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5th EEGS-ES Meeting
- Conference date: 06 Sep 1999 - 09 Sep 1999
- Location: Budapest, Hungary
- ISBN: 978-94-6282-119-4
- Published: 06 September 1999
21 - 40 of 196 results
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Geophysical monitoring for the Hungarian uranium remediation
Authors Zs. Berta, J. Csicsák, Zs. Koleszár and A. VárhegyiAfter the abandonment of uranium mining and ore processing activity in Mecsek mountains, a throughout close-out and remediation work has been started which is one of the biggest environmental protection project of Hungary. The sources of pollution are the objects created during the 40 years mining and milling operation, the open-air deposition of the waste material from the exploited cca. 18 million m3 rock. The most important polluting sources are the underground mines, the mining and milling facilities and areas, the transporting roads and pipelines, the waste rock piles, the alkaline heap leaching piles and first of all the two tailings pounds. In addition to the conventional mining remediation tasks the neighbourhood of a big city (Pécs), the contamination risk of its only underground drinking water reservoir and the radioactive pollution of the environment are also arise. The operated environmental monitoring system is unichronic with the Mecsek uranium industry. In that respect it preceded the usual practice of the age, and recently plays a particular role in the Hungarian environmental geophysics. Using the historical results of the monitoring system, completing with other surveys, a comprehensive environmental study has been created, which determine the most important tasks for the next 5 years. The study summarises the features all of the polluting objects (quantity, composition, radioactivity, horizontal and vertical delineation etc.) and risks, outlines the basic principles of environmental (first of all water) and radiation protection, and determines the main remediation tasks together with the related monitoring system.
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High resolution electrical monitoring of fluid flow through the unsaturated zone of a sandstone aquifer
Authors V. Hatzichristodulu, R. Barker and J. TellamThe unsaturated zone is very important because it influences recharge to the underlying aquifer and controls the transfer of pollutants to it. The typical methods by which the unsaturated zone has been studied are tensiometer and neutron probe field measurements. However these methods are invasive, laborious, time-consuming and do not provide spatially continuous information. Surface geophysics goes some way to eliminating these disadvantages. The aims of this work are to test the usefulness of surface high resolution electrical tomography in mapping the 3D movement of water in the unsaturated zone and to provide further understanding of the processes involved in the unsaturated zone flow.
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A wire net design for electrical monitoring of barriers
Authors C. Bernstone, T. Dahlin and P. UlriksenA wire net concept for monitoring of environmental barriers, aimed for permanent installations, is described. This presentation includes results from a laboratory study and from an installation constructed below a brine pond mineral liner at the Filborna landfill in Southern Sweden. A widespread leachate management strategy is ”containment and collection”, which implies that the detection of leakage at early stages from containers of hazardous liquids is a matter of great concern. Development work to construct effective and reliable systems for leakage detection based on geophysics has been underway since mid 1980s, commonly by using DC resistivity. The work on developing a new concept for electrical monitoring of impermeable barriers has been a part of research program at Lund University, with funds from the SEPA. The work started in late 1996 and runs as “Geophysical mapping and monitoring of waste deposits”. A Licentiate thesis has been produced [1] and this spring monitoring results from Filborna has been presented.
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Integrated geophysical survey for the detection of hydrocarbon pollution
Authors A. Godio and G. MorelliGeophysical methods provide useful tools to detect and map large areas potentially subjected to hydrocarbon pollution; many technical papers on the use of different geophysical techniques are reported in literature (Atenkwana, 1998, Godio and Morelli, 1998). Electromagnetic mapping, GPR investigation and electrical methods have proven to be effective for detecting polluted areas. An extensive survey has been undertaken on contaminated sites in Northern Italy over the last two years. The site assessment using the hydro-geological approach and destructive test (drillings) was completed with integrated geophysical investigations. The survey involved the characterisation of the subsoil assessment in some test sites, where oil recovering wells, tanks and pipelines were located. The main goal of the combined investigations was to find the presence of hydrocarbon pollution due to the leakage from the old tanks and pipelines. Because of the geological setting of the region, the main aim was to localise the free hydrocarbon phase above the shallow aquifer (2-3 meters in depth) and the presence of contamination in the vadose zone. Due to the long permanence of hydrocarbons in the soil (probably more than 20 years), chemical and biological reactions could have occurred in the polluted zones; these phenomena could complicate the interpretation of the geophysical response. A short description of the results of the geophysical tests is here reported: the selected examples relates to the survey performed on an area of about 50 x 100 m, previously employed to collect gas and oil. Most of the cultural features, such as tanks and pipelines, were removed from the site. The soil and water sampling confirmed the suspicion of the existence of a high contamination. The main goal of the geophysical survey was to verify the extension of the polluted zone, in order to plan a subsequent recovering of the area. Different methods were employed: a preliminary fast mapping was carried out using the low induction technique (LIN); the CM031 (Geophyzica) instrument in a dipole vertical mode was used. Ground probing radar (GPR) acquisition, which operates in the range between 100 MHz up to 500 MHz, was performed. Finally, electrical resistivity tomography (ERT) was carried out to confirm the response of the former geophysical investigations. Electrical measurements were performed employing the geo-electric intelligent nodes scanning system Syscal R2 (IRIS instrument). An electrode spacing of 2 meters was adopted in the dipole-dipole and Wenner array configuration for a total of 32 electrodes. The data processing involved the mapping of the in-phase and conductivity (quadrature component) of the electromagnetic response of the LIN acquisition; the GPR data were processed in order to obtain the time-slices of the reflection amplitude. ERT data were processed using a least square inversion (2D) with smoothness constrains, according to the procedure described by Morelli and Labrecque (1996).
