- Home
- Conferences
- Conference Proceedings
- Conferences
Near Surface 2010 - 16th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 06 Sep 2010 - 08 Sep 2010
- Location: Zurich, Switzerland
- ISBN: 978-90-73781-88-7
- Published: 06 September 2010
1 - 50 of 153 results
-
-
Examination of Seismoelectric Observations at the Test Site Schillerslage and Laboratory
Authors J. Holzhauer, T. Guenther and U. YaramanciWithin the development of a new test-site in Schillerslage near Hannover with typical north-german geology of shallow sandy aquifers alternating with till deposits, we used the opportunity of having a well-defined site, already investigated by various geophysical methods and boreholes, to test the observability of the seismoelectric signals. The electrical signals related to seismic excitation are caused by an electrokinetic coupling occurring at the grain-fluid interface and can be of two kinds: First, a coseismic electromagnetic signal highly similar to the compression wave and travelling with it, and second a converted electromagnetic signal originating from interfaces with discontinuities in hydraulic properties as in porosity, hydraulic conductivity, fluid salinity and further parameters. The field data at Schillerslage displays both types of seismoelectric signals. Analysing the amplitude distribution of the converted signal, the depth of an interface could be detected at 2.1 ± 0.3 m, implying a P-wave velocity around 240 m/s in the upper layer. Laboratory experiments, which are in the initial stage, allow to study the dependence of both coseismic and converted signals on various hydraulic parameters more thoroughly.
-
-
-
Improving the Signal-to-noise Ratio of Surface-NMR Measurements by Reference Channel Based Noise Cancellation
Authors M. Mueller-Petke and U. YaramanciThe technique of surface-NMR (also called Magnetic Resonance Sounding) as unique hydro-geophysical prospection method has shown several improvements in data processing, inversion and interpretation during the last years. But the applicability of surface-NMR is often limited due to bad signal-to-noise ratio, i.e., the measurable NMR signal is masked by electromagnetic noise. Therefore, improving s/n ratio is a demanded development. According to newly introduced improved instrumentation providing simultaneous multi-channel recording the development of reference techniques is possible. We present and describe a noise cancellation approach based on predictive filter. Our approach allows to handle amplitude and phase differences of the signals recorded at the detection and reference loop. Furthermore, size and shape of the loop may not be equal. The technique is assessed with synthetic and field examples that clearly show the signal improvements obtained using the presented noise cancellation approach.
-
-
-
Aquifer Characterisation by Magnetic Resonance Field and Laboratory Measurements
Authors R. Dlugosch, M. Müller-Petke, T. Günther and U. YaramanciIn order to evaluate the potential and reliability of surface-NMR measurements for aquifer characterisation we compare a field dataset collected on a well known test site with sandy aquifers to the laboratory-NMR and hydrogeological measurements of porosity and hydraulic conductivity using cores samples and grain size analyses. The decay time distribution derived from the QT-inversion of the surface-NMR data shows good agreement to laboratory-NMR results. A comparison of the water contents and T2* decay times achieved from different inversion approaches, i.e., using lately developed QT-Inversion and SAMOVAR show similar results. For most parts of the subsurface, the hydraulic conductivity from field and laboratory-NMR agreed very well with values estimated from grain size analyses or measured in the laboratory. Disagreements especially in parts of the first aquifer need to be further evaluated.
-
-
-
Improving Surface-NMR Estimates of Nuclear-spin Relaxation (T1)
Authors J.O. Walbrecker, M. Hertrich and A.G. GreenSurface nuclear magnetic resonance (NMR) is a useful tool for hydrological investigations of shallow aquifers. An important parameter is the NMR relaxation time T1, from which information on pore structure or even hydraulic conductivity can be inferred under favourable circumstances. T1 data are conventionally acquired using a scheme that involves two sequential pulses of electromagnetic energy, the second of which is phase-shifted by pi relative to the first. We have discovered that variations of the excitation field with distance from the transmitter introduce a significant bias to conventional estimates of T1. Here, we propose a novel yet simple modification to the conventional scheme that is theoretically capable of resolving this problem. The proposed scheme comprises a conventional double-pulse sequence followed by an additional double-pulse sequence in which the 2nd pulse is in-phase with the 1st pulse. Subtracting the voltage signals measured during the two double-pulse sequences eliminates the bias. This strategy of continuously cycling the phase of the 2nd pulse between pi and 0 in sequential double-pulse experiments and then subtracting the resulting voltages is a highly promising step towards recording more reliable T1 data under general field conditions.
-
-
-
Experimental Design of Parallel 3D Geoelectric Measurements
Authors M. Blome, H.R. Maurer, M. Hertrich and S.A. GreenhalghWe have developed ETH DCMES II, a new, fully distributed, smart electrode geoelectric data acquisition system that allows parallel measurements to be performed efficiently. This is an important prerequisite for a novel data acquisition concept that encompasses two main aspects. Firstly, roll-along strategies can be employed to cover a large virtual grid with a limited number of electrodes. Secondly, within each electrode deployment, pole-bipole measurements can be acquired efficiently using the parallel recording capabilities of ETH DCMES II. For a set of n electrodes only 2n parallel pole-bipole scans (using 2 variable reference electrodes) are required to reconstruct any other pole-bipole configuration, even in the presence of noise. The performance of the system is demonstrated with a field experiment over a former waste disposal site in Switzerland. The 3D data set of several hundred thousand measurements could be acquired in about 7.5 hours, which allowed critical features within and outside of the waste pits to be delineated.
-
-
-
Zonal Cooperative Inversion of Data Sets with Partially Co-located Model Areas
Authors H.C. Paasche, J. Tronicke and P. DietrichWe extend the zonal cooperative inversion (ZCI) approach based on fuzzy c-means cluster analysis and conventional single-input data set inversion algorithms for the cooperative inversion of data sets with partially co-located model areas. This is done by considering recent modifications made to the fuzzy c-means cluster algorithm. We apply the extended ZCI to cooperatively invert two crosshole tomographic traveltime data sets with partly co-located model areas. Additionally, we demonstrate how additional a priori information can be incorporated in the ZCI methodology. The ZCI results in a single geophysical model outlining the major subsurface zonation. Available ground-truth data support our zonal geophysical model and, thus, prove the applicability of the proposed inversion approach.
