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Near Surface Geoscience 2013 - 19th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 09 Sep 2013 - 11 Sep 2013
- Location: Bochum, Germany
- ISBN: 978-90-73834-38-5
- Published: 09 September 2013
81 - 100 of 116 results
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Investigation of Landslide Kinematics Derived from UAV- and Ground-based Imagery - Precision Analysis of 3D Point Clouds
Authors M. Walter, R. Rothmund, U. Niethammer, J.P. Malet and M. JoswigDuring an eight week field campaign at the Super-Sauze landslide (South French Alps), several UAV- and ground-based photographs have been acquired. Additionally, high-resolution terrestrial laser scanning (TLS) has been performed in order to capture a precise reference data set. The High-resolution and multi-temporal, but also disordered and oblique images were used to generate 3D point clouds from Structure-from-Motion and Multi-View Stereo (SfM-MVS) algorithms. These multi-temporal digital surface models (DSMs) were required for a precise ortho-rectification, as well as for morphometric and kinematic investigations. In order to determine the quality of the point clouds, all photogrammetric data were compared to the time corresponding TLS-based DSMs. The analysis of DSMs and orthophotos is one part of a field experiment that has the objective to characterize the relationship between fracture processes (slidequakes and local tremor events), slope movements, fissure developments and pore water pressure build-up. The main focus of this contribution is a comprehensive evaluation of the SfM-MVS-based point clouds in order to demonstrate the potential but also the limits of such approach.
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Combined Active and Passive Seismic Investigations on Railroad Embankments
Authors F. Koellner, E. Pomponi, C. Sens-Schönfelder and A. SchuckThe present study introduces combined active and passive seismic investigations on a railroad embankment that is constructed on a soft layer of peat clays. This type of sediments causes the embankment to be in a dynamic movement whenever trains are passing. As long term consequence the stability of the embankment is affected. Based on the concept of seismic interferometry temporal changes in the medium due to the heavy dynamic force load induced by the train can be identified. Furthermore, velocity information of travling body waves that have been retrieved by cross-correlating the signals of train passages are compared to results from seismic crosshole measurements.
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Imaging Subsurface Cavities Using Geoelectric Tomograpgy, Ground-penetrating Radar and Microgravity
By M. SobhIn the past few years, construction extended to the southeast of Cairo, Egypt, where limestone caves occur. The existence of caves and sinkholes represents a hazard for such new urban areas. Therefore, it is important to know the size, position, and depth of natural voids and cavities before building or reconstruction. In this paper, geoelectric-resistivity tomography, ground-penetrating radar (GPR) and microgravity have been applied to image shallow subsurface cavities. Microgravity measurement was done to detect density variations in rock strata. Several gravity anomalies were found in the residual map. Semi-automated interpretation techniques including the Euler deconvolution, analytic signal have been used to investigate the depth and size of anomalous sources. Resistivity survey was conducted along three profiles over an exposed cave with unknown extensions. Results from 27 GPR profiles obtained by a SIR-2000 instrument equipped with a 200 MHz antenna were visualized in the form of horizontal time slices and vertical time sections. As a result, the cave at a depth of about 2 m and a width of about 4 m was detected using the geophysical data, which correlates with the known cave system.
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Optimised Sequential Experimental Design for Geoelectrical Resistivity Monitoring Surveys
Authors P.B. Wilkinson, S.S. Uhlemann, J.E. Chambers, P.I. Meldrum, L.S. Oxby and O. KurasSequential experimental design methods use previous data and results to guide the choice and design of future experiments. This paper describes the application of a sequential design technique to produce optimal resistivity imaging surveys for time-lapse geoelectrical monitoring experiments. These survey designs are time-dependent, and are optimised to focus a greater degree of the image resolution on the regions of the subsurface that are actively changing than static optimised surveys that do not change over time. The sequential design method is applied to a synthetic 2.5D monitoring experiment comprising a well-defined cylindrical target moving along a trajectory that changes its depth and lateral position. The data are simulated to be as realistic as possible, incorporating survey design constraints for a real resistivity monitoring system and realistic levels and distributions of random noise, in order to match a forthcoming experimental test of the method. The results of the simulations indicate that sequentially designed optimal surveys yield an increase in image quality over and above that produced by using a static (time-independent) optimised survey.
