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4th EEGS Meeting
- Conference date: 14 Sep 1998 - 17 Sep 1998
- Location: Barcelona , Spain
- ISBN: 978-94-6282-127-9
- Published: 14 September 1998
221 - 236 of 236 results
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Quantitative interpretation of thermo-electric selfpotential anomalies due to a dipping sheet-like structure
Authors G. Skianis, D. Vaiopoulos and T. PapadopoulosWhenever water flows along a boundary of adjacent rocks with different thermoelectric coupling coefficient, a self-potential field (thermo-electric SP) may be produced, because of the difference in temperature between water and surrounding medium. According to Fitterman (1979), the SP field source can be described in terms of a series of current dipoles perpendicular to the boundary. This model of the polarised sheet has been often elaborated for SP prospecting of geothermal fields and the interpretation is usually carried out in space domain (Corwin et.al. 1981, Fitterman and Corwin 1982, Noutsis and Skianis 1987, Lagios and Apostolopoulos 1995).
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Discriminating between true and pseudo-anisotropic ground using azimuthal resisitivty soundings
More LessIn an azimuthal resistivity sounding, ground resistivity is measured as a function of both depth and azimuth. The resultant distribution of apparent resistivity recorded at a particular electrode spacing is indicative of the nature of the subsurface at that associated depth of investigation. An elliptical pattem of apparent resistivity implies that the surveyed ground might be electrically anisotropic. Fractured rock constitutes a hydraulically anisotropic medium. Hydrogeologists drawan analogy between these two properties and so employ azimuthal resistivity soundings to identify and then characterise fracture induced anisotropy. Fracture systems are of considerable importance to hydrogeologists as they can represent significant sources of groundwater and have considerable implications as regards waste disposal, groundwater protection and contaminant transport.
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Experiments with surface NMR for improved measurements and inversion
Authors U. Yaramanci, G. Lange and K. KnödelWith the rather new method of surface nuclear magnetic resonance (SNMR) for which meanwhile a commercial equipment is available, direct proof of the existence of groundwater and also study of aquifer parameters is now possible (Schirov et al. 1991, Beauce et al. 1996). In SNMR signal the amplitudes are directly related to the water content, while the decay times are linked to pore size, grain size and, therefore, to hydraulic conductivities. The phases are related to the electrical conductivity, however, this is only used qualitatively.
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New step in anisotropy studies: Arrow - type array
Authors D. K. Bolshakov, I. N. Modin, E. V. Pervago and V. A. ShevninAnisotropy of rocks' resistivity is a weak enough phenomenon in comparison with inhomogeneity influence. That is why we are trying to find an array being the most sensitive to anisotropy and to develop field survey and data processing technology for anisotropy and inhomogeneity influence estimation and separation. New step in this study gives arrow-type array.
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3D geoelectric mapping of subsurface area by means of radiowave geointroscopy technology
Authors V. A. Istratov, M. G. Lisov and I. V. ChibrikinSurveys by means of interweIl geointroscopy of rocks (RWGI) method permits to determine precisely and reliably rock conditions during projecting and constructing of metro railway tunnels under complicated geological setting, to study subsurface area under the basements of big buildings and to carry on monitoring of banks zone of water storage. RWGI method is based on multiplying raying of the interweIl area by means of high frequency electromagnetic waves during relative movements along wells of dipole transmitter and receiver. Data processing technology takes into account electric anisotropy, diffractive and interferentive effects of local unhomogenities. Sufficient degree of electric contrast of the objects searched for by means of RWGI is 1:2 in relation to the host rocks. 3D geoelectric maps represents morphological features of the rock media structure, and using this maps it is possible to detect tectonic displacements, to estimate the degree of fracturing and fluid saturation. Specially designed equipment, special data acquisition technology and data processing software of the method were applied to engineering geological surveys in the Moscow metro and in various industrial and water-development projects.
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Focussing effect of an averaged three-electrode configuration
Authors A. Kampke, A. Weller and G. PeschelGeoelectrical measurements with multi-electrode arrays are performed to obtain an image of the subsurface resistivity structure. Conventional configurations like Wenner, dipole-dipole or pole-dipole yield pseudosections of apparent resistivity showing a more or less distorted image of the real structure. Consequently, inversion procedures have to be applied to transform the measured data into a realistic model. Practical experience has shown that a simple averaging of the apparent resistivities measured with a configuration of three collinear, equispaced electrodes in the forward (AMN) and in the backward mode (NMA) by changing the position of the current electrode A and the potential electrode N results in an improved pseudosection reflecting the main features of the subsurface. This configuration has proved to be an efficient tool for data acquisition combining sounding and profiling (Peschel, 1967). Based on this arrangement, a special continuous imaging technology was developed which has been successfully applied to engineering geological problems and at archaeological sites (Peschel and Lubenow, 1995). The available tools for modelling and inversion have been used to investigate the resolving .power ofthe averaged three-electrode configuration.
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Comparison of GPR-measurements of the ground wave with FDTD-modelling
More LessAccurate in-situ measurements of soil water content is of utmost importance for agriculture, hydrology and environmental research. For instance, soil water plays an essential role in an ecosystem for the growth of the plants and limits the erop yield more than any other soil or meteorological variable. Existing methods for soil moisture measurement (i.e. gravimetric method by oven drying or time domain reflectometry (TDR) are unsuitable for large areas because of the high variability of soil moisture. In various large-scale field measurements Sperl et al. (1997) evaluated the capability of the ground wave of ground-penetrating radar (GPR) for the determination of the volumetric soil water content (Bv). For a better understanding of the frequency dependenee of the influence depth of the ground wave we designed physical experiments. In addition to these experiments simulations of the electromagnetic wave propagation based on a Finite-Difference- Time- Domain (FDTD) scheme were carried out to prove the physical experiments. Here we will show, that slight variations in the depth of the capillary fringe can have an extreme influence on the propagation of the electromagnetic waves in the subsurface and on the measurement of the velocity of the ground wave which is used to determine 0v.
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New place for MSU students field geophysical training - Alexandrovka
Authors I. N. Modin, A. G. Jakovlev, A. A. Bobachev, V. A. Kulikov and V. K. KhmelevskojGeophysical practice of MSU geological faculty students for many years passed in Crimea in 1500 kms from Moscow. After disintegration of the USSR the realization of practice became complicated. Therefore in 1992 it was decided to establish new place for geophysical practice in 200 kms from Moscow. This area geologically belongs to Russian platform with crystalline basement at the depth about 1200 m. Sedimentary cover includes Devonian, Carboniferous, Jurassic, Cretaceous and Quaternary layers.
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Non-unity reflection coefficient at the open end of a coaxial TDD probe
Authors B. L. Nguyen, E. Slob and J. BruiningIn the last two decades, Time Dornain Reflectometry (TOR) has become a extremely versatile method to measure soil volumetric water content and bulk soil electric conductivity. Recent research has also proved the applicability of TOR for measuring subsoil petroleum contaminant saturation. Next to the traditional time domain approach, many researchers use the frequency domain scatter function to describe the measured signal with a network analyser. Existing models of the scatter function are based on the assumption that the soil in the cross-section of the TOR probe is homogeneous and that the end of the probe can be described as an open ended circuit [1]. The latter assumption implies that all energy is reflected at the end of the probe and hence, all energy loss is the modelIed as due to an intrinsic loss mechanism in the soil. This assumption would work if the probe would be ended in air. As the TOR probe is usually ended in the soil which can have relatively high complex and frequency dependent electric perrnittivities, the reflection at the end of the probe is expected to deviate from unity and to be frequency dependent as weIl. In this paper we show that this is indeed the case and that the unity end reflection coefficient assumption leads to wrong values of the electric perrnittivity
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Two-dimensional resistivity inversion using a new topographical correction method
Authors M. Varga, A. Kovacs, C. Gaspar and S. GisbertIn shallow geoelectric exploration, e.g. in problems occurring in environmental and engineering geophysics, the topographical effects can be larger in measured values than changes caused by resistivity inhomogenities to be found. In the simplest topographical correction method the forward problem is solved for homogeneous halfspace having actual topographical relief and the measured apparent resistivities are corrected before interpretation with coefficients calculated for homogeneous halfspace. This method is inaccurate if large resistivity variations occur near to the surface and it is problematic to represent the results of two-dimensional inversion (section prepared from cel-resistivities) taking into account the topographical data, too. Therefore it is very important to use real topography both in the direct and forward modeling but using conventional 2-D methods it is very time-consuming. To solve the reconstruction (inverse) problem by e.g. a Newton-type method a number of direct problems have to be solved. Therefore the choice of the applied solution method of the direct problem is of great importance. Here we used a numerical method adopted from the computational flow modeling which makes it possible to reduce the computational cost by a remarkable amount. The method is based on a non-equidistant, non-uniform but Cartesian cellsystem (computational) generated by the so-called quadtree algorithm. This algorithm came from the computational graphics and is a heart of the "unconstructed grid generation ". Special finite difference schemes have been defined on the above computational grid. To speed up the computations a simple but efficient multigrid technique has been developed in the quadtree context. It is shown that to solve the direct problem (2-D Poisson equation) the number of the necessary arithmetic operations is proportional to the first power of the number of the surface points only which results in a much more economie method compared with the traditional solvers.
