addition, to reduce time for calculating sensitivity, a moving footprint approach was also used. In numerical experiment with simple 3D model containing conductive targets, subsurface model was successfully reconstructed.
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Saltwater Intrusion Monitoring with the SAMOS System as a Basis for Groundwater Management of Coastal Aquifers
Authors M. Ronczka, T. Günther, M. Grinat and H. WiederholdSummarySaltwater intrusions are a major problem for the freshwater supply in coastal regions. The project go-CAM is part of a research initiative to develop a platform for groundwater quality assessments. The main objective of the presented study is the monitoring of the freshwater/saltwater interface with the vertical electrode chain SAMOS in the vicinity of the North Sea. The first system was installed in December 2018 near Jever in the Sandelermöns region, where ongoing chloride monitoring detected an increasing chloride concentration at 50 m depth. ERT and HEM surveys provide an overview about the resistivity distribution in the investigated region and are used to find other SAMOS locations. The data are influenced by the drilling process indicating that the equilibrium is still not reached after four months of monitoring. An inversion procedure separates drilling effects from undisturbed conditions. First results show that the transition zone towards saline groundwater starts at approximately 40 m depth and reaches a minimum resistivity of 25 fim at 50 m depth.
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Incorporating Petrophysical Variability to Coupled Hydrogeophysical Modelling of DC Resistivity in Heterogeneous Coastal Aquifers
Authors A. Gonzalez Quiros and J. ComteSummaryGeophysical techniques are well suited to provide additional data for the parametrization of groundwater models. Joint use of geophysical and hydrological information through coupled modelling and inversion relies in an appropriate transfer function, the petrophysical model. Due to uncertainties in the spatial distribution of petrophysical model parameters, constant distributions are often used as a simplifying assumption, even when heterogeneity is present in the subsurface. When using this simplification, discrepancies can arise in the computed geophysical response. In this work, we show how spatially variable properties and simultaneous petrophysical models can be utilized in a coupled hydrogeophysical modelling. The developed coupled modelling relies in the definition of adequate transfer functions between models (hydrological and geophysical), providing direct correspondence between the spatial distribution of the hydrogeological and electrical properties. It has been observed that the effects of lithological heterogeneity can have as much importance as fluid salinity variations only. Therefore, when assuming constant petrophysical parameterizations geophysical results might be interpreted incorrectly. The understanding of these effects is a necessary step for a correct application of the petrophysical model in hydrogeophysical studies and shows the importance of petrophysical parameter variability when using coupled modelling methodologies for accurate imaging and quantification of saltwater intrusion.
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Investigating the Saturation State of Higher Strength Rock (HSR) by Geoelectrical Imaging at the Core Scale
Authors O. Kuras, T. Debouny, P. Wilkinson, L. Field, A. Milodowski, R. Metcalfe and S. NorrisSummaryThis study demonstrates how the capacitive resistivity imaging (CRI) technique can be used at the laboratory scale to derive a spatially resolved 3D model of water content in a core sample of Higher Strength Rock (HSR). Understanding groundwater flow and the transport and retardation of any radionuclides in fractured HSR is a key research aim associated with the geological disposal of radioactive waste. Retardation and incorporation of radionuclides from groundwater into the surrounding rock mass is important for the safety case for a geological disposal facility (GDF) in HSR. For a rock with dual porosity, such retardation and incorporation may be enhanced if the radionuclides are able to diffuse from fractures that support advection into effectively stagnant porewater within the rock matrix. This diffusion is termed Rock Matrix Diffusion (RMD). Improved non-destructive estimation and spatial mapping of water content in HSR under “fresh rock” conditions may offer new insights in RMD research.
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Visualising Groundwater Flow Using Time-Lapse Electrical Resistivity Tomography
Authors M. Karaoulis, W. Bakx, P. Doornenbal, P. Kruiver and S. RijpkemaSummaryWe performed Electrical Resistivity Tomography (ERT) measurements around a groundwater extraction well in a field of drinking water extraction wells. Oxygen enriched water is injected in a well during short periods as a purification step to remove iron from the groundwater. The goal of the pilot is testing the performance of ERT as a monitoring tool. Cross-hole ERT measurements were performed in a time-lapse mode to link changes in resistivity to groundwater flow expressed by variations in temperature and possibly oxygen content from the treatment. We show one example of the change in resistivity over a period of 16 hours. The changes in resistivity are small, in the order of 0.5 %, but proven to be above the noise level. When the time-lapse results are visualized in a movie (instead of just one snapshot), the patterns of change are consistent and can be linked to groundwater flow. We conclude that the ERT methods is suitable to be used as a monitoring method for groundwater flow. An additional future dataset will consist of temperature measurements using fibre optics Active Distributed Temperature Sensing. The next step will be to use the information from both methods to optimize well operation.
