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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
21 - 40 of 153 results
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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).
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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.
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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.
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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.
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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.
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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.
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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.
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