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A new futering technique for correcting time variations in magnetic data
By M. N. HasanThe earth's magnetic field fluctuates with time. The short-duration and small-amplitude fluctuations, known as Time Variations, distort the anomalies due to the geology during magnetic surveys. The standard method for removing time variations from the magnetic data is based on the assumption that the time variations measured at the base station and those encountered at the field stations are exactly the same, and accordingly the time variations are removed from the field data simply by subtracting the base station reading from the corresponding field station reading. This assumption is practically true for very local land-based surveys, in which the base and field stations are close to one another.
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Z/H Measurement in the radio-frequency range
Authors T. Radic and L. AschmannShallow subsurface explorations requires substantial and efficient geophysical methods. Among the geoelectrical techniques are the passive EM methods which use existing radio transmitters as the signal source. Consequently they can be set up very compactly, thus being versatile. Compared to the radiomagnetotelluric method (RMT), the weIl known VLF method is a contactless one, so that a high progress in measurement can be achieved. The Z/H method is a further development of the VLF method by using a wide frequency range, thus giving the possibility of avertical sounding. The aims of the present work are as follows: • calculating the magnetic transfer functions in radio frequency range • increasing the frequency range to 10kHz up to 2000 kHz.
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Improved characterization of unconsolidated sediments under field conditions based on complex resistivity measurements
Authors T. Radic, D. Kretzschmar and E. NiederleithingerThe pore space of unconsolidated rocks could, in spite of the several attempts within the last few years, still only roughly be characterized by de geoelectric surface measurements. Important parameters like the permeability coefficient could at best only be estimated with much uncertainties. The reason for this is, that the de resistivity is mainly determined by the fluid conductivity and the saturation of the pore space; the pore space geometry, which is important for the hydraulic conductivity, is only of secondary importance. The complex resistivity method (CR) measures the capability of rocks to accumulate electric energy (induced polarization method lP). Since this feature is related to the geometry of the pore space, the hydraulic parameters can be estimated much more reliably than it is possible with the de resistivity method.
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The effect of low pass filters in time domain electromagnetic soundings
Authors E. Auken, F. Effersø and K. SørensenIn time domain electromagnetic, TDEM, it is a wide spread practice to model the current waveform (e.g. Asten, 1987), where as the effect of low pass filters (LP) in the receiver system is neglected. In this paper we show that when the TDEM method is used in hydrogeological and environmental investigations it is mandatory to include the filters in the forward and inverse modeling scheme to obtain avalid geological model for the survey site. IDEM systems is normally band-limited as input signals from amplitude modulated radio transmitters operating in the target bandwidth of the system often dominate the noise characteristic of the system over several decades in time. Band-limiting the input signals significantly improves the signal-to-noise ratio and thereby the depth of exploration. However, the LP filters distort the early and medium part of the decay curve due to damping of the high frequency content. The level of distortion depends on several factors such as cut-off frequency and subsurface resistivities. Therefore, the LP filters must be incorporated in the inversion scheme such that the calculated forward response is subject to the same filtering as the measured response. In this paper we (1) quantify the distortion of responses due to band-limitation for a number of typical resistivity models. The band-limitations are based on cut-off frequencies typically used in commercial instruments. (2) We demonstrate that distortion due to band-limitation can be modeled in the inversion scheme by including the LP filters in the forward response. This approach is tested on theoretical data.
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Transient EM sounding in the shallow study
More LessTransient EM method was developed in Russia. In 1969 Dr. Vladislav Sidorov and Pr. Vyacheslav Tikshaev from Nizhnevolzhsky Geology and Geophysics Research Institute for the first time conducted field measurement of transient EM processes in case when a distance between a transmitter and a receiver was considerably shorter than the depth of investigations. Simultaneously in the sixties Dr. Obukhov in Moscow and Pr. Alexander Kaufman in Novosibirsk, the latter lives now in the USA, theoretically grounded the ability of TEM sounding.
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Geoelectric survey of Fundáo Island, Rio de Janeiro - Brasil
Fundäo Island, located in Guanabara Bay, and part of Rio de Janeiro city, Brasil, is the result of an artificial embankment of a former small archipelago, built in the early 1950's. The aim of this work is to identify, by means of a geoelectric survey, the cores of the original islands. Normally, deep structures near the seacoast cannot be detected through geoelectric measurements. This is due to the presence of salt water in sediments. However. in the case of Fundäo Island, the high conductivity segments have a lenticular form and correspond to the embankments.'This circumstance allows the detection of deep layers and resistive nuclei. Results of vertical electric soundings using Schlumberger method with a maximum aperture (AB/2) of 100 m at the UFRJ campus (Fundäo Island) show that the resistive nuclei are strongly correlated to the original islands of the archipelago. Data were processed by means of RESIX-IP (1993) software.
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