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Testing ERT and Fiber Optic Techniques at the Laboratory Scale to Monitor River Levees
Authors A. Hojat, D. Arosio, I. Di Luch, M. Ferrario, V. Ivov Ivanov, L. Longoni, A. Madaschi, M. Papini, G. Tresoldi and L. ZanziSummaryIn this paper we present the results of laboratory tests using electrical resistivity tomography (ERT) and fiber optic techniques to monitor river levees. A small-scale levee was constructed with the scale of 1:12 based on the earthen levee of an irrigation canal in San Giacomo delle Segnate, Italy, where a customized ERT monitoring system has been operating since September 2015. The most important mechanisms affecting the stability of river levees were simulated during different tests. To explore the possibility of using fiber optic sensors to monitor the integrity of levee structures, Fiber Bragg Grating (FBG) technology was adopted and four Organic Modified Ceramics (ORMOCER) coated 250μm-diameter fibers were deployed at the water exit side of two leakage zones. ERT data from a miniaturized electrode spread show that changes in the water level in river and the rainfall events significantly affect the resistivity distribution in the levee. Using TDR data, the relationship between water content and resistivity values was calibrated to be able to translate the inverted ERT images into water content maps. The results of fiber optic measurements showed the feasibility of fiber optic sensors to detect the deformations of the levee body in response to water infiltration.
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Incorporating Spatial Heterogeneity in Stability Analyses of an Unsaturated Soil Embankment - Extended Role of Integrated Geophysics
More LessSummaryExtreme climate events like frequent and heavy rainfall and extreme dry weather make slopes, dykes and embankments extremely vulnerable. The shallow subsoil in such cases are generally in unsaturated condition. In all common failure mechanisms of unsaturated slopes or embankments, the spatiotemporal distribution of water saturation and matric suction plays a crucial role. With input data from integrated geophysics, we have developed a new inversion scheme to invert in-situ soil-water characteristic curve (SWCC), linking saturation to suction. The new inversion uses total least squares to minimize the data-model residual simultaneously for water saturation, confining stress and smallstrain shear modulus. Applied to data obtained from integrated geophysics, the inversion offers quite accurate estimates for in-situ SWCC. For the first time, it has been possible to incorporate deterministically obtained heterogeneous SWCC to make reliable stability estimates for unsaturated slopes/embankments. For an embankment containing a clay layer underlying surficial silty-sand, the estimated factor of safety for Coulomb failure and internal erosion and piping can be dangerously overestimated if a mean value of SWCC is used, instead of a more accurate SWCC considering heterogeneity. For a clay-cored sand embankment, the mean value of SWCC can overestimate the safety factor.
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Levee Characterization by Means of Data Fusion of In-Situ Geophysical and Geotechnical Information
Authors T. Dezert, Y. Fargier, S. Palma-Lopes and P. CôteSummaryTo prevent catastrophic consequences imputed to levee breakage, the identification of the subsoil constitutive materials, as well as the detection of interfaces and anomalies are crucial. During investigation campaigns, complementary geophysical and geotechnical methods are usually used. These two sets of methods provide data with very different spatial scales, levels of incompleteness, uncertainty and inaccuracy. The effective combination of these two different types of data is challenging. In this work, we propose the fusion of geophysical and geotechnical data by means of Belief Functions, implementing Smets and Dempster-Shafer rules. We demonstrate our approach on a real case study, at Saint-Clément-des-Levées (France) including one geophysical method (electrical resistivity tomography) and two geotechnical ones (cone penetrometer and grain size analysis after drilling). This work also aims at supplying a better visual representation of a levee section with related degrees of belief. Our results highlight the ability of our fusion methodology to properly characterize the considered materials as well as to indicate the positions of the interfaces and the associated levels of confidence.
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Electrical Resistivity Tomography (ERT) Study in the Xochimilco District - Mapping the Subsidence in Southeastern Mexico City
SummaryThe district of Xochimilco is located towards the Southeast of Mexico City. The area is found within the transition zone between the basalts of the Chichinautzin formation and the lacustrine sediments of the ancient lake of Xochimilco. This paper presents the preliminary results of two electrical resistivity tomography (ERT) studies conducted in two different areas of the Xochimilco district (Mexico City). The first one is carried out in the hilly zone to identify the subsoil structures that are causing a severe affectation to the urban zone in the area, just over the transition between the igneous extrusive rocks of the Chichinautzin formation and the sediments of the ancient lake. The second study was carried out in the low-lands, where a sinkhole appeared in a pre-Hispanic canal of Xochimilco (Zacapa pier), which drained it completely.
The results obtained in both studies show the applicability of the ERT technique, and for making decisions regarding remediation. It was observed that both events are closely related to the superposition of two different phenomena, the presence of an abrupt transition zone between two materials of different mechanical properties (i.e. basalts and sediments) and the excessive groundwater extraction that exacerbates the phenomenon of subsidence.
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Delineation of Dead-End Structure of Thin Sand Layer by ERT and Cylinder Electrode CCR Survey
Authors C. Konishi, Y. Yamashita, K. Takahashi, D. Kurata and A. ShinseiSummary2D ERT and cylinder electrode capacitively coupled resistivity (CCR) survey has been conducted to detect deadend structure of a thin sand layer that may cause piping and a river levee collapse in the worst case. The survey lines were placed at an inland side of Tokoro river where a flood occurred by unprecedented heavy rainfall in 2016. We planted electrodes with 0.5 m spacing to increase the vertical resolution for ERT, and cylinder electrode, which is equivalent to 0.5 m dipole cable, was employed for the CCR survey. As a result, high resistivity thin layer was detected at 1 m depth, and dead-end structure was observed by 2D ERT and CCR survey. Soil samples were collected by auger drilling at several places around the observed dead-end point of the high resistivity layer and lateral variations of the soil were confirmed. It suggested the sand layer was terminated and the capability of ERT and the cylinder electrode CCR for delineating the thin sand layer was verified.
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Dynamical Changes of Foundation Displacements and Local Seismic Activity Occurred During Construction of Enguri Arch Dam
SummaryIn present research we aimed to analyze dynamical characteristics of high dam foundation displacement under the construction of large hydro power plant and during periodic changes of water in artificial lake behind the dam. We used data sets collected at international test area around Engury high dam from 1974 to 2013. We also analyzed changes occurred in the dynamics of local seismic process during the construction of hydro power plant and its functioning. Data sets of dam foundation displacements and seismic data sets have been collected in period started from the beginning of Enguri high dam construction (1974) to 2013. In our analysis we used modern methods of data analysis such as recurrence quantification analysis, algorithmic complexity testing, singular spectrum analysis as well as multivariate data analysis combined with the surrogate data testing. It was found that dynamics of dam foundation displacement is strongly influenced by process of HPP construction and especially by water level change in artificial reservoir behind Enguri high dam. It was also shown that the same factors leaded to quantifiable changes in the dynamics of local seismicity around Enguri hydro power plant.
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Combined 3D Inversion of Ground and Airborne Electromagnetic Data over Iron Ore in Kiruna
Authors M. Smirnova, N. Juhojuntti, M. Becken, M. Smirnov, P. Yogeshwar, A. Steuer, R. Rochlitz and M. SchifflerSummaryWe present the first results of a semi-airborne survey conducted in 2018 at Per Geijer iron ore in northern Sweden (Kiruna). In our semi-airborne system, the transmitter is positioned on the ground and magnetic field receivers are airborne. To support airborne measurements, we complemented the survey with an array of ground electric field receivers, magnetotelluric (MT) stations, and SMARTem measurements. The objective of the study is to combine different types of electromagnetic (EM) data in order to derive a common 3D conductivity model of the exploration area. We process data in frequency domain using a robust processing scheme. The airborne data are additionally corrected for rotation. The final 3D model is derived using combined ground and airborne data from three source positions (Controlled-Source EM inversion). Additionally, a 3D model from MT data is obtained separately and used as a starting model for CSEM inversion. Both models match the well-known surface geology and other geophysical information yet bring new insights into the general model of the camp. The next step is a joint inversion of the MT and semi-airborne CSEM data.
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ERT Survey for Identifying Mine Shafts in a Area of Municipality of Höganäs, Sweden
Authors J. Siikanen, E. Lindvall and A. Fouskopoulos LarssonSummaryAn ERT survey has been carried out in an area of Höganäs municipality, Sweden. The aim of the survey was to locate old mine shafts where coal was previously mined. The result should be used as a basis for facilitating the municipality's detailed planning work so that future exploitation of buildings is carried out in a safe manner. The ERT survey consist of four lines performed with gradient configuration and a electrode separation of 2 meter. The direction of the mine shafts is presumed to have a north-easterly direction. All ERT lines have been placed to measure perpendicular to the direction of the mine shafts. The result show anomalies that initially was interpreted as mine shafts and later verified with geotechnical drilling.
