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Near Surface 2009 - 15th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 07 Sep 2009 - 09 Sep 2009
- Location: Dublin, Ireland
- ISBN: 978-90-73781-72-6
- Published: 07 September 2009
21 - 40 of 139 results
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High Resolution Seismic Profiling for Tunnel Engineering at Olkiluoto, Finland
Authors C. Cosma, N. Enescu, T. Ahokas and E. HeikkinenThe ONKALO tunnel is the underground rock characterisation facility being built for rock characterisation for the final disposal of spent nuclear fuel at Olkiluoto, in Finland. The bedrock is studied, during the construction phase, by geology, geophysics, hydro-geology and geochemistry and rock mechanics. The research is meant to help ensure the suitability of the bedrock for the final disposal. A pilot test of high resolution seismic was done, consisting of reflection imaging ahead and aside of the tunnel and refraction imaging along the same line, 100m long, with sources stations spaced at 1m. Refraction imaging was done from two groups of 10 shots, recorded by the 100m long receiver array. Fractures making an angle of 30° or more with the tunnel axis were imaged several tens of meters away. Vertical fractures perpendicular to the tunnel could also be imaged, but only in the vicinity of the tunnel. Gently dipping fractures were imaged below and above. With only one line along the wall, it may not be possible to resolve between the above–below locations.
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Tunnel Seismic Investigations – 3D Imaging
Authors J. Tzavaras, K. Groß and S. BuskeThe aim of our work is to determine the spatial locations of faults, fractures and heterogeneities by seismic imaging and then to investigate the seismic properties of these heterogeneities by numerical modeling. We performed a 3D ”Kirchhoff Prestack Depth Migration” (KPSDM). P- and S-wave velocity models were used to migrate the receiver gathers. We extended the 3D KPSDM by applying “Reflection-Image-Spectroscopy” (RIS) to the data. Slowness and polarization analysis is used to calculate the emergence angle of incoming reflections. Direction estimates are derived by the cross power matrix method. “Fresnel Volume Migration” (FVM) takes into account the emergence angle and restricts the imaging to the region around the actual reflection or diffraction point.
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Investigation of Lateral Lithological Heterogeneities and Groundwater Salinity Using MRS and TEM Methods
Authors A. Legchenko and M. EzerskyTransient Electromagnetic (TEM) and Magnetic Resonance Sounding (MRS) methods were applied jointly to investigate lithological variations in the Dead Sea coast of Israel. The subsurface in this area is heterogeneous and composed of intercalated sand and clay layers over a salt rock, which is partly karstified. Groundwater is very saline, with a chloride concentration of 100-225 g/l. TEM is known as an efficient tool for investigating electrically conductive targets like saline water, but it is sensitive to the salinity of groundwater, the clay content and the porosity of rocks. MRS, however, is sensitive primary to groundwater volume but also to lithological variations in the subsurface. MRS is much less sensitive to variations in groundwater salinity in comparison with TEM. We show that MRS enables us to resolve the fundamental uncertainty in TEM interpretation caused by the equivalence between groundwater resistivity and lithology. Combining TEM and MRS in Nahal Hever area we have identified lateral extensions of different lithological formations.
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Dependency of NMR Relaxation Behavior from Water Saturation Degree in Loose Sediments
Authors S. Costabel, F. Alali and U. YaramanciNuclear Magnetic Resonance (NMR) applications in geophysics (in the lab, in boreholes and the field, respectively) provide estimations of the water content and, in the case of full water saturation, the saturated permeability. The NMR relaxation behavior at full saturation contains the structure information of the pore space, i.e. the pore size distribution. In principle, this information should also allow for estimating the unsaturated permeability as a function of the saturation degree. The key to do this is to understand and predict the relationship of the NMR relaxation times and the saturation degree. We introduce a reliable model to quantify this relationship and show its plausibility for various loose materials covering a broad range of grain sizes. We show the similarities of our approach to the Brooks-Corey model that is a standard model in soil sciences to estimate the relative hydraulic conductivity. Furthermore we show that the saturation dependency of the relaxation time distribution can be predicted from a single NMR measurement at full saturation. For such predictions we assume the pore space to consist of capillaries with various cross-sectional areas (circular and triangle-shaped cross-sections).
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Improving Aquifer Characterization Using Magnetic Resonance Sounding
Authors J.M. Vouillamoz, A. Legchenko, J. Hoareau and M. GrammareDuring the last decade several attempts have been made to quantitatively characterize aquifers from magnetic resonance sounding (MRS) results. However a reliable relationship between MRS water content and hydrogeological storage-related parameters has never been reached, and estimate of aquifer transmissivity from MRS results is not always reliable. In some cases theses failures can be explained by the heterogeneity of the geomagnetic field that corrupts the currently measured free induction decay (FID) signal of MRS. To overcome the limitation of MRS in non-homogeneous geomagnetic field, we adapted the spin-echo (SE) methodology routinely used for laboratory measurements. We present examples of sounding carried out in a sandy aquifer of Southern India where both SE and FID signals have been recorded. We found that the volumetric groundwater content estimated from SE measurements is higher than that derived from FID measurements (from 20% to 70%). Results of a groundwater model are in good agreement with SE measurements. Analysis of our results shows that underestimate of groundwater content and poor estimate of aquifer transmissivity could easily take place when performing FID measurements because small heterogeneity of the geomagnetic field can be unnoticed. Thus spin-echo measurements could be a way for improving aquifer characterization in many geological contexts.
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Large Scale Mapping of Groundwater Resources Using a Highly Integrated Set of Tools
Authors V.S. Søndergård, E. Auken and A.V. ChristiansenThe aim of this abstract is to give a short description of the essential ideas of the Danish national strategy for large scale mapping of the groundwater resources. Emphasis will be put on a description of the advantages obtained by combining acquirement of spatially dense geophysical data covering large areas with information from an optimum number of new investigation boreholes, existing boreholes, logs and water samples to get an integrated and detailed description of the groundwater resources and their vulnerability. Development of more time efficient and airborne geophysical data acquisition platforms (e.g. SkyTEM) have made large-scale mapping attractive and affordable in the planning and administration of groundwater resources. The handling and optimized use of huge amounts of geophysical data covering large areas has also required a comprehensive database, where data can easily be stored, documented, extracted, interpreted, recombined and reused one time after the other.
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Monitoring Saline Intrusion of a Coastal Aquifer with Automated Electrical Resistivity Tomography
Authors R.D. Ogilvy, O. Kuras, P.I. Meldrum, P.B. Wilkinson, J.E. Chambers, M. Sen and P. TsourlosAn Automated time-Lapse Electrical Resistivity Tomography (ALERT) system has been developed for the remote long-term monitoring of vulnerable earth systems. This ALERT technology has been permanently installed in the River Andarax, Almeria, Spain to monitor and manage the impact of climatic change and land-use practice on the underlying Quaternary aquifer. An electrode array, nearly 1.6 km long, has been buried below the normally dry river bed. The maximum depth of investigation is about 150 m below ground level. An unmanned, permanent control station, in a secure location, allows the aquifer to be interrogated remotely from the BGS office in the UK. Spatial geoelectric images of the subsurface can be obtained “on demand” or at regular intervals; thereby eliminating the need for expensive repeat surveys. The entire process from data capture to image on the office PC is automated and seamless. The ALERT technology provides early warning of potential threats to the aquifer such as over-exploitation, rising sea levels, anthropogenic pollutants and seawater intrusion. The electrical images obtained (in space and time) are interpreted in terms of the hydrogeologic features including the seawater-freshwater interface. The timely detection and imaging of groundwater changes can help to regulate pumping and irrigation schemes.
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Some Observations on Imaging Inter-aquifer Leakage Using Airborne EM Technologies
Authors T.J. Munday, A. Fitzpatrick, A. Viezzoli, E. Auken, V. Berens and K. CahillKey to the effective management of natural ecosystems that characterise the floodplains of the Murray Basin in south eastern Australia, and maintenance of river health (a critical water resource) in a setting where severe salinisation is an ever-present threat, requires a sound understanding of surface water-groundwater processes. This paper presents results from an examination of hydrogeophysics, specifically airborne electromagnetics (AEM) data acquired by the SkyTEM time domain helicopter EM system, as a means for improving our knowledge of spatial patterns associated with inter-aquifer mixing where groundwater flow is complex. In the south-eastern part of the Murray Basin, AEM data shows considerable promise as a means for understanding of groundwater quality and its lateral variability. In the Bookpurnong and Loxton irrigation areas the high moment capability of SkyTEM permits us to investigate variations in the quality of groundwater at depth (>100m), which in turn allows us to visualise how groundwater may be moving across aquitards and within particular aquifer systems.