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Computation of Optimized Arrays for 3-D Electrical Imaging Surveys
Authors P.B. Wilkinson, M.H. Loke and J.C. ChambersThree-dimensional surveys and inversion models are required to accurately resolve structures in areas with very complex geology where 2-D models might suffer from artifacts. Many 3-D surveys use a survey grid where the number of electrodes along one direction (x) is much greater than in the perpendicular direction (y). Frequently, due to limitations in the number of independent electrodes in the multi-electrode system, the surveys use a roll-along system with a small number of parallel survey lines aligned along the x-direction. The 'Compare R' array optimization method previously used for 2-D surveys is adapted for such 3-D surveys. Offset versions of the inline arrays used in 2-D surveys are included in the number of possible arrays (the comprehensive data set) to improve the sensitivity to structures in between the lines. By using PCs with modern graphics cards incorporating a fast Graphics Processing Unit (GPU) and using an improved single-precision 'Compare R' algorithm, the 3-D optimized arrays can be calculated within a reasonable time despite the comprehensive data set possibly have millions of arrays. A comparison with data sets using inline measurements made using conventional arrays show that structures located between the lines are much better resolved with the optimized arrays.
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Source and Receiver Array Simulations for Near Surface Seismic Investigations
Authors A. Tognarelli and E. StucchiWe present a procedure for enhancing the data quality of shallow seismic data based on two different steps: 1) an acquisition step that requires the recording of closely spaced common source records with standard source and receiver equipment and 2) a processing step where weighted and/or un-weighted source and/or receiver arrays can be simulated on the basis of required needs for source related noise attenuation and depth penetration. The data acquisition can be carried out employing single source – single geophone recordings, with a standard 24 or 48 channels equipment. Simple energy sources such as weight drops or sledgehammer are considered. The design and application of the spatial filters in the processing phase is very flexible and can be tailored to the specific needs: in fact, the simulated source and/or receiver arrays can be time and/or space variant and, if needed, can be weighted to provide the desired responses. Optimal weights can be determined by means of Chebyshev polynomials. Real data examples show the increase in the data quality in terms of better coherent noise attenuation and of enhanced depth penetration.
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Software for Development of Seismic Models of Rock Masses Employing 2D-3D Seismic Exploration
Authors O.O. Glukhov, A.I. Kompanets, A.V. Antsiferov, V.A. Antsiferov and L.A. KamburovaA vital task of seismic exploration in Ukraine is prediction of tectonic structure of coal-rock masses in conditions of thin-bedded near-surface (up to 1,500 m) geologic section taking into account low-amplitude tectonic faulting. In our paper, software developed in UkrNIMI of the National Academy of Sciences of Ukraine (NAS of Ukraine) for construction of models for such coal-rock masses based on the results of 2D – 3D seismic exploration is briefly described. Software units enable to execute reading and visualization of initial seismograms; to estimate record quality and preprocessing of seismic traces, input of acquisition configuration and seismic data tying; to make estimation of 3D survey quality, computation and visualization of seismic sections and cubes, computation and visualization of the surfaces of geologic structures.
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Estimation of the P-wave Velocity Profile of Elastic Real Data Based on Surface Wave Inversion
Authors A.V. Ponomarenko, B.M. Kashtan, V.N. Troyan and W.A. MulderRecently, we proposed an analytical approach to invert for a smoothly varying near-surface P-wave velocity profile that has a squared slowness linearly decreasing with depth. The exact solution for such a velocity profile in the acoustic approximation can be expressed in terms of Airy functions and leads to a dispersion equation. The method was successfully applied to synthetic elastic data with small Vs/Vp-ratio. Here, we apply the method to land data. The result agrees with that of multi-layered inversion, confirming its potential to provide an initial P-wave velocity model for acoustic full waveform inversion. Compared to multi-layered inversion, the method is simpler to use and produces a smooth model characterized by three parameters. In some cases, having a smooth rather than a blocky initial model for full waveform inversion is more appropriate.
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Relationship between Surface Conductivity and Induced Polarisation Parameters
Authors A. Weller, L. Slater and S. NordsiekInterpretation of resistivity measurements is often uncertain as resistivity is controlled by pore fluid chemistry, moisture content and lithology. Induced polarisation measurements may help to reduce this uncertainty as they are primarily controlled by lithology and, relative to resistivity measurements, weakly dependent on pore fluid chemistry. In order to exploit this potential, we have investigated the relationship between the surface conductivity appearing in petrophysical models for resistivity and the imaginary conductivity measured with induced polarisation. It has previously been suggested that imaginary conductivity can be used to estimate the surface conductivity if the ratio of surface polarisation to surface conduction (l) is known. However, a detailed investigation of this ratio has not hitherto been reported. We performed a study on an extensive database of sandstone and unconsolidated samples where resistivity and induced polarisation measurements were acquired as a function of salinity. The surface conductivity was estimated assuming a simple parallel conduction model where conduction through interconnected pores and conduction along interconnected pore surfaces add in parallel. The surface conductivity exhibits a weak salinity dependence that is similar in shape to the salinity dependence of the imaginary conductivity. This finding suggests that l is only weakly dependent on salinity.
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The Salinity Dependence of Spectral Induced Polarization Studied with an Extended Model of Membrane Polarization
More LessIn order to estimate hydraulic parameters from SIP measurements, it es essential to understand the dependence of the electrical impedance not only on structural parameters describing the pore space, but also on electrochemical properties. Here, we investigate the influence of fluid salinity on the IP parameters based on an analytical model to simulate membrane polarization. The model was originally developed for a one-dimensional pore system, where the membrane effect is postulated to be caused by varying ion mobilities in two different pore types. The model was recently extended to explicitly include finite pore radii and the properties of the electrical double layer, and the membrane effect comes out naturally as a result of varying ion concentration with distance from the pore wall. We carry out simulations to study the influence of salinity on the IP effect and compare the results with experimental observations. While our results are consistent with several trends observed in experiments, such as an increasing imaginary conductivity with fluid salinity, some features, such a decrease of the imaginary conductivity at high salinities, were not yet reproduced.
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Estimation of van Genuchten-Mualem Parameters from Spectral Induced Polarization Measurements
Authors S. Nordsiek, A. Hördt, E. Diamantopoulos and W. DurnerThe application of spectral induced polarization (SIP) surveys to estimate hydrological properties of the subsurface is promising. For the interpretation of the SIP data, the knowledge of the relationships between soil hydraulic properties and parameters obtained from SIP measurements is essential. Therefore, we performed hydrological and SIP measurements in the laboratory on seven natural soils and a medium sand sample with a narrow grain size distribution. The SIP spectra were interpreted with the Debye decomposition, which yields integrating parameters for the resulting relaxation time distribution. To derive soil hydraulic parameters from the hydrological experiments, we fitted the van Genuchten-Mualem model, which is widespread in soil physics, to the measured hydrological data. In this study, we concentrate on the relationship between two parameters: the normalized chargeability from the Debye decomposition of SIP spectra and the parameter alpha from the van Genuchten-Mualem model. We present an approach to estimate the parameter alpha from the normalized chargeability.
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Prediction of GPR Performance in Soils Using Broadband Dielectric Spectroscopy
More LessOne of the soil properties influencing the sensing depth of ground-penetrating radar (GPR) is intrinsic attenuation. Especially in high-frequency investigations, as for explosive ordnance and landmine detection, it plays a major role. In many cases attenuation is the limiting factor that determines the applicability of the GPR system. In order to investigate the frequency-dependence of electromagnetic soil properties, different soil samples from Germany and Afghanistan were analyzed. For this purpose, we used a coaxial transmission line together with a vector network analyzer in the laboratory. Two coaxial line cells were used in order to determine the complex dielectric permittivity in the 1 MHz to 10 GHz frequency range. The complex permittivity curves were fitted by a generalized model, which accounts for different dielectric relaxation mechanisms. The inversion of the model was carried out by means of the Geophysical Inversion and Modelling Library (GIMLi). Splitting the measured complex dielectric permittivity data using the generalized model made it possible to investigate the different energy loss contributions to the intrinsic attenuation, which are in turn attributed to certain soil components. Based on the laboratory results, we intend to create a simple method for the prediction of the GPR performance in the field.