-
-
-
Geoacoustic Model for the Inversion of Seismic Profiling Data
Authors K.C. Leurer and C. BrownWe present a geoacoustic model to predict sediment physical parameters from single-channel seismic profiling data. The model uses the concept of a simplified sediment structure, modeled as a binary grain-size sphere pack. The seismic/acoustic response is formulated using Biot’s poroelastic theory as the general framework that is extended by two viscoelastic models. These extensions describe the mechanisms that we consider to have the most significant influence on wave propagation through soft sediment. Viscoelastic response arising from local fluid flow in expandable clay minerals leads to frequency-dependent elastic moduli of the grain material. A heuristically modified Hertz-Mindlin/Walton based viscoelastic-contact model describes local fluid flow at the grain contacts, resulting in frequency-dependent elastic moduli of the sediment frame. Porosity, density and the structural Biot parameters (permeability, pore size, structure factor) follow from the binary grain-size sphere-pack model. The remaining input parameters to the geoacoustic model consist solely of the effective pressure, the mass fractions and the known mechanical properties of each mineral constituent. We will show an example of a successful application of this model for the inversion of single-channel seismic profiling data using a neural network inversion scheme.
-
-
-
Determination of the Damping Ratio in the Soil from SASW Tests Using the Half-power Bandwidth Method and Arias Intensity
Authors S.A. Badsar, M. Schevenels, W. Haegeman and G. DegrandeThis paper focuses on the determination of the material damping in the soil by means of the SASW test. Two new methods are proposed. The first method is based on the half-power bandwidth method applied to the width of the peaks in the $f$--$k$ spectrum to determine the attenuation of Rayleigh waves. The second method uses the spatial decay of the Arias intensity at the surface to determine the material damping ratio. Both methods are used to determine the material damping ratio in the soil at a site in Belgium. The identified soil profiles are used to simulate the wave field registered in the SASW test, which is confronted with the experimental data. Good results are obtained with both methods, the second method performing slightly better than the first.
-
-
-
Source Wavelet Estimation during Full-waveform Inversion of Ground Penetrating Radar Data
Authors F.A. Belina, J. Irving, J.R. Ernst and K. HolligerA major issue in the application of waveform inversion methods to crosshole ground-penetrating radar (GPR) data is the accurate estimation of the source wavelet. Here, we explore the viability and robustness of incorporating this step into a recently published time-domain inversion procedure through an iterative deconvolution approach. Our results indicate that, at least in non-dispersive electrical environments, such an approach provides remarkably accurate and robust estimates of the source wavelet even in the presence of strong heterogeneity of both the dielectric permittivity and electrical conductivity. Our results also indicate that the proposed source wavelet estimation approach is relatively insensitive to ambient noise and to the phase characteristics of the starting wavelet. Finally, there appears to be little to no trade-off between the wavelet estimation and the tomographic imaging procedures.
-
-
-
Full-waveform Time-domain Inversion of GPR Data by Progressive Frequency-bandwidth Expansion
Authors G.A. Meles, S.A. Greenhalgh, A.G. Green and J. van der KrukGround penetrating radar (GPR) is a popular and important subsurface imaging tool for environmental and engineering site investigations. Full-waveform inversion of crosshole data offers greatly improved resolution over standard ray-based tomography, but still suffers from limited angular coverage of the target and convergence problems due to the high degree of non-linearity of the forward problem. Here, we present a modification to the standard time-domain full-bandwidth inversion approach that mitigates the latter problem. We start with a low frequency filtered version of the radargrams to avoid getting trapped in a local minimum and progressively expand the bandwidth as the iterations proceed. This conveys stability and at the same time builds resolution as the shorter wavelength features of the permittivity and conductivity distributions are sequentially added. We illustrate the improved performance of this scheme over the traditional approach by means of two synthetic examples – the first involving two small embedded high/low permittivity and conductivity bodies in a uniform background and the second involving multiple low conductivity / low permittivity bodies in a layered background with superimposed stochastic fluctuations.
-
-
-
Stochastic Inversion of Vadose Zone Properties: Impact of Parameter Correlation on Uncertainty Estimates
Authors M. Scholer, J. Irving, A. Binley and K. HolligerA number of studies have shown that time-lapse crosshole geophysical data can provide valuable information regarding the hydraulic properties of the unsaturated zone. The stochastic inversion of such data can yield estimates of uncertainties in such properties, which are valuable for hydrological characterization. Here, we investigate the effect on output parameter uncertainties of accounting for realistic correlation between the hydraulic model parameters in the inversion procedure. We do this within a Bayesian framework using a Markov-chain-Monte-Carlo (McMC) strategy, and we investigate the particular problem of estimating vadose zone hydraulic properties from ground-penetrating radar (GPR) data collected during a 1-D infiltration experiment. Our results clearly indicate that prior information on the correlation between model parameters has the effect of noticeably reducing posterior parameter uncertainties and hence, if available, should be included in such inversions.
-
-
-
Mixed-phase Deconvolution of Ground-penetrating Radar Data
Authors C. Schmelzbach, F. Scherbaum, J. Tronicke and P. DietrichGround-penetrating radar (GPR) surveying has the potential to provide detailed images of the shallow subsurface. However, these images are distorted due to effects of the source wavelet. Standard deconvolution procedures, which are routinely used to increase the vertical resolution of seismic data by removing the source wavelet, are often not effective when applied to GPR data. It is commonly suspected that this problem is linked to the fact that standard stochastic deconvolution algorithms are based on the assumption of a minimum-phase source wavelet, whereas GPR source wavelets generally are mixed-phase. We propose a new approach to deconvolve GPR data that is based on the observation that a mixed-phase wavelet corresponds to the convolution of a minimum-phase wavelet and a dispersive all-pass filter. Consequently, the successful deconvolution of GPR data involves estimating two inverse filters, one that cancels the minimum-phase wavelet and one that removes the effects of the all-pass filter. We demonstrate the potential of this approach using synthetic and field GPR data. The results show that our deconvolution approach significantly increases the vertical resolution compared to standard processed GPR data.
-
-
-
A Global Measure for Depth of Investigation
Authors A.V. Christiansen and E. AukenDepth of investigation (DOI) is a commonly requested parameter in geophysical surveys. For diffusive methods, such as groundbased or airborne EM, there is no specific depth below which there is no information on the resistivity structure, but the question is to what depth the model is most reliable. We present a new robust concept for the calculation of DOI that is valid for any 1D EM geophysical model. The method is based on the actual model output from the inversion and includes the full system response, contrary to assuming e.g. planar waves over a homogeneous halfspace. Equally important, the data noise and the number of data points is integrated in our calculation. Our methodology is based on a recalculated sensitivity (Jacobian) matrix of the final model and it can thus be used on any model type for which a sensitivity matrix can be calculated. Contrary to other sensitivity matrix methods we define a global and absolute threshold value contrary to defining a relative, say 5%, sensitivity limit. The threshold limit will apply to all 1D inverted data and will thus produce comparable numbers of DOI.
-
-
-
Non-linear Smoothness-constrained Model Error and Resolution Estimates
Authors T. Kalscheuer, M. Garcia, N. Meqbel and L.B. PedersenA comparison of error and resolution properties of 2-D models of electrical resistivity from single and joint inversions of direct-current resistivity (DCR) and radiomagnetotelluric (RMT) data is presented. Linearized model resolution and error estimates are computed from the Jacobian and its smoothness-constrained generalized inverse. As a novelty, linearized model errors are compared to most-squares error estimates to better account for non-linearity. For a synthetic example, linearized analyses yield model errors up to 30 to 40 per cent for data errors of two per cent and resolving kernels spread over several cells in the vicinity of the investigated cell. Most importantly, linearized errors are in good agreement with most-squares errors and, hence, linearized model errors can be representative. DCR data can constrain both resistive and conductive structures whereas RMT data provide superior constraints for conductive structures. For structures within the depth ranges of exploration of both methods, error and resolution of joint inverse models are equal to or better than those of single inversions. For structures outside the depth range of exploration of one method, error and resolution of joint inverse models are close to those of the best single inversion given appropriate data weighting.
-
-
-
4-D Inversion of Resistivity Monitoring Data Using L1 Norm Minimization
More LessInversion artifacts, one of the major problems which may lead a misinterpretation of geophysical monitoring data, can be reduced through incorporating the time domain regularization by defining both the subsurface model and the time-lapse data in the space-time domain. Applying this time regularization, however, may result in a model too smoothly varying in the time domain, and as a result, underestimate the real subsurface changes. To alleviate this problem and to find a way to accurately evaluate the ground truth, we developed a new inversion algorithm where we can flexibly adopt either L1 norm or L2 norm minimization of each of the three penalty values in the four dimensional inversion: data misfits, model roughness in the space domain and that in the time domain. Synthetic data experiments of crosshole resistivity monitoring were conducted using the developed algorithm. Further tested was the application to field resistivity monitoring data obtained for the assessment of the ground condition changes caused by a tunnel excavation. All these tests showed that the 4D inversion of minimizing the L1 norms of both the data misfit and the model roughness in the time domain produced the results which closely resemble the true ground condition changes.
-
-
-
Methods to Reduce Banding Effects in 3-D Resistivity Inversion
More LessThree-dimensional surveys are required to accurately resolve structures in very complex areas. In many cases the 3-D data set is collated from a series of parallel 2-D survey lines, and frequently the distance between the lines is more than the electrode spacing along the lines. The inversion of such data sets frequently produce models with banded near-surface anomalies that are aligned parallel (or perpendicular) to the lines. It is shown that such banded structures can be caused by the data acquisition geometry. Several modifications to the smoothness-constrained least-squares inversion method are examined to reduce such artifacts in the inversion model. One method is to use a higher damping factor for the model cells in the near-surface layers. The banding effects can also be reduced using a model discretization such that the width and length of the model cells are similar although the spacing between the lines is larger than the in-line electrode spacing. The remaining banding artifacts can be further reduced by modifying the horizontal roughness filter used such that it has components in the diagonal directions as well as in the directions along and perpendicular to the 2-D survey lines.
-
-
-
Aquifer Characterization by Spectral Induced Polarisation - Field and Lab Measurements
Authors R. Holland, T. Günther and U. YaramanciIn this study porosity (Φ) and hydraulic conductivity (K) estimations are conducted on the basis of independent frequencies of 1D, 2D and lab Spectral Induced Polarisation (SIP) data. We compare SIP 1D and 2D models with lab results, before we discuss different approaches to derive hydraulic parameters i.e. classical and modified Archie for Φ estimation and Slater and Lesmes (2002) and Boerner et al. (1996) for K estimation. Both approaches for P estimation yield similar results, while lab and field data show differences. For K estimation lab and field SIP data agree better. While K values after Boerner vary strongly within depth, results after Slater vary in a smaller range but are slightly too low, probably due to the limited validity range of this empirical relation.
-
-
-
Application of Multi-frequency Electromagnetic Induction for Monitoring of Soil-moisture Dynamics at the Hillslope Scale
Authors P. Dietrich, D. Altdorff and S. Popp-HofmannHydrogeological issues play a major role for slope stability in the context of natural hazards. Therefore, the investigation of subsurface structures related to soil-moisture pattern as well as monitoring of soil-moisture dynamics is essential for the assessment of landslide risks. In this study, we present first results of ongoing soil-moisture monitoring in the western Austrian Alps based on the application of electromagnetic induction. We use a multi-frequency device that delivers electrical conductivity data from various depths. The results of two surveys in different seasons show a depth-related and temporal heterogeneity of conductivity data, related to spatial soil heterogeneity and different hydrogeological properties of the ground. Further mapping surveys for a more detailed analysis of relative soil-moisture dynamics are planned.
-
-
-
Introducing the Okavango Delta, Botswana, Airborne TEM Survey
Authors J.E. Podgorski, L. Kgotlhang, T. Ngwisanyi, C. Ploug, E. Auken, W. Kinzelbach and A.G. GreenThe Okavango Delta, Botswana, is one of the largest inland deltas on the planet and is unique in that surface water remains remarkably fresh despite evapotranspiration being the dominant water removal mechanism. To help understand the phenomena of salt sequestration, airborne transient electromagnetic (TEM) data with 2 km line spacing were acquired over the entire delta in 2007-2008. These data have been inverted using a quasi-2D least-squares linear optimization scheme to produce a 5-layer resistivity model to 250 m depth. Comparison of the model with available borehole data shows a strong correlation between resistivity and lithology and salinity. Further analysis should contribute significantly to understanding the hydrogeology of the delta.