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Full-waveform Modeling and Joint Inversion of Electromagnetic Induction and Ground Penetrating Radar Data
Authors D. Moghadas, F. André, H. Vereecken and S. LambotExisting ground penetrating radar (GPR) and electromagnetic induction (EMI) sensors present some limitations related to their calibration procedure and the strong simplifying assumptions in modeling the antenna-subsurface system. We present a new EMI model in which a zero-offset, off-ground loop antenna is efficiently modeled using frequency-dependent, complex linear transfer functions. The air-subsurface is described by a Green’s function for wave propagation in three-dimensional multilayered media. We validated our model in laboratory conditions by performing measurements at different heights above a copper sheet as a perfect electrical conductor. Then joint inversion of full-waveform mono-static GPR and EMI data was carried out for the particular case of a one-layered medium. Analysis of the objective functions demonstrated the benefit of combining the two sources of information. Yet, due to the variations of the GPR and EMI model sensitivities with respect to the properties of the medium, the formulation of an optimal objective function is not straightforward.
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Using Electrical Resistivity Tomography and Self-potential Methods for Wells Implementations in Fractured Limestones
Authors T. Robert, A. Dassargues, S. Brouyère, O. Kaufmann, V. Hallet and F. NguyenElectrical resistivity tomography (ERT) and self-potential (SP) investigations were conducted in fractured limestones in Belgium. The aim of this study was to find suitable positions for high yield water wells. Large ERT profiles (640 meters) allowed us to image the resistivity distribution of the first 60 meters of the subsurface and to detect and characterize (in terms of direction, width and depth) fractured zones expected to be less resistive. Data errors, DOI indexes and sensitivity models were analysed in order to calculate the depth of investigation of ERT and to avoid the misinterpretation of the resulting images. Self-potential measurements were performed along electrical profiles to narrow the possible locations given by the electrical images. Some negative anomalies possibly related to preferential flow were detected. ‘Ground truth’ geological data as well as pumping tests information gave us a way to assess the contribution of geophysics to a drilling programme. Wells implemented in low resistivity zones associated with SP anomalies have very high yields. Inversely, wells drilled in resistive zones or outside SP anomalies have poorer capacities. An apparent coupling coefficient between SP signals and differences in hydraulic heads was also estimated in order to image the water table.
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A Geographical Information Systems based Analysis of Resistivities of Carboniferous Lithologies in Ireland
Authors S. O' Rourke and P. O' ConnorCarboniferous limestones and associated lithologies are the most abundant rock types in Ireland. The few published summaries of geophysical properties of common ground materials in Ireland have qualitatively assigned the resistivities of these rocks across large ranges There is a necessity for a more precise analysis of values of resistivity across general lithologies and for individual formations, based on a systematic analysis assessment of available field resistivity results. Inverted resistivity values were selected from profiles which were interpreted to be representative of fresh unweathered bedrock not subject to major faulting or karstification. Thematic maps and histograms of the median resistivities of formations and rock types were then generated to show variations of resistivity for Carboniferous lithologies, and variations with location within individual formations. Overall median resistivity values for each of four divisions were as follows: 140 Ohm-m for Namurian mudstones, shales and siltstones; 507 Ohm-m for shaly limestones; 1108 Ohm-m for intermediate muddy limestones; 1882 Ohm-m for massive/oolitic limestones. Selected other results include a median resistivity of 522 Ohm-m for Lucan Formation and Calp combined, and 2038 Ohm-m for Waulsortian Limestones.