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Contamination plume evaluation of two ditches using frequency domain electromagnetic method
Authors F. P. O. O. Figueiredo, F. A. Ferreira and M. J. Senos MatiasThe study site is located at 6 km NW of Coimbra, central Portugal, near two ditches: North ditch and the old Mondego river bed (Fig. 1). These ditches have been receiving sewage water from Coimbra municipality, causing severe environmental problems, such as ground-water contamination. This contamination is affecting agricultural and arboretum activities, which are the main use of the surroundings areas. In this area, the upper bedrock is of Cretaceous age and consist of siliciclastic sediments, limestones and marls, overlaid by recent sediments of the Mondego river. A geophysical survey, using electromagnetic method in frequency domain, was made near the two ditches with contaminated water. The purpose of this survey is to obtain information about the location of the conductive plume induced by organic and chemical pollution. This information will be used to locate fresh water aquifers, in order to provide the suitable location sites of water wells drilling.
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Comparison between geological models based on resistivity data and resistivity-IP data
More LessA geophysical study of a contaminated land area is presented. The main objective of this paper lies in the IP (induced polarization) technique — compared with EM and PVES (profile-vertical electrical soundings), what additional information on the geology and contamination is obtained when also the IP data is acquired? In glacial sediments lacking clay minerals, the induced polarization effect depends mainly on the grain-size distribution of the material and the resistivity of the pore-water. Vanhala (1997) measured large phase shifts to silts and fine-grained sands (Fig. 1), but small values to coarse sands and gravels, and also to clays. Very similar behavior has been reported also by Iliceto et al., (1982) and Dudas et al., 1991.
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Environmental examination of tailing ponds by geophysical methods
Authors I. Kaszás and J. StickelThe "Alkaloida" pharmaceutical factory located at the village of Tiszavasvári was established in 1927. Its main profile was to produce morphine from dry poppy-heads. During the decades, the activity of the factory and production of goods grew continuously, thus parallelly the amount of industrial wastes increased as well. The largest modification in the production circle took part in 1970 when the production of synthetic plant-protecting agents, fungicides and herbicides started, in addition to the traditional products of pharmaceutical industry. Until 1964, the communal and industrial waste water produced by the factory has been run into the Hortobágy Main Canal, after a. simple precipitation. Later the toxic waste waters which could not be cleaned biologically, were deposited in the abandoned clay pits of the neighbouring brickworks. The remediation program of the site started in 1997. The general task was to dewater the whole area, to separate it from the surroundings by slot walls, and later a complete recultivation. We carried out reconnaissance geophysical measurements in the site, the results of which were sufficient to start a detailed survey. The morphology of the site is quite plain, surface shaping by rivers can be detected, although these traces have been mixed with eolic sediments. The waste water tailing ponds are located on a plain ridge. The ponds are surrounded by earth dams. The River Tisza runs about 10 km far from the site, the most important irrigation canal of the country, the East Main Canal can be found at a distance of 1.8 km. Near the ponds to the south there are apple plantations, and about 200 m to the southwest fish pond are to be found. Thickness of the strongly stratified Pleistocene sediments is about 150 m. According to the different phases of fluvial sedimentation highly permeable sand and gravel layers alternate with impermeable layers of clay, silt and mud. A number of information on the near surface layers are available: archive drilling data and results of Engineering Geophysical Soundings, an advanced version of Cone Penetrating Tests. Moreover, aerial photos of the site taken in·1956 and 1994 are available in a scale of M = 1 : 5000. On the basis of investigation of the monitoring wells the groundwater table is between 1.0 and 2.0 m in the average, the direction of the groundwater flow is W. The task of geophysics was to delineate the deposited contaminating material both in horizontal and vertical directions, and to detect the pollution occurred. Moreover, the depth, thickness and hydraulic conductivity of the impermeable clay layer should also be determined. An important requirement was that the shallow impermeable clay layer be unsettled in the possible slightest degree. Ground geophysical methods meet these demands; four of them were chosen:
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Wavelet technique applied to localisation and interpretation of electrical signal distortions produced by pollutants in soils
More LessOn the bases of previous laboratory results, we performed and tested a Geoelectrical Integrated Method (GIM) to investigate and monitor the presence of chemical and organic pollutants near riverbed, water tables, and urban waste deposits. The GIM is based on the electrical parameters obtained both at frequency and DC conditions in laboratory and in `in situ'. So doing no destructive and low cost investigations allow find the presence of pollutants and evaluate the volume of polluted terrain. In particular it will be possible discriminate macromolecules of organic pollutants from metallic ions. To find the class and the quantity of pollutants in soils and rocks we can use the different electrical behaviour of organic compounds and metallic ions at the solid-liquid interface as function of electrical frequency in the DC÷200kHz range. In particular the electrical behaviour of polluted soil samples resulted strongly no-linear and consequently the output waveform are distorted. We used the classical THD% (% Total Harmonic Distortion) and the HD%(ω) parameters to quantify the deformation level. This method allows to identify the frequency interval of nonlinear electrical behaviour, that is strictly dependent of the pollutant molecular size and give an "electrical signature" of the pollutant-mineral interaction.
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Electrokinetic treatment of cadmium spiked clays. influence of the heterogeneities.
Authors P. Marceau, P. Broquet and P. BaticleRecently, attention has focused on electrokinetic techniques that make it possible to move and extract contaminants from fine-grained soil under an electric field (Figure 1). The application of a constant electric current has several effects: (1) electrolysis of water, plating reactions and gas formation occur at the electrodes. H+ is produced at the anode, and OH- at the cathode (2) the electric potential difference leads to electroosmosis , the pore water flow is toward the cathode, since most soils have a negative surface charge (3) the electric field initiates electromigration of species available in the pore fluid and of those introduced at the electrolytes. These phenomena change the chemical pore fluid composition and induce sorption reactions in the soil. Bench scale studies showed that inorganic species and heavy metals such as arsenic, cadmium, chrome, copper, iron, lead, mercury, nickel and zinc can be efficiently extracted from polluted or spiked soils (Pamucku and Wittle, 1992; Acar and al., 1995; Cox and al., 1996; Reed and al., 1995; Li and al., 1996; Acar and Alshawabkeh, 1996; Marceau and al., 1999). Extraction rates of over 90 % are reported. But species seem to precipitate with OH- near the catholyte. Organic species such as benzene, phenol and acetic acid can also be removed (de Marsily and al., 1992; Bruell and al., 1992; Acar and al., 1992).
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Time-Lapse resistivity imaging inversion
By M. Heng LokeIn studying the changes of the subsurface resistivity with time, two-dimensional resistivity imaging surveys are often repeated over the same line at different times. Such studies include the flow of water through the vadose zone, changes in the water table due to water extraction, flow of chemical pollutants and leakage from dams. Normally, the data from the surveys conducted at different times are inverted independently, frequently with a smoothness-constrained least-squares inversion method (deGroot-Hedlin and Constable 1990). The changes in the subsurface resistivity values are then determined by comparing the model resistivity values obtained from the inversions of an initial data set and the later data sets. In many cases, such an approach has given satisfactory results. However, in theory, since the inversion of each data set is carried out independently, there is no guarantee that the differences in the resistivity values are only due to actual changes in the subsurface resistivity with time. Each inversion attempts to minimise the difference between the observed and calculated apparent resistivity values for an individual data set without taking into account the model obtained from the initial data set. The use of joint inversion techniques using three different types of cross-model constrains is investigated in this paper. The model obtained from the inversion of the initial data set is used as a reference model to constrain the inversion of the later time-lapse data sets. Firstly, a simple damped least-squares constrain to minimise the differences in the model resistivity values between the initial model and the time-lapse model was used. The second method uses a least-squares smoothness constrain to ensure that the differences in the model resistivity values vary in a smooth manner. Thirdly, a robust (Claerbout and Muir 1973) smