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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 - 20 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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