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Near Surface 2006 - 12th EAGE European Meeting of Environmental and Engineering Geophysics
- Conference date: 04 Sep 2006 - 06 Sep 2006
- Location: Helsinki, Finland
- ISBN: 978-90-73781-62-7
- Published: 04 September 2006
61 - 80 of 142 results
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Comparison of the Spatial Resolution of Standard and Optimised Electrical Resistivity Tomography Arrays
Authors O. Kuras, P. B. Wilkinson, P. I. Meldrum, J. E. Chambers and R. D. OgilvyTwo new ERT array optimisation strategies are presented that attempt to maximise the spatial resolution of the image for a given number of measurements. Synthetic modelling studies and inversions are used to assess the performance of the optimised arrays against arrays of standard Wenner-Schlumberger and Dipole-Dipole configurations. For the same number of configurations, the optimised arrays are found to perform at least as well as, and mostly better than, the standard arrays.
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Kelvin Inversion of PEC and PMC Spheres for Geoelectric Imaging
Authors E. H. Eloranta and J. J. HänninenThe classical Kelvin inversions in spherical geometry are revisited in the context of geoelectric imaging. A point source of current is outside a perfect electrically conducting (PEC) sphere or outside perfect magnetically conducting (PMC) sphere. Perfect electric conductor is very good conductor with zero electric resistivity. Perfect magnetic conductor corresponds to very good insulator with infinite resistivity. The sphere is embedded in a homogeneous whole space which has constant electric conductivity. Analytical solutions can be obtained by the method of images. The methods are described and numerical demonstrations based on them are presented.
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3D Resistivity Monitoring - An Effective Electrode Array to Follow Preferential Flow during Rainfalls
Authors L. Marescot, N. Denchik and D. ChapellierThe aim of this study is to select an appropriated array type to figure localized horizontal movements of fluid in the subsurface for 3D resistivity monitoring using a permanent electrode device. Due to technical limitations, the number of current and potential electrodes is limited in the field. Therefore, the choice of the appropriated array for the survey is crucial. Using numerical modelling, the most appropriated array (pole-dipole) has been selected to detect lateral variations of the electrical resistivity in the framework of a permanent surveying installation system. A good correlation between results of numerical modelling and a field data example is shown and preferential infiltration can be observed in the field.
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Target Detectability Depths of DC Arrays for Various Models
More LessWe have compared target detectability depths for six different DC geoelectric arrays. Five various 2D inhomogeneity models and two noise levels (5 pc and 10 pc) were assumed. The maximum detectability depths were determined by using the RES2DMOD software. Although the results are model-dependent (they depend both on geometry and resistivity contrast), the best results (namely: the maximum detectability depths) were obtained usually with the pole-dipole (P-DP) and the dipole axial (DP-ax) arrays. The worst results (namely: the smallest detectability depths) were obtained (with one exception) in case of the pole-pole (P-P) and the Wenner-alfa (W-alfa) arrays. The results by using the Wenner-beta (W-beta) and dipole equatorial (DP-eq) array groups are slightly below or above the average. Detectability depth values are comparable exclusively for the same model (partly due to the variable resistivity contrast), but in case of a certain model it can be unambiguously declared, which array is the most effective one.
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Optimizing Resistivity Array Configurations by Using a Non-Homogeneous Background Model
Authors E. Athanasiou, P. Tsourlos, C. B. Papazachos and G. N. TsokasIn this work an algorithm designed to find optimum resistivity arrays is examined and tested for various synthetic data. Further, tests with real data are presented as well. The algorithm is based on existing model resolution matrix approaches for finding optimum arrays, but it differentiates since it studies the effect of using an initial model to calculate an optimum resistivity array. Results obtained with this approach are compared to those obtained from existing algorithms which operate for homogeneous ground and to the most commonly used traditional arrays. The optimum array type acquired by using this approach seems to represent the subsurface structure in the best manner, since it takes into account all existing information. The described approach has practical application mainly in time-lapse ERT surveys.
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Resistivity Imaging Using Flexible Electrode Combination Data Collected with a Multi-Channel, Full Waveform System
Authors L. Marescot, J. Zhe and S. GreenhalghIn this paper we suggest a major break from the usual practice of resistivity imaging using standard electrode arrays, such as Schlumberger, Wenner or dipole-dipole. Rather, one should take as many measurements as practical from a fixed set of electrodes and record from numerous combinations of current and potential electrodes. With this intention, a new prototype multi-channel and multi-electrode, full waveform resistivity acquisition system which permits such recording strategy has been developed. It allows for significant data collection capacity and flexibility and is therefore suitable for 2D or 3D surface or cross-hole resistivity tomography and for general electric monitoring.
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Geophysical Investigations for Hydrogeological Characterization at the Kappelen Test Site (Switzerland)
Authors Y. Hacini, L. Marescot and D. ChapellierVertical electric soundings, 2D resistivity imaging and several logging measurements were performed at Kappelen test site to identify the various geolelectric facies that allowed determining the tabular and horizontal structure of the aquifer. The surface-based geoelectric methods allowed for a reliable characterization of the overall structure and the geometry of the aquifer, while geophysical logging methods allowed for inferring detailed hydrogeophysical characteristics, such as the electrical resistivity, total porosity, global and matrix density and hydraulic conductivity. The synoptic interpretation and integration of this broad and diverse database allows for constraining the key hydrological characteristics and hence forms the basis for the detailed hydraulic modelling of flow and transport process.
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Improving the Signal-to-Noise Ratio of Surface NMR Data Due to the Remote Reference Technique
By T. RadicA new instrument was developed for Surface NMR measurements in noise-contaminated areas. Heart of the new measuring concept is the Remote Reference Technique (RRT), which helps to minimize cultural and natural noise in the SNMR data. The RRT is a very efficient tool because it needs no additional measuring time.
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Feasibility Study of AMT Application to Sedimentary Structure of Tidal Flat
Authors G. Park, B. D. Kwon, C. K. Lee, S. Y. Choi and S. HuhWe have applied the audio-frequency magnetotelluric (AMT) method to the intertidal deposits of Gunhung Bay, west coast of Korea, and analyzed the field data both qualitatively and quantitatively to investigate the morphology and sedimentary stratigraphy of the tidal flat. In order to test the sensitivity of AMT responses, the AMT inversion results were compared with the seismic section. The AMT survey and seismic survey both show the three-layered structure comprising unconsolidated deposits, semi-consolidated sedimentary rocks, and metasedimentary basement. The inversion of AMT data well reveals the upper sedimentary layer of Holocene intertidal sediments. However, it is thought that the AMT inversion tends to overestimate the depth of basement when compared with the seismic section. Since MT responses are not significantly sensitive to the resistivity of middle layer or the depth of basement, the AMT inversion result for the basement may have to be adjusted using other geophysical information like a seismic section or logging data if possible.
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GPR Structure of the Parana Delta (Argentina) - Preliminary Results
Authors A. Casas, F. Colombo and L. RiveroAn important research object when studying river deltas is predicting their internal structure, stratigraphy, lithofacies, depositional history and volume of sand or gravel accumulated. Until recently, prediction was based on morphologic-depositional models developed by comparing surface sediment texture and morphology with subsurface structure in carved exposures and/or drill cores. While this approach is commonly used to make a first approximation about delta structure and processes, ground-penetrating radar now provides a more accurate alternative, at low cost and without disturbance to the ground.
Ground-penetrating radar (GPR) was used to image the internal structure of the Paraná River Delta (Argentina). The topographically low parts of some deltas are unsuitable for ground penetrating radar method because the existence of marine conditions and fine-grained sediments deposited in restricted environments, creating an electrically conductive medium unfavorable for the penetration of radar waves. Then, the second aim was test the ability of GPR to acquire enough subsurface information for radar-facies imaging of the complete structure.
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First Results from Spaceborne Radar Interferometry for the Study of Ground Displacements in Urban Areas
Authors L. F. Loddo, C. M. Crosetto, C. A. Casas, R. G. Ranieri, A. I. Atzori and M. M. ManuntaThe Differential Synthetic Aperture Radar Interferometry (DInSAR) is a remote sensing technique that allows us to produce spatially dense deformation maps with centimetre to millimetre accuracy and to detect and follow the temporal evolution of deformations via the generation of time – series. To do this, the information available from each interferometric data pair must be properly related to those included in the other acquisitions, via the generation of an appropriate sequence of DInSAR Interferograms. In this work we process a large set of data acquired by the European Remote Sensing (ERS) satellites on the Palau y Solitari de Plegamans area located near Barcelona (Spain) and on the Cagliari area (Italy). In the second work we apply an algorithm referred to as Small Baseline Subset (SBAS) proposed by I.R.E.A. - C.N.R. The results demonstrate that the DInSAR technique is a fast and precise method to detect and follow the temporal evolution of deformations, so it is a powerful technique for monitoring the territory and to reduce its vulnerability.
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Information Enhancement for Peatland Investigations - Case Study Torronsuo
Authors G. van den Dool and J. LerssiGeological Survey of Finland (GSF) has contributed considerably to the rapid expansion of the Finnish peat industry and simultaneous helped to implement the National Mire Conservation Programme. In order to process this growing demand of information GSF has enhanced their procedures with modern data / information processing techniques.
To support the peatland investigations the GIS-orientated Data Base Management System (DBMS) was enlarged. This database (Geotietoydin / GTY) holds geological theme-based layers and includes now also information about all peatlands (over 20 hectares) in Finland.
The Peatland Information Layer contains field-, depth- and laboratory-measurement data. In the planning process GTY is used to select the most urgent peatlands (either by industrial or conservational interest), using all available spatial information. The fieldwork procedures are modified to enable the collection of borehole information through hand-held GPS devices. In the field bases this information is automatically validated and transferred into the structure of GTY. While reporting the findings of the fieldwork, with the use of computational data manipulations, the form and structure of the investigated peatlands are more accurate estimated.
This information utilisation has benefited to a more efficient planning, direct information use and knowledge integration in GSF.
Additional information about the methods and techniques used in this presentation can be found at http://www.eracnet.fi/publications/
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An Algorithm for Fast 3D Inversion of Parallel 2D ERT Data-Sets
Authors N. G. Papadopoulos, P. Tsourlos, C. Papazachos, G. N. Tsokas and A. SarrisIn this work, a new automated 3d resistivity inversion algorithm, for parallel 2D ERT lines which is at least two times faster than a standard inversion algorithm, is presented. The basic idea of this algorithm is based on the examination of the Jacobian matrix entries and the fact that due to the 2D arrangement of measuring electrodes there are many 3D parameters for which the calculated Jacobian matrix values are practically zero. These values are automatically identified and excluded from the Jacobian calculation, which is the most time-consuming part of forward modeling. Further, to take full advantage of the sparseness of the resulting Jacobian matrix, inversion is preformed using the LSQR (Least Squares Regression) method which results in faster inversion and has less memory requirements than the conventional approach. The algorithm is compared to a standard 3D inversion routine and proved its efficiency in both synthetic and field data.
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Multi-Salinity Measurements of Spectral Induced Polarisation on Sandstones
More LessThe polarisation effects are studied while saturating selected sandstone samples with brines of different cations. The real part of conductivity shows - as expected - a linear relation to the brine conductivity. The imaginary part of conductivity depends on the kind of cations in the brine with the strongest effect for sodium chloride. At higher concentration, a saturation is observed.
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Inversion of SIP Data for Cole-Cole and Constant-Phase-Angle Relaxation Model Parameters
Authors R. Blaschek, A. Hördt and A. KemnaWe present a tomographic inversion procedure for the computation of intrinsic Cole-Cole or Constant-Phase-Angle (CPA) model parameters from spectral induced polarization (SIP) data. The idea is to invert data at all frequencies simultaneously while using a subsurface model which consists only of the parameters of the relaxation model for each cell, instead of inverting each frequency separately for complex resistivities and fitting a relaxation model to each cell afterwards. Connecting the information from all frequencies during the inversion is expected to yield more consistent models. Both approaches are compared and also applied to a set of surface field data.