-
-
-
An Automated Electrical Resistivity Tomography System to Monitor the Freshwater/saltwater Zone on a North Sea Island
Authors M. Grinat, W. Südekum, D. Epping, T. Grelle and R. MeyerAn automated electrical resistivity tomography system was developed to monitor changes in the transition zone between the freshwater lenses and the underlying saltwater on the North Sea island Borkum. Main part of the system is a vertical electrode chain of about 20 m length comprising 78 stainless steel ring electrodes mounted on a rigid PVC pipe. The distance between adjacent electrodes is 0.25 m. On the island of Borkum two of these electrode chains were installed in 45-65 m depth in the water catchment areas Waterdelle and Ostland in September 2009. Borehole measurements ascertained the installation depths. The ongoing monitoring measurements are carried out using a modified commercial resistivity meter with active electrodes (4point light 10W). The power is supplied by solar panels and the data are transmitted to Hannover by telemetry. Within the last seven months more than 800 complete pseudosections using a Wenner-alpha array were recorded with each of the two vertical electrode chains. Significant resistivity changes were observed mainly in depths dominated by saltwater, but up to now these changes are considered to be caused by the readjustment of the disturbed conditions at the drilling locations to the normal situation.
-
-
-
Combined Electrical Imaging and Distributed Temperature Sensing to Characterize Groundwater/surface-water Exchange
Authors F.D. Day-Lewis, L.D. Slater, D. Ntarlagiannis, R.D. Henderson and J.W. LaneAccurate understanding of the spatial and temporal distributions and rates of groundwater/surface-water exchange is critical to addressing numerous hydrologic and engineering problems, including, for example, the calculation of contaminant loading to rivers; stream temperature dynamics and aquatic habitat; and stormwater management and flooding. Hydrogeophysical approaches for both characterization and monitoring, respectively, can provide valuable information to delineate areas of enhanced exchange and to monitor such exchange over time. Here, we review recent case studies from the Columbia River, Washington, USA, and Waquoit Bay, Massachusetts, USA. These studies showcase three highly complementary methods: (1) continuous waterborne electrical imaging; (2) electrical resistivity imaging with fixed electrodes; and (3) fibre-optic distributed temperature sensing.
-
-
-
A Multi-borehole 3-D ERT Monitoring System for Aquifer Characterization Using River Flood Events as a Natural Tracer
Authors I. Coscia, S. Greenhalgh, N. Linde, A. Green, T. Günther, J. Doetsch and T. VogtWe are using 3D time-lapse borehole electrical resistivity tomography (ERT) to investigate the hydrological properties of a producing aquifer in north-eastern Switzerland. During the frequent flooding of an adjacent river, relatively high-resistivity river water infiltrates the gravel aquifer. As a consequence, the electrical characteristics of the infiltrating water can be used as a natural tracer to delineate preferential flow paths through the 7 m thick aquifer. For this research we have installed eighteen monitoring boreholes that completely penetrate the underlying aquifer. Each borehole has been instrumented with 10 electrodes that span the thickness of the aquifer. A multichannel resistivity system, programmed to cycle through various four-point electrode configurations of the 180 electrodes in a rolling sequence, allows the collection of approximately 15,500 apparent resistivity measurements every seven hours on a continuous basis. Three-dimensional static ERT inversions at periods of stable hydrological conditions were carried out to investigate the resolving capability of our acquisition scheme, to define the main lithological structures within the aquifer, to study the superposed time-varying effects (e.g. water table height fluctuations, changes in salinity and temperature) on the measurements, and to provide a base model for future time-lapse inversion studies.
-
-
-
Observation of Infiltration Processes by 3D- ERT Measurements: Results and Problems
Authors U. Noell, T. Günther and S. AltfelderAt two sites 3D electrical resistivity tomography (ERT) is used to study the process of infiltration of water through the soil. After the infiltration the sites are excavated and the water content and the water tensions are measured. Characteristic reference functions are derived and these will be fed into a soil water transport model aiming at remodelling the infiltration experiment. The soil type at the field sites is podsol under a plough layer. For the first experiment 110 l of water is infiltrated within 4.6 h on an area of 1.6m x 0.4m. The ERT inversion shows that about 1 day after the infiltration 46 l of water has reached the groundwater table (1.7m). During the second experiment 80 l of coloured water is infiltrated on an area of 1m x 0.4m within 8 h. The ERT inversion reveals that the water is kept relatively close to the surface for the first three hours. Then the infiltration plume moves deeper and seems to come close to the groundwater table (3.2m) 8 h after the start of the infiltration.
-
-
-
A Saline Tracer Test Monitored with ERT to Detect Preferential Flow/transport Paths in Limestones
Authors T. Robert, D. Caterina, J. Deceuster, O. Kaufmann and F. NguyenThe success of a tracer test highly depends on the number and the localisation of the sampling wells. When preferential solute transport paths are expected, one needs to set up carefully the tracer test to recover information such as the local groundwater flow direction and an estimate of the transport velocities. In this work, we used electrical resistivity tomography (ERT) to monitor a saline tracer test. This experiment was performed in fractured limestones where high transport velocities and strong dilution effects were expected. This required a continuous injection and fast ERT acquisition. Two different salt concentrations (40 and 160 g/l) were injected to deal with dilution effects. We also tested the resolution and the depth of investigation of our dipole-dipole sequence by changing the electrode spacing. Two transversal (and a longitudinal) profiles were placed every 20 m from the injection well. During the first test, a maximum of -8 % (-16 % in the second) change of electrical resistivity was observed in the nearest ERT profile while no change occurred in the other ones. We were then able to estimate the transport velocities in addition to the local groundwater flow direction even if the dilution effects were important.
-
-
-
Very High Resolution Hard Rock Seismic Imaging for Excavation Damage Zone Characterisation
Authors C. Cosma, N. Enescu and E. HeikkinenSeismic and Radar investigations performed in 2009 at Olkiluoto in a niche excavated on the side of the ONKALO access tunnel. The objectives have been to confirm the existence of the EDZ at Olkiluoto, to evaluate its extent and to determine its physical characteristics. Three crosshole and one singlehole seismic sections have been measured before and after the excavation. The results of these investigations are presented and discussed. As direct consequence of the EDZ along the tunnel wall, a decrease of acoustic P and S wave velocities and an increase in attenuation can be observed both from direct cross-hole measurements done in the immediate vicinity of the excavated wall and from remote sensing by side-scan measurements done in a borehole parallel to the tunnel wall and away from it. The existence of the EDZ has been proven beyond doubt by both transmission and reflection seismic imaging.
-
-
-
Geophysical Mapping Techniques of Excavation Damage Zone for Quality Control of Drill and Blasting in Crystalline Rock
Authors E.J. Heikkinen, T. Lehtimäki, M. Silvast, P. Kantia, N. Enescu and C. CosmaThe Excavation Damage Zone in crystalline bedrock was mapped under controlled conditions. The work was carried out in drill and blasting excavated Investigation Niché at depth level -350 m along access tunnel to ONKALO underground characterization facility, Olkiluoto. The work belongs to geological disposal project of spent nuclear fuel in Finland, carried out by Posiva. The work was co-operated with SKB, Sweden. EDZ may form hydraulic flow path on the tunnel surface and thus impact to retardation and transportation of radionuclides and further to long term safety of disposal. Quality control of drill and blasting excavation is advised for ensuring that the constructed disposal tunnels will meet the set requirements. Disposal tunnels need to express low hydraulic conductivity. The excavation method to be applied has to enable discontinuous EDZ along the tunnel surface. It was found necessary to develop a reliable mapping and assessment technique of EDZ. Geophysical investigations were applied before and after excavation on same locations. The results were compared between baseline and repeat surveys. Results were compared also between several geophysical techniques including seismic tomography and GPR mapping to understand EDZ property. Mapping and processing technique of GPR dispersivity was developed for efficient EDZ characterization.
-
-
-
Appraisal of Waveform Repeatability and Fidelity for Crosshole Seismic Monitoring of Potential Radioactive Waste Reposit
Authors S. Marelli, E. Manukyan, H.R. Maurer, A.G. Green and S.A. GreenhalghAn experimental study into the viability of remote crosshole seismic monitoring of potential nuclear waste repositories has been undertaken. Full-waveform repeatability measurements to assess source, receiver and coupling effects were carried out at the Grimsel hard rock laboratory in Switzerland. Numerical modelling simulations showed that changes in the anomalous feature to be monitored (bentonite plug) resulted in measurable changes in the seismic waveforms. However, the use of waveform inversion to extract changes in medium properties requires that the changes not be overshadowed by recording variations. We found that a sparker source was highly repeatable up to frequencies of several kilohertz for propagation distances out to several tens of meters. In contrast, we observed large variations of the hydrophone coupling to the host rock when the hydrophone streamer was removed and re-inserted into the boreholes. Our investigations have outlined a quantitative methodology to assess the data quality requirements for successful monitoring. We suggest that seismic full-waveform tomography can be used to monitor radioactive waste repositories provided that careful attention is paid to receiver coupling differences.
-
-
-
Combined Seismic Waveform Inversion for Source Functions, Medium Parameters and Receiver Coupling Factors
Authors H.R. Maurer, S.A. Greenhalgh, S. Marelli, E. Manukyan and A.G. GreenThe quality of receiver-to-ground coupling can be highly variable in surface, surface-to-borehole, and crosshole seismic experiments. Poor coupling can affect the recorded seismic traces in a major way, such that ignoring variable coupling conditions can lead to severe problems when waveform inversions are attempted. To address this issue, we have developed a novel scheme that estimates medium properties and frequency-dependent source functions and frequency-dependent receiver-coupling factors. We demonstrate the efficacy of the new scheme via a synthetic crosshole experiment in which realistic receiver-coupling factors are simulated. Our simulations indicate that a combined inversion for source functions, medium parameters and receiver coupling factors yields results that are comparable to those obtained by using known receiver coupling factors (of course, this is only possible for simulations) and inverting for the source functions and medium parameters.
-
-
-
Data Fusion of Ground Penetrating Radar and Electromagnetic Induction for Reconstruction of Soil Electrical Properties
Authors D. Moghadas, F. Andre, E.C. Slob, H. Vereecken and S. LambotWe jointly analyzed the ground penetrating radar (GPR) and electromagnetic induction (EMI) synthetic data to reconstruct the electrical properties of multilayered media. The GPR and EMI systems operate in zero-offset, off-ground mode and are designed using vector network analyzer technology. We compared different approaches for GPR and EMI data fusion. As a first approach, we weighted the EMI and GPR data using the inverse of the data variance. The ideal point method was also employed as a second weighting scenario and the third approach is the naive Bayesian method. Synthetic GPR and EMI data was generated for the particular case of a two-layered medium. Analysis of the objective function response surfaces from the two first approaches demonstrated the benefit of combining the two sources of information. However, due to the variations of the GPR and EMI model sensitivities with respect to the medium electrical properties, the formulation of an optimal objective function based on the weighting methods is not straightforward. While the Bayesian method relies on assumptions with respect to the statistical distribution of the parameters, it may constitute a relevant alternative for GPR and EMI data fusion.
-
-
-
Cross-gradients Joint Inversion of Time-lapse Crosshole ERT and GPR Data
Authors J.A. Doetsch, N. Linde and A. BinleyTime-lapse geophysical monitoring is a valuable tool in hydrogeology, but the resulting models can suffer from inversion ambiguities. These ambiguities may be reduced by structural joint inversion of multiple geophysical methods. Joint inversion using cross-gradients constraints has been successfully applied to static geophysical measurements. Here we present the first application to time-lapse data. We show on a synthetic and field vadose-zone water-injection experiment how joint time-lapse inversion of crosshole ERT and GPR data can improve the estimated plume shape and the mass recovery. The 3-D layout and coverage of the ERT measurements help to constrain the GPR model, which is based on 2-D measurements; the GPR data are able to resolve sharper boundaries and allow better estimation of changes in water content. The recovered mass fraction within a plume defined by thresholding is 91% for the individual and 99% for joint inversion of the synthetic data. For the field example, the recovered mass fractions are 84% and 91% for individual and joint inversions, respectively. The method appears to works well for water-tracer experiments, but needs further development before it can be applied to flow and transport problems in unsaturated media.
-
-
-
Borehole GPR Fracture Imaging of a Crystalline Rock Aquifer, Stang-er-Brune, Brittany, France
Authors C. Dorn, N. Linde, J. Doetsch, T. Le Borgne and O. BourThe spatial organisation of fractures and their properties dictate the hydraulic behaviour of fractured rock aquifers. Identification and characterisation of the exact flow paths between the boreholes is an unresolved problem. In an extensively studied crystalline aquifer in Brittany (Stang-er-Brune), borehole logging and hydraulic testing have been used to characterise fractures intersecting boreholes. The results indicate that only a few well-connected fractures are important for describing the hydraulic behaviour. In an attempt to improve our understanding of potential flow paths at this site, we have acquired, processed and interpreted GPR single-hole and cross-hole data using 100 and 250 MHz antennas (down to 100 m depth). To separate the reflected signals from direct wave energy and source-generated noise, we performed a data processing scheme that included bandpass filtering, F-X deconvolution, eigenvector filtering, muting and depth migration. The final stacked and migrated GPR sections clearly image fracture zones and single fractures (dipping 30-90°, 2-14 m away from the boreholes) including large subvertical fractures that were not identified by borehole logging. Most of the fractures previously identified as being hydrologically important and dipping 30-90° could be recognised as moderate to prominent reflections.