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Application of Radiomagnetotellurics on Geomorphological Studies
By B. TezkanA radiomagnetotelluric (RMT) survey is carried out to solve geomorphological studies concerning the evolutionary history of the terraces of the river Rhine in the Lower Rhine Bay next to Cologne, Germany. Geomorphological studies concern e.g. the evolutionary history of the valleys and the classic geomorphologic approach is to take drilling samples to examine the spreading of the covering sediments. The geophysical task is to map continiously the two boundaries between tertiary sands, fuvial gravel and loess. The measurements were realized using the new tensor RMT-device developed recently. Horizontal components of the magnetic and electric fields were observed in the frequency range from 10 kHz to 1MHz. Military and civilian radiostations broadcasting in this frequency range were used as transmitters. Transfer functions (e.g. apparent resistivities and phases) are determined by spectral analysis using a newly developed processing software. The observed apparent resistivities and phases were reliable and they showed a smooth frequency dependence. The data was inverted for 2D conductivity models and compared with two-dimensional inversion results of DC measurements and with existing boreholes. The good comparability indicates that radiomagnetotellurics can be optimally applied to this kind of geomorphological questions.
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Near Surface Case Studies Using RMT Measurements in Sweden
Authors M. Bastani, L. Persson and L.B. PedersenFour examples from the radiomagentotelluric (RMT) measurements in Sweden are selected to present application of the method in the near surface studies. Detection and mapping the details of a low resistivity water-bearing fracture zone in resistive crystalline bedrock is shown in the first example. Estimation of the depth and lateral extension of hazardous sulphidic sediments is the aim in second example. Results of 2D inversion of RMT data along a profile show that the contaminated sediments are considerably less resistive than the surrounding sediments. Use of RMT method in the engineering investigation at a bridge construction site where the modelled overburden thicknesses are in good agreement with those determined by the geotechnical tests is presented in the third example. At the end the results from RMT measurements with the purpose of mapping the depth extent of a resistive and homogeneous reef limestone that has a variety of industrial use are presented.
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Pipeline Insulation Quality Estimation with Superficial Electromagnetic Methods
Authors V.A. Shevnin, A.A. Mousatov, E.K. Nakamura and O. Delgado-RodriguezEstimation of insulation quality from the earth surface with the help of magnetic and electrical measurements is important task for pipeline industry. Pipelines are under operation in underground conditions during tens of years and periodical inspection should be performed. Some pipes can't be inspected from inside, only superficial inspection is possible. Pipeline inspection works with very long profiles and in the same time needs high resolution. In DC electrical methods only electrical signals are measured while in AC electromagnetic methods mainly magnetic field measurements are used. In Mexican Petroleum Institute new technology of pipeline characterization named Surface Electromagnetic (SEM) Technology for Pipeline Inspection was developed based on the approximation of a metallic pipeline by heterogeneous transmission line with variable distributed parameters. Developed field operations and interpretation procedure allow us quantitative determination of insulation resistance and uncoated area rate evaluated on both magnetic field and electrical field data. Magnetic and electrical observations have different measurement time (magnetic survey is about an order faster) and different spatial resolution to insulation damage. Their integration is quite reasonable and provides complete pipeline characterization because includes reconnaissance survey and detailed study of anomalous zones. The examples presented demonstrate the efficiency of the SEM technology.
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Fully Automated, Data Driven Suppression of Surface Scattering in Large Volumes of GPR Data
Authors S. Carpentier and H. HorstmeyerThe occurrence of undesired surface scattering in GPR data is a well-known problem in GPR studies. Resulting false interpretation of such events is not uncommon. Two categories of approaches to suppress these types of air diffractions have been proposed by previous authors: a migration (semblance) based detection method combined with synthetic forward modelling plus subtraction of air diffractions, and a 2D (directional) filtering method. Both methods fell short for us in treating a large volume (32+ km of lines) of GPR field data from New Zealand. For the migration based approach, too much leftover surface scattering energy was observed in case of clustered and distorted events, and the 2D filtering approach was not an option given the tedious manual application and risk of removing real dips from the original data. Therefore, an improved, fully automated, data driven surface scattering suppression algorithm was developed. Because it uses the complex amplitude- and phase patterns of the original surface scattering for the forward modelled diffractions and subtraction thereof, this algorithm can better suppress the aforementioned clustered and distorted air diffractions. Application on the large volume of GPR field data yields exciting results, where desired subsurface features emerge from underneath the suppressed surface scattering.
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Use of Principal Component Images for Classification of the EM Response of Unexploded Ordnance
By M.W. AstenThe location and identification of unexploded ordnance (UXO) is a major challenge for environmental rehabilitation of former military firing ranges and bombing target areas. EM methods are in widespread use for the location of metal objects, however the presence of large quantities of scrap metal from successful detonation of munitions makes discrimination between munitions and scrap and munitions of various sizes a necessity in order for efficient location, digging and removal of UXOs to proceed. Several recent papers show that detailed three-component EM measurements followed by inversion to dipole moments of an EM target is effective in characterising a target, however such techniques require precise data, usually from stationary data acquisition. EM data acquired from a moving ground platform for UXO detection is typically high in motion-induced noise which limits the usefulness of decay-curve analysis in target characterization. We use a data set from the Australian Air Force Newholme UXO Test Range, Armidale, NSW, and show that false-color images of principal-component transforms of the data are superior to images of individual time windows, or images of adaptive decay index, for the purpose of differentiating between different types of munitions.
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Negative Apparent Resistivity Effect by a Steel Casing Borehole Near Electrical Dipole-Dipole Survey Line
By H. JungIn DC resistivity field measurements negative readings can occur, but it has been explained until now that these are not caused by subsurface formations if standard electrode arrays are used, therefore these are due to measurement errors. But un-noisy negative data exist even in shallow cases of dipole-dipole spread n = 1 or 2. There was not any numerical study showing the possibility of actual negative apparent resistivity different from negative IP effect. In this study negative apparent resistivity effect due to a steel casing borehole is examined by verified 3-D numerical modeling program. The apparent resisitivity pseudosection shows pants-leg effect, and abnormal high apparent resistivity trends occur closely to negative data. From various numerical modeling results, -ρa by one steel casing borehole near resistivity survey line can be acquired with dipole-dipole array or pole-dipole array. Obviously, negative apparent resistivity anomalies even in the flat area are subsurface information. Because the misinterpretations lead to seriously wrong pitfall, inversion techniques for those effects need to be developed in the near future.
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Optimizing Electrical Resistivity Array Configurations by Using a Method Based on the Sensitivity Matrix
Authors E.N. Athanasiou, P.I. Tsourlos, C.B. Papazachos and G.N. TsokasIn this work a strategy for optimizing ERT data sets based on the sensitivity matrix is examined and compared with the existing optimization schemes as well as with the most commonly used traditional arrays. Synthetic data tests illustrate that existing ERT data optimization approaches are highly dependent on the subsurface resistivity. As a result we also propose optimizing ERT measurements for an average subsurface background resistivity model rather than homogeneous ground. This approach is not practical for routine data collection but is highly suited to time-lapse ERT monitoring. The effectiveness of the approach is demonstrated with synthetic examples run with an ERT inversion algorithm that is based on a finite element forward solver. Further, the presented algorithm is tested on field data. The results demonstrate that the optimum data sets can provide improved subsurface images in relation with existing arrays.
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High Resolution Observations of 137Cs in Northern Britain and Ireland from Airborne Radiometric Data
Authors C. Scheib and D. BeamishThis study presents high-resolution airborne geophysical estimates of the distribution of 137Cs across three areas of northern Britain and Ireland. The radiometric spectra were acquired as part of a program of modern resource and environmental surveying. The largest survey area considered covers the whole of Northern Ireland. All three data sets display some clustering on high ground together with regional scale banding features. The two main banding directions are approximately NNW-SSE and NW-SE. Our interpretation of the regional scale features is in relation to existing knowledge of the wet deposition model of the Chernobyl release of 1986. The airborne estimates, obtained at 200 m flight line intervals, add significant detail in relation to the meteorological and atmospheric interactions involved in the fallout from the Chernobyl plume over Western Europe.
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Combining Airborne Electromagnetics and Drillings to Construct a Stochastic 3D Lithological Model
Authors J.L. Gunnink and B. SiemonAirborne (helicopter) electromagnetics (HEM) together with drillings were used to construct a three-dimensional model of the subsurface in an area where glacial incisions of Elsterian age occur. HEM data showed marked variations in resistivity in the upper 70 m of the soil that could be ascribed to variation in lithological properties of the sediment. Linking borehole data to resisitivity enables the development of a three dimensional lithological model of the area.
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