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Study on Fractal Structure of Pore Space of Sandstones Based on NMR and Capillary Pressure Curves
Authors Z. Zhang, W. Debschuetz and A. WellerWe apply the fractal equation of pore distribution and NMR spectrum to 25 sandstone samples from several boreholes in China, and we show the results of the relations between NMR, fractal dimension and capillary pressure. The fractal dimensions can be calculated from NMR and capillary pressure data, and the value of correlation coefficient could indicate whether the pore structure shows fractal features. For the same core sample, there are different fractal dimensions in different pore radii ranges, indicating that core pore space has multiple fractal structure. The fractal dimensions determined from NMR data is lower than the values determined from capillary pressure data, this is due to the different pore throat space reflected by these two methods. The fractal dimension from NMR data can better represent the rock’s physical properties.
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Lab-scale Tests in the Detection of a Multi-phase DNAPL
Authors L. Orlando and B. RenziIn order to improve the understanding of the sensitivity of geophysical investigation to different phases of contaminant, GPR, electric resistivity and TDR measurements performed to detect and monitor DNAPLs spilled in a saturated media are analyzed. The experiment was performed in a Plexiglas cell filled with saturated grain glasses, simulating a permeable sand media, and hydrofluoroether (HFE-7100) simulating the contaminant. The results showed that GPR is more sensitive to the separate phase of contaminant because the dissolved phase does not significantly influence the electric permittivity. These results were confirmed by TDR measurement. On the contrary the electric resistive measurements resulted sensitive to all phases of contaminant (gaseous, separate and dissolved). The water flow and the variation of the water table contribute to the dissolution and migration of the DNAPL, even if the lab experiment showed that it cannot be easily removed within a saturated medium because its separate phase remains trapped within the pores and, very slowly, it moves downwards, also in the presence of a water flow.
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Integrated Investigation of the Instability-prone Landfill Site
Authors V Frid, A. Averbakh and G. LiskevichThe study was motivated by the landfill surface fracturing and grate doubts regarding to the landfill body stability. Three geophysical methods were used for the site study as follows: electrical resistivity tomography, ground penetration radar and seismic refraction. Application of the geophysical methods was combined with borehole drilling campaign, DPSH testing and inclinometer measurements. As a results of common geophysical geotechnical data processing 3D model of the landfill body was built, the regions of leachate concentration were found and waste physical properties were studied. The weak zone observed in the landfill bode and very a low values of P-wave velocity certainly indicated intensive waste disintegration and hence confirmed our work hypothesis on the lower waste body stability that was aggravated by the leachate location at the landfill body bottom. Inclinometer measurements indeed corroborated high values of waste instability. Application of the integrated methodology allowed one to assess waste and leachate condition and not only to evaluate the landfill stability in general, but also to locate the most sensitive its parts.
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Using Geophysical Tomographical Techniques for the Weak Point Analysis on a Potash Mining Dump
Authors K. Dünnbier, T. Schicht and F. BörnerThe objective of the presented research project is to develop a combination of geophysical and petrophysical methods for the non-invasive characterization of the potash mining dump material. It includes tomographic methods such as geoelectrics and seismics as well as ground penetration radar (GPR), spectral induced polarization (SIP) and nuclear magnetic resonance (NMR) methods. The resulting geophysical models should detect and display weak areas due to, for example, abnormal strengths, hydraulic permeability and water content. Previously acquired data has given first impressions of a part of the waste dump. The project aims to give the operating company a tool to characterize the different areas of a potash waste dump. The results could help the company to decide whether to reuse the material (backfill material for stabilization of salt mines) or to cover the waste dump effectively.