-
-
-
Permeability Estimates from Poro-elastic Dispersion Analyses of Sonic Logs
Authors L. Baron and K. HolligerModern sonic logging tools designed for shallow environmental and engineering applications allow for P-wave phase velocity measurements over a wide frequency band. Methodological considerations indicate that, for saturated unconsolidated sediments in the silt to sand range and source frequencies ranging from approximately 1 to 30 kHz, the observable poro-elastic P-wave velocity dispersion is sufficiently pronounced to allow for reliable first-order estimations of the underlying permeability structure. These predictions have been tested on and verified for a surficial alluvial aquifer. Our results indicate that, even without any further calibration, the thus obtained permeability estimates as well as their variabilities within the pertinent lithological units are remarkably close to those expected based on the corresponding granulometric characteristics.
-
-
-
Retrieving 2D Structures from Surface Wave Data by Means of a Space-varying Spatial Windowing
Authors P. Bergamo, D. Boiero and L.V. SoccoThe Surface Wave (SW) techniques are mainly used to retrieve 1D subsoil models. However, in 2D environments the 1D approach usually neglects the presence of lateral variations and, since the SW path crosses different materials, the resulting model is a simplified description of the site. We propose a processing technique to retrieve 2D structures from SW acquired with a limited number of receivers. Our technique is based on a two step process: first of all several local dispersion curves are extracted along the survey line using a spatial windowing based on a set of Gaussian windows with different shape; the windows maxima span the survey line so that for every window a dispersion curve can be extracted from the seismogram, thus retrieving a set of dispersion curves each of them referring to a different subsoil portion. This space-varying spatial windowing provides a good compromise between wavenumber resolution and the lateral resolution of the obtained local dispersion curves. In the second step of our procedure the retrieved set of dispersion curves is inverted using a laterally constrained inversion (LCI) scheme. This procedure has proven to be effective for the processing of both real and synthetic data.
-
-
-
Controlled Seismic Imaging in Determining AVA Curves under Shallow Complex Structures
Authors J. J. Silva, D. M. Soares Filho, M. A. Martins and L. LandauOne of the main objectives of seismic imaging is to generate maps of the Earth's reflectivity function, which can be inferred about the physical properties of rocks in their locations. The amount of energy that is reflected on an interface depends on the incident’s wave field angle, called AVA (Amplitude vs. Angle) function. In the methodology proposed in this work, the seismic data recorded on the surface is extrapolated to a region just above the datum of the exploratory interest. This redatum is achieved by a generalization of the concept of reverse time migration in which common focus point gathers are extrapolated. The synthesis’ operators and extrapolations of the direct and reverse fields are made for solutions of acoustic complete wave equation by finite differences method. It was proposed a new imaging condition based on the sum of modules of amplitudes around the wavelet of maximum amplitude. As a result of this methodology, we have obtained graphs of reflectivity, i.e., amplitude based on incidence’s angle, corrected from effects of propagation (specifically, spherical divergence, transmission and conversion).
-
-
-
Stoneley Wave Modeling in Heterogeneous Porous Media with Viscous Pore Fluids
Authors R. Sidler, J.M. Carcione and K. HolligerWe implemented Biot-type porous wave equations in a pseudo-spectral numerical modeling algorithm for the simulation of Stoneley waves in porous media. Fourier and Chebyshev methods are used to compute the spatial derivatives along the horizontal and vertical directions, respectively. To prevent from overly short time steps due to the small grid spacing at the top and bottom of the model as a consequence of the Chebyshev operator, the mesh is stretched in the vertical direction. As a large benefit, the Chebyshev operator allows for an explicit treatment of interfaces. Boundary conditions can be implemented with a characteristics approach. The characteristic variables are evaluated at zero viscosity. We use this approach to model seismic wave propagation at the interface between a fluid and a porous medium. Each medium is represented by a different mesh and the two meshes are connected through the above described characteristics domain-decomposition method. We show an experiment for sealed pore boundary conditions, where we first compare the numerical solution to an analytical solution. We then show the influence of heterogeneity and viscosity of the pore fluid on the propagation of the Stoneley wave and surface waves in general.
-
-
-
Building 3D Exploration Models from Borehole Geophysical and Petrophysical Data
Authors E. Bongajum and B. MilkereitWe demonstrate how geostatistical tools can be applied to a petrophysical database for constructing a 3D earth model that honours the statistics of the physical properties observed from borehole logs and cores. The framework for using geostatistics in this study is primarily supported by the presence of a relatively dense spatial sampling of the study area with various borehole information which is otherwise absent or sparsely available in most exploration projects. Conditional sequential Gaussian simulation based on cokriging can be useful to constrain the 3D earth model by incorporating the correlation between existing information of various categories based on a preestablished hierarchy. We show an application where derived stochastic density models can be used for mineral exploration. The proposed strategy for building 3D stochastic models honours the information at all available borehole locations. Thus, this approach is applicable to a wide range of near surface exploration problems in groundwater, environmental and geotechnical investigations.
-
-
-
Geophysical Imaging of Discontinuous Permafrost in Northwest Canada
Authors A.F. McClymont, L.R. Bentley, M. Hayashi, B. Christensen and W.L. QuintonIn order to improve predictions of how hydrological processes in regions of low-relief discontinuous permafrost will respond to future climate warming, it is necessary to understand how discrete bodies of permafrost thaw in the subsurface. Using data collected within the Scotty Creek research basin in the Northwest Territories of Canada, we demonstrate how electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) surveying can be used to delineate accurately the two-dimensional structure of permafrost bodies to depths of around 15 m. Our ERT and GPR images reveal that the permafrost bodies have thicknesses of around 7-12 m, are characterized by steep lateral boundaries and have variable thaw depths beneath permafrost plateaus. Beneath one permafrost plateau, a thaw-depth depression confines some groundwater within an anomalously thick layer of saturated peat. We suggest that enhanced thawing at this location will eventually cause groundwater to 'punch' through the permafrost and provide a conduit for surface runoff to infiltrate into the deep groundwater system.