The inverted resistivity model of line 1, 2 and 3 indicates a low value in residuals. Line 4 indicates high residual that probably is caused by high conductive material in the ground in or close to the line. The results from this investigation have not shown clearly that the mine shaft consist of a cavity in the rock, but rather a change in the quality of the rock, probably have the mine shafts been filled and/or have collapsed.
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Assessment of Water-Protective Sequence at the Verkhne-Kamsky Deposit of Salts from 3D Inversion Results of EMS-IP
Authors A. Belova, Y. Davydenko, A. Zhukov, D. Bogdanovich, S. Bukhalov, M. Shkirya and K. AbornevSummaryApplication of ground-based methods of electromagnetic sounding in geology-and-engineering survey allows assessment of geological structure in terms of discovering potentially dangerous low-resistivity zones. For this purpose, the method of electromagnetic sounding and induced polarization (EMS-IP) turned out to be highly efficient. The method implies excitation of transitional processes with grounded dipole and registering with receiver lines. Considering inversion within 1D model, it was feasible to specify the horizontally bedded structure of sedimentary sequence above the potassium mine, and to outline potentially dangerous objects of increased conductivity. At the next stage, the obtained model produces initial approximation for 3D inversion of transitional processes. As a result, position of the increased conductivity field was rectified both in plan and over depth.
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Multimethod High-Resolution Geophysical Imaging of Landslides in Mountain Area - A Case Study from Cisiec, Poland
Authors A. Marciniak, S. Kowalczyk, T. Gontar, B. Owoc and M. MajdańskiSummaryThe presented paper shows a case study of Multimethod high-resolution geophysical imaging of landslide. The study area was located in the SW Poland Beskid Zywiecki Mountain range. The area is a clearance, where ski lift is located. To obtain high-resolution images, a proven combination of seismic (Surface Waves Analysis, Seismic travel-time Tomography and Reflection Imaging) methods with ERT tomography were used. The final interpretation shows the whole characteristics of the landslide with accompanying faults. The proposed methodology is cost-effective, and in comparison to a standardly used stand-alone ERT or other single method approaches, delivers the much more detailed image with minimal additional effort
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An Overview of High Spatial Resolution Geophysical Methods for Landslide Characterisation and Monitoring
Authors J. Whiteley, M. Kendall, C. Sujitapan, C. Inauen, R. Swift, A. Watlet, M. Raines, J. Boyd, M. Cimpoiasu, J. Holmes, L. Sibbett, S. Uhlemann and J. ChambersSummaryCombining geophysical methods allows for the characterisation and monitoring of subsurface processes in landslide systems at unprecedented spatiotemporal resolutions. For high spatial resolution characterisation and monitoring of the subsurface, methods that provide maps, cross-sections and three-dimensional volumes of data are preferred. An overview of the results of various long-term monitoring campaigns using such geophysical methods at the Hollin Hill Landslide Observatory in the UK are presented. These methods include electrical resistivity and seismic tomography, self-potential mapping and cross-sections of horizontal-to-vertical ratio measurements of ambient seismic noise. Repeating these surveys over time results in the production of time-lapse data, making these approaches effective monitoring tools. Variations in these measurements show relationships to changes in environmental conditions, for example, decreases in seismic velocity and resistivity values associated with decreases in soil moisture content. Critically, the use of geotechnical-geophysical relationships can provide information between, and beyond the depth of, shallow geotechnical and surface environmental sensors. Using such time-series of high resolution spatial data can help achieve a better understanding of the moisture and kinematic dynamics of unstable slopes, and provides subsurface information for incorporation in to local landslide early warning systems.
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Seismic Noise Polarization Analysis for Unstable Rock Monitoring
Authors D. Arosio, M. Taruselli, L. Longoni, M. Papini and L. ZanziSummaryIn this work, we discuss three different approaches to investigate the polarization characteristics of ambient vibrations collected on unstable rock slopes for monitoring purposes. We consider the Principal Component Analysis (PCA) of the time-domain covariance matrix, the Horizontal-to-Vertical Spectral Ratio as a function of Azimuth (HVSRA), as well as the Singular Value decomposition (SVD) of the Hermitian spectral density matrix. A simple test taking into account seismic noise datasets collected in two acquisition sessions on a potentially unstable rock pillar suggests that the latter method is able to provide more information with respect to the PCA and HVSRA approaches. Processing of the spectral density matrix allows to estimate frequency-dependent polarization parameters, namely the degree of polarization and four angular quantities describing the particle motion in the 3-dimensional space. Provided that processing is properly tuned, frequency-dependent polarization analysis may be able to track subtle changes of unstable rock vibration modes and so to increase our rock failure forecasting capabilities.