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Geophysical Characterisation of a Former Waste Disposal Site in the Context of Landfill Mining
More LessIn the context of landfill mining, we conducted electrical resistivity tomography (ERT), borehole electromagnetics (EM39), and magnetometry campaigns to characterise the subsurface of a waste deposit. The electrical resistivity structures in the landfill appear tabular with a first layer (12-15 m) characterised by higher resistivities (> 10 ohm.m; with spots > 30 ohm.m) whereas the saturated zone (below 15 m) presents lower resistivities (< 4 ohm.m). Based on ERT images, we positioned 6 drillings, collected waste samples, and recorded in-situ measurements (distributed temperature and EM39) to confirm the ERT interpretation and bring out some limitations of the method. Magnetic field data were recorded in the same area. We noticed a clear distinction between the inside (high magnetic field gradient) and the outside (low variation) of the landfill. The large scale variation of the total magnetic field is compatible with the existence of a clinker dam below the landfill bottom membrane (known from historical information). Therefore, magnetometry could be used to detect variation in the waste composition (clinker, metallic drums, or municipal waste) but is also a cheap and powerful tool to map unknown landfill boundaries. Both aspects are crucial in the context of landfill mining.
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3D ERT Monitoring of the Reactivation of Waste Biodegradation with Fresh Leachate Injection
More LessThe aim of this study is to monitor (bio) physical processes occurring in a landfill. The experiment consists in injecting leachate towards a drain in unsaturated and not yet digested waste to reactivate (or activate) waste biodegradation. The target is the first 15 meters of the studied landfill subsurface. The visualisation of the wet front arrival (short term effect) is crucial because we want to ensure that waste is entirely humidified to allow the reactivation of waste digestion. The second process is a long term effect consisting in the increase of the internal temperature of the landfill which is synonymous of the reactivation of biodegradation processes. We currently use 3D time-lapse ERT on a monthly basis to capture the decrease of electrical resistivity related to the increasing temperature. We also collect ground truth data, including distributed temperatures in a borehole to validate results. For short term effects, we monitored the wet front arrival with three 2D ERT profiles during an entire day. Preliminary results, corroborated by ground truth data, show that leachate flow is anisotropic (more rapid horizontally than vertically). So far, waste was completely humidified but only slight changes of temperature occurred.
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Landfill Leakage Detection Using ERT
Authors P. Tsourlos, G. Vargemezis, I. Fikos and A. GiannopoulosIn this work we applied the electrical resistivity tomography technique in an attempt to locate areas of leakage in a lined landfill. A systematic ERT survey was carried out using dipole-dipole and pole-dipole forward and reverse arrays. Based on the explanation of the different inversion images obtained over the same line for the the different electrode arrays we propose an interpretation approach which can be used for identifying leakage within a geomembrane liner landfill. Further, the systematic survey with dipole-dipole and pole-dipole (forward and reverse) arrays and the combined graphical interpretation can be used to locate potential leakage area.
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Combining Redox Potential Measurements and IP Signatures to Improve the Monitoring of a Chlorinated Solvents Plume
Authors A. Etienne, J. Deceuster and O. KaufmannRemediation strategies for contaminated sites require accurate mid and long term monitoring methods. This is especially the case when groundwater is polluted by soluble organic contaminants such as chlorinated solvents or LNAPL (Light Non-Aqueous Phase Liquids). In general, the monitoring of remedial actions is based on water samples analyses from which geochemical parameters are derived. However this information is often limited in time and space for technical and economical reasons. As a result, a spatiotemporal uncertainty may remain on the evolution of the plume. Complementary methods, such as geoelectrical methods, can be used to reduce this uncertainty. A field experiment was conducted on a former industrial site mainly contaminated by trichloroethylene (TCE) and exhibiting natural bioremediation phenomena. A carbon source was injected to enhance the reductive dechlorination process. Geoelectrical and geochemical monitoring were carried out on the test site during this experiment. Geochemical data were integrated to geophysical results. A linear correlation between the redox potential and the IP (Induced Polarization) signatures was observed during this study.
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