-
-
-
Determination of Ice-thickness Using GPR at Brandner Glacier in Vorarlberg, Austria - Case Study
Authors I. Kreutzer and A. FreudenthalerThe discharge of the Brandner glacier (Austria) is being caught for electricity generation. As the glacier has been rapidly shrinking in the last few years the existing and now dry gallery should be replaced. Therefore a geophysical survey with ground penetrating radar and reflection seismic was carried out in 2009 to determine the thickness of the glacier and the lowest level of the bedrock. A 40MHz antenna was used for GPR and 4m geophone spacing for the seismic. The depth of the bedrock from GPR and seismic data correlates very well. Reflections in the GPR data down to 100m depth are identified. The seismic velocity for ice is about 3700m/s, the bedrock velocity 4600m/s. The GPR data show that moraine material and stones lie within the ice body. A 2D map was made with kriging-technique based on the line data identifying two troughs. The estimated ice-volume is 14 million m³. The thickest part of the glacier is in the upper trough and goes down to 80 m. In the lower trough the main thickness is around 20 m. The lowest elevation of the bedrock surface is situated at the eastern end of the glacier.
-
-
-
Geophysics in Glacial-hazard Initiation Zones, Russian Caucasus
Authors I. Lecomte, F. Köllner, D. Petrakov, S. Chernomorets, M. Shakhmina, S.E. Hamran, H. Juliussen and A. KääbNumerous glacier lakes have formed in recent decades due to worldwide glacier retreat induced by climate change. These lakes, dammed by glaciers and moraine ridges, are hazardous because of potential glacial lake outburst flows (GLOF). The GLOF probability is increasing in the Russian Central Caucasus, like at the Bashkara glacier which has been extensively studied, but detailed information about the ground is missing. A pilot geophysical campaign carried out during summer 2009 tested GPR and resistivity profiling at this site, using towed-systems to facilitate acquisition. The GPR measurements were successful with penetration depth down to 70 m on icy ground, though the acquisition was difficult due to rough ground terrain. The results show that GPR measurements would greatly improve the knowledge of the internal structure of that complex zone, thus helping for hazard assessments, but more field work is needed, including CMP measurements. The resistivity measurements were not that successful, the towed system requiring repeating each profile with increasing offset, the progression on the ground being heavy. Only the very first meters of the ground were retrieved, i.e, not really providing useful information. Results and experience gathered in 2009 are now analysed to plan another campaign summer 2011.
-
-
-
Locating Bulk Water within the Tête Rousse Glacier (French Alps) Using the MRS Method
Authors A. Legchenko, M. Descloitres, H. Guyard, C. Vincent, S. Garambois, K. Chalikakis and M. EzerskyUnder mountain conditions accumulation of water in high reservoirs may present a natural hazard for local population and infrastructures. One of the possible water sources could be glaciers with melting ice. Examples of water accumulation that caused catastrophe events are known in history. The main objective of our study was to investigate the possibility of accumulation of non-frozen water within the Tête Rousse glacier (Massif of Mont Blanc, altitude of 3200 m). The fieldwork was carried out in September, 2009. For detecting bulk water we applied the Magnetic Resonance Sounding method (MRS) selectively sensitive to groundwater. We performed nine soundings with 80×80 m2 square loop that covered the major part of the glacier. MRS measurements allowed us to detect bulk water with a high degree of reliability and to locate the principal water storage area. The total volume of the water was estimated with MRS between 60000 and 70000 m3. This result will be verified during the summer 2010 by using additional geophysical measurements and boreholes.
-
-
-
Combined GPR-FWD Survey to Detect Low Compaction Zones in a Harbour Area
By J. EndomSudden subsidence and occurence of voids and cavities in a harbour area lead to the developement of a new combination of two well known non destructive test methods - GPR and FWD - to identify areas of low compaction in a fast and cost effective way.
-
-
-
Geophysical Survey of a Municipal Waste Landfill Using Electrical Resistivity and Induced Polarization Methods
By P. HolubAbandoned gravel pits have been often used as landfill area for the municipal and industrial waste. From 1971 to 1990 the ancient gravel pit En Colliare (north of the city of Lausanne, western Switzerland) was filled with slags of incineration, garbage from car shredding and municipal waste. The base of the landfill is in its central part close to groundwater table and landfill leachate polluted groundwater and the near river Venoge. Objective of this survey was to test the applicability of the geophysical method at this particular landfill site. Electrical resistivity and induced polarization 2D tomographic survey has been carried out to add quantitative data about the landfill content and its volume. In addition to the photogrammetric data, these results have served for the remediation project of the landfill. Since 2005 a new project has been prepared. The preparatory works have started in November 2008. Induced polarization (IP) method seemed to be more effective than electrical resistivity method due to variable lithological conditions and heterogeneous landfill content. Nevertheless, the combination of both methods proved to be valuable for the assessment of the landfill dimensions.
-
-
-
Geophysical Assessment of Soil Volumes Polluted with Metal Elements from Long-term Waste Water Irrigation
Authors A. Tabbagh, J. Thiesson, R. Guerin, M. Dabas, M. Thiry and F. van OortAt WNW of Paris, a large area of coarse textured soil have been irrigated during one century (until 1999) with urban waste water so that large amount of soil organic matter (SOM), dissolved mineral compounds and metal pollutants accumulated in the surface horizons. One must now identify the volumes of polluted soils in order to favour as the stabilization or decontamination of these sites. Resistivity mapping and complementary magnetic properties measurements are used to assess the volume of polluted soil.
-
-
-
A Hierarchic Approach for the Characterization of CO2 Permeable Pathways Using Geophysical and Geochemical Methods
Authors C. Schuetze, H. Lamert, U. Schneidewind, U. Werban and P. DietrichBefore large-scale underground CO2 storage can take place, it will be necessary to demonstrate that the processes are well understood, risks to the environment and population are low, and environmental disturbances are negligible. One of the main research questions is to understand the transport processes of CO2 from the reservoir through the earth’s crust to the surface. The locations of degassing areas are linked to faults acting as preferential pathways for the fluids. An important issue for the acceptance of the CO2 storage in geological formations is the availability of methods for the detection and monitoring of potential CO2 degassing. For an efficient and fast as well as cost effective assessment of different types of potential CO2 degassing different methods and technologies from chemistry, hydrogeology, geophysics and biology should be combined. The here presented study was carried out with the purpose to develop a sophisticated approach to investigate and to observe potential CO2 emanations in large areas with high resolution. For the secure monitoring of future geological CO2 storage sites an adapted concept for the investigations is crucially needed. The recent results obtained at natural CO2 degassing areas represent the successful application of the underlying basic hierarchical concept.
-
-
-
Geophysical Assessment of Contamination from a Wastewater Treatment System in the Milltown Lake Catchment, Ireland
Authors S. Donohue, R. Flynn, V. McCarthy, A. Orr, P. Rafferty and K. GalvinResidential on-site wastewater treatment systems (OSWTS), where contaminated wastewater discharges to the subsurface, act as the dominant means of domestic wastewater disposal in rural Ireland; septic tanks constitute the most common technology employed. The objective of this study was to determine the effectiveness of a number of non-invasive geophysical techniques, employed in conjunction with hydrogeological data, for characterising the three dimensional extent of a contaminant plume generated by septic tank effluent discharging to glacial-till subsoils at a test site within the Milltown Lake Catchment, Co. Monaghan, Republic of Ireland. It was found that the integrated use of three geophysical techniques, electromagnetics, electrical resistivity tomography and seismic refraction with existing hydrogeological and water quality data, significantly improved our understanding of the contaminant plume and associated subsurface contaminant pathways. The geophysical results, when combined with hydrogeological data suggest that the majority of wastewater contamination originating from the septic tank is being transported through a fractured zone of bedrock, far quicker than originally expected.
-
-
-
A High Sensitivity Impulse Neutron Method for Real-time Detection of Elements and Compounds in Soil and Subsoil Water
More LessThe PNN method was run as continuous monitoring under various conditions with different tasks like pollution control for organic and inorganic compounds. For many elements are established regressions for quantitative estimations. It does not matter under which conditions the PNN is performed, along the individual approach of the decay curve disturbing factors can be deleted and therefore always a representative result may be achieved. The PNN method is still undiscovered for a wide application to environmental problems. Technological adaptations may allow an continuous monitoring and even a neutron tomography on the whole polluting path water-soil-air.
-
-
-
Airborne Geophysical Survey and Innovative Landslide Monitoring at Gschliefgraben, Austria
Authors R. Supper, I. Baron, A. Ita, E. Winkler, B. Jochum and K. MotchkaNew geophysical approaches for landslide investigation and monitoring were tested in the site of Gschliefgraben by the end of September 2009. Multiparameter airborne geophysics is a promising method for landslide investigation which could quickly determine the most susceptible areas to mass wasting (electromagnetics) and areas with active shallow mass movements (passive microwave); the content of radioactive elements could contribute to studying displacement or weathering degree. The introduced new techniques for mass movement monitoring and early warning (GEOMON4D and D.M.S. automated monitoring systems) proved, that such a combination of high-accuracy monitoring of displacement, ground-water level and ground temperature together with monitoring of the ground resistivity and self-potential temporal changes could definitely be an effective approach for early warning.
-
-
-
Airborne EM Mapping of Rock Slides and Tunneling Hazards
Authors A.A. Pfaffhuber, E. Grimstad, U. Domaas, E. Auken, M. Halkjaer and N. FogedThe inner Aurland fjord and the adjacent Flåm valley (Western Norway) are subject to potential rock slides comprised of creeping rock- and debris masses. Based on indications that precipitation drives the sliding movements, the local municipality and regional hydroelectricity company are evaluating the option to drain the unstable area with a more than 10 km long drainage tunnel to a nearby hydropower reservoir. Both rockslides and tunnel corridor encounter phyllite, a low grade metamorphic rock type that is potentially reworked to clay in disturbed zones. Water saturated clay is a strong conductor and thus an ideal target for an electromagnetic (EM) survey. We conducted an airborne EM mapping survey to find indications for the sliding planes and to assess the tunnel corridor for potential tunnelling hazard areas. Spatially constrained inversion of the 250 line km data set reveals extended conductive zones that we interpret as sliding planes and/or gneiss / phyllite interface. Detailed follow up of initial results is planed with limited percussion drilling and ground resistivity surveys.
-
-
-
Mudslide-bedrock Interaction at Super-Sauze (French Alps) by UAV-based Remote Sensing and Nanoseismic Monitoring
Authors M. Joswig, U. Niethammer, S. Rothmund and M. WalterThe Super-Sauze (French Alps) mudslide was observed by high-resolution aerial photography from UAV (unmanned Aerial Vecicle) and Nanoseismic Monitoring in 2008. Remote sensing could resolve mudslide movement, and different types of superficial fissure patterns. Nanoseismic Monitoring identified seismic event types of ‘rockfall’, ‘fracture’ and ‘scratch’. The spatial distribution of the 'fracture' epicenters and ‘scratch’ source areas indicate the generation of these events close to the 'in-situ' crests of bedrock topography, most of them hidden below the mudslide material today. Seismic source areas correlate to the spatial distribution of observed fissure patterns which diverse into extension and shear fissures of varying size, and to secondary scarps. The superficial fissure patterns reside at stable position despite the moving mudslide; their spatial distribution, dimension, shape and orientation relates directly to the in-situ crests and the lateral bedrock boundaries. Our hypothesis from joint analysis of both observation data is that ‘fracture’ impulses are induced by brittle failure of the uppermost, dried mud layer, while ‘scratch’ sequences are initiated by “scratching” and “grinding” of single rock particles within the mudslide against the hard rock in-situ crests.
-
-
-
Tracking the Movements of Electrodes on an Active Landslide over Time Using Only Time-lapse Resistivity Data
Authors P.B. Wilkinson, J.E. Chambers, P.I. Meldrum, D.A. Gunn, O. Kuras and R.D. OgilvyThe movements of permanently installed monitoring electrodes on an active landslide will cause artefacts in the resulting resistivity images if their positions are not continuously updated and incorporated in the inversion. In this paper we investigate the effects of electrode movements on time-lapse resistivity tomography using a simple analytical model and real data. The correspondence between this model and the data is sufficiently good to be able to predict the electrode movements with reasonable accuracy. We show that the model can be used to invert the downslope displacements of the electrodes from their original baseline positions using only the time-lapse ratios of the apparent resistivity data. The example datasets are taken from an electrode array on an active lobe of a landslide. We show that the electrode positions can be recovered to an accuracy of 4% of the baseline electrode spacing, which is sufficient to correct the artefacts in the resistivity images. Using a time-lapse sequence of resistivity data, we demonstrate that this technique can be used to track the movement of the landslide over time to the same level of accuracy.
-