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Linking Geoelectrical Monitoring to Shear Strength - A Tool for Improving Understanding of Slope Scale Stability
Authors J. Boyd, J. Chambers, P. Wilkinson, S. Uhlemann, A. Merritt, P. Meldrum, R. Swift, M. Kirkham, L. Jones and A. BinleySummaryLandslides pose a risk to both infrastructure and wider society, managing the geohazard requires and understanding of factors driving slope instability, in particular the response to moisture content. Traditional methods of slope investigation involve discrete point geotechnical measurements which are not spatially sensitive. Recent studies have shown the spatial sensitivity of geoelectrical methods to both the internal geometry of unstable hill slopes and moisture dynamics, demonstrating their value in landslide investigation and characterisation. In this study we present the use of a geoelectrical monitoring system on an active landslide in Lias mudrocks, North Yorkshire, UK. Building on previous studies of the field site, subsurface resistivity distributions determined from time-lapse electrical resistivity tomography (ERT) are converted into shear strength estimates through calibration between electrical resistivity and matrix suction. Geotechnical parameters are determined from shallow borehole samples. Shear strength distributions show agreement with field observations of the slope, relatively low shear strength values (<100 kpa) correspond to the parts of the slope which are actively moving. We suggest there is scope for further work identifying regions of potentially instability on hillslopes through coupled hydromechanical and geoelectrical modelling.
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Imaging Near Surface Scatterers by Scattered Surface Waves - Revamping an Inverse Scattering Code
Authors U. Harmankaya and A. KaslilarSummaryWe use an inverse scattering method for imaging the near surface scatterers by scattered surface waves. The method involves three dimensional elastodynamic wave propagation and scattering in a layered medium. The scattered wavefield is due to density contrast only, and is given as an integral representation by considering Born approximation. For efficient modeling and inversion, we converted our legacy computer code, developed for the method, into a newer programming language. Also, parallel programming techniques were introduced into the code to increase its efficiency. A simple parallelization structure was implemented, which resulted in a significant decrease in program runtime. We validated the code by comparing its modeling output with a finite-difference modeling of wave propagation program. We used half space models with scatterers that have positive and negative density contrasts in both programs and compared the results. Scattered wavefields obtained from the finite-difference modeling are used in our revamped code for imaging. Inversion results for both scatterer cases show that the location and density contrasts of the scatterers are reasonably well estimated.
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Near-Surface Geophysical Imaging of the Permafrost — Initial Result of Two High Arctic Expeditions to Spitsbergen
Authors A. Marciniak, B. Owoc, T. Wawrzyniak, A. Nawrot, M. Glazer, M. Osuch, W. Dobiński and M. MajdańskiSummaryIn presented work, authors applied multiple geophysical methods, to monitor and estimate the seasonal changes in both the active and permafrost layers. The study site, located near the Polish Polar station in Svalbard, is unique due to its location and the environmental processes occurring in this region. The two seismic lines were deigned to image the structure between coast and mountains and post galcial sediments in formt of the retreating glacier. The acquisition of the same lanes was repeated in two consecutive seasons in Autumn 2017 and Spring2018, to develop optimal acquisition and processing methodology. To image seasonal changes, multiple geophysical methods (Seismic Imaging, Travel time tomography, surface waves methods, ERT, Thermal monitoring) were used, in both seasons. As a result, the optimal acquisition was designed. The optimal processing, assumes the use of ERT, GPR, MASW, Seismic travel-time Tomography and Reflection Imaging in presented order. The interpretation of the data shows high variability of the active layer and possibly bottom of the permafrost.
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Seismic Monitoring and Site-Characterization with Near-Surface Vertical Arrays
Authors E. Ruigrok and B. DostSummaryThe northeast of the Netherlands is covered with a dense network of seismic stations. The prime use is monitoring of induced seismicity from the Groningen field and a string of surrounding gas fields. The near-surface consists of unconsolidated sediments. After extensive testing, a standard station design was chosen with an accelerometer at the Earth's surface and geophones at 50, 100, 150 and 200 m depth. Clearly, these vertical arrays cost more than single sensors and do not add in the network coverage. However, they also bring multiple advantages. The combination of a strong motion sensor and at least one weak-motion sensor is needed to record a wide spectrum of earthquake magnitudes. The installation of weak-motion sensors at depth reduces the seismic noise up till about 30 dB. Sensors are placed at different depth levels to enable near-surface characterization and to facilitate phase identification. Furthermore, the installation of multiple buried sensors provides redundancy in case one of the geophones breaks down.
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High-Resolution Crosshole Seismic Tomography (CST) Studies on Nuclear Power Plant Foundation Site
Authors V. Ignatev, A. Konkov, I. Polikarpov, A. Shuvalov and A. OshkinSummaryThis paper describes an example of crosshole seismic tomography (CST) fulfillment on the construction site of one nuclear power plant (NPP). The main objective was to characterize the site's subsurface condition and calculate its elastic dynamic properties. Tomographic measurements were conducted among eight borehole pairs (i.e. among one central and four side wells). Deployment of specific approach for data processing and visualization allowed us to combine the obtained data in 3D mode in order to facilitate the interpretation process. According to the CST results, a dense system consisting of intersecting velocity sections that provide detailed information on the studied structure was constructed.
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Automated Fracture Point Calculation Procedure
Authors I. Ravimo, K. Tiensuu and E. HeikkinenSummaryPosiva Oy is responsible for preparation of final disposal of spent nuclear fuel in Olkiluoto and has done extensive research to ensure the over-all safety of the final disposal. Final disposal is scheduled to start in 2020's. Extensive geophysical survey programs have been executed in both deep boreholes and pilot holes located in the vicinity of planned disposal site. Scope of the automated fracture point calculation procedure was to automatically detect the most significant fractures from geophysical data based on known fracture locations and geophysical anomalies. This approach sets high demands for the processing and depth correction accuracy of the data. Automation was implemented using R programming language. Calculation procedure is applied to one hole at the time and processing constants were determined using existing data from pilot holes. Fracture point calculation procedure returns scaled parameter specific points and scaled total points for each fracture. Fractures are then classified based on their total points in order to determine their relevancy. The most relevant ones get the highest total points but in some cases it can also be beneficial to concentrate only on certain methods. Results are used in Rock Suitability Classification process together with geological and hydrogeological data.
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Integrated Geophysical Studies on the Archaeological Sites without Relief Features (Ust-Tartas and Tartas-1)
Authors E. Balkov, O. Pozdnyakova, P. Dyadkov, A. Alymov, Y. Karin, G. Panin, D. Kuleshov, N. Evmenov and L. NgomayezweSummaryGeophysical survey techniques have become important tools for archaeological research. Integrated geophysical studies were conducted on two archaeological sites, Ust-Tartas and Tartas-1. The electromagnetic, electro-tomography and multiple-height magnetic survey methods were used. In these studies, the electromagnetic survey methods were used to determine the geometrical parameters of the buried objects, and also were most useful in areas of low contrast of the magnetic properties and contamination. Additional methods, the electro-tomography and multiple-altitude magnetic survey were used to carry out a more detailed study about the geometry and depth of objects. Obtained results from the electrical and electromagnetic surveys significantly complement the magnetic survey results. The integrated use of different geophysical methods is always useful for confidently identifying an anomalous object and reduce the ambiguity in the interpretation. The studies showed that the combination of geophysical data makes it possible to solve complex tasks so as to achieve the degree of information required for archaeological research.
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Geophysical Survey at a Coastal Area of Artemida City, East Attica, Greece, with Archaeological Interest
Authors G. Apostolopoulos, G. Amolochitis, A. Sakalidi, E. Garbi, D. Karaiskos and S. KarizonisSummaryA geophysical survey was carried out at a coastal area of Artemida City, East Attica, Greece, with its main purpose to detect any anthropogenic structures of archaeological interest covered by loose sediments. The archaeological excavation has already revealed the foundation structure of the Temple of Artemis Tavropolou and southerly a small temple near the coast. Two areas have been covered with EM, ERT and GPR measurements, one in the vicinity of the Temple and the other the whole sandy coast connecting the two temples. Saline water intrusion plays the key role in the conductivity distribution in all methods which can show where dense impermeable structures exist either being anthropogenic or foredune or bedrock. Even the later can reveal the paleo-environment and exclude areas for further archaeological exploration. EM maps have provided the best position of ERT and GPR profiles which finally have indicated local structures and their continuity.
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