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2D and 3D Resistivity Imaging in an Investigation of Boulder Occurrence and Soil Depth in Glacial Till
Authors R. Wisen, F. Linders and T. DahlinThe uncertainty regarding depths to bedrock and occurrence of boulders in glacial till can make the planning of a major road construction more difficult. Despite the fact that geophysics is frequently used in many applications and also occurs as a natural method to solve problems in geotechnical engineering, it has often a subordinate role as a site investigation method prior to road construction in Sweden. This project aims at evaluating 2D and 3D resistivity imaging to determine soil depths and track boulders in glacial till. Hopefully, the extent of the excavations preceding a road construction will thus be better estimated in the future, as the geotechnical investigations are suitably complemented. When compared with existing geotechnical data and brought together, both the 2D and 3D resistivity methods are regarded as good producers of reliable soil depth models. However reference data from alternative methods must always be used for validation and calibration of results. It is shown that a 3D resistivity dataset, consisting of a number of parallel CVES profiles, in some cases can give significantly improved resistivity models.
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Least-Squares Inversion of DC Resistivity Data Acquired over Dynamically Changing Earth Model
By J. H. KimIn the geophysical monitoring to understand the change of subsurface material properties with time, the time-invariant static subsurface model is commonly adopted to reconstruct a time-lapse image. This assumption of static model, however, can be invalid particularly when fluid migrates very quickly in highly permeable medium in the brine injection experiment. In such case, the resultant subsurface images may be severely distorted. In order to alleviate this problem, we develop a new least-squares inversion algorithm under the assumption that the subsurface model will change continuously in time. Instead of sampling a time-space model into numerous space models with a regular time interval, a few reference models in space domain at different times pre-selected are used to describe the subsurface structure continuously changing in time; the material property at a certain space coordinate are assumed to change linearly in time. Consequently, finding a space-time model can be simplified into obtaining several reference space models. The regularization along time axis is introduced assuming that the subsurface model will not change significantly during the data acquisition. The performance of the proposed algorithm is demonstrated by the numerical experiments using the synthetic data of crosshole dc resistivity tomography.
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Resistivity Monitoring of an Irrigation Experiment at Högbytorp, Sweden
Authors T. Dahlin, P. Aronsson, A. Hagevi, E. Wall and M. ThörnelöfAn irrigation experiment was carried out close to the Högbytorp waste site outside Stockholm, Sweden, in which leachate water from the waste site was used for irrigation of a Salix plantation. Three different irrigation levels were used, plus no irrigation as control. Resistivity imaging was used at a number of time steps with the objective to try to monitor changes in water saturation and salinity in the ground. The monitoring results show strong potential for the method. Variation in soils properties are indicated by the variation in resistivity. Changes in resistivity that correlate well with differences in irrigation quantities and plant growth are evident. The experiment is planned to continue, and more in-depth analysis of all available data will be carried out. It is beyond doubt, however, that a lot of information related to variation in water quantity and quality is contained in the resistivity imaging results.
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The Use of the CVES Method in a Basaltic Environment - A Case Study from the Faroe Islands
Authors T. Varming and F. P. DalsgarðA CVES survey has been conducted in order to test the method and locating a coal-bearing sedimentary layer in an otherwise dominated basaltic environment showing considerable topography. The data has been initially interpreted using the smooth 2D minimum structure inversion. The results showed that data did not contain much information in the deeper section and the sedimentary section could therefore not be confidentially mapped. However good estimates of the thickness of the overburden and the presence of a shallow underground down-hill water flow can be mapped with confidence.
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1D Inversion of Resistivity and Water Content of Magnetic Resonance Sounding
Authors U. Yaramanci and M. BraunMagnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance, SNMR) is used for groundwater exploration and aquifer characterization. It provides valuable information about geometry of the aquifer, water content and hydraulic conductivity.
In MRS, since the excitation magnetic field depends on the resistivity of the subsurface, it has to be taken into account in the inversion. Currently, resistivity is provided by a priori information from supplementary geoelectrical measurements or by assumptions. We developed an inversion scheme for MRS to invert also for the resistivity directly from the MRS data.
Results for synthetic data show that the water content, layer boundaries and resistivity can be derived using only the amplitude of the MRS data. However, using amplitude and phase of the MRS signal improves the inversion result. The feasibility of the new inversion scheme has been demonstrated by the successful implementation of field data. The resistivities derived from SNMR are comparable with those derived from conventional electromagnetic methods.
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