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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 - 20 of 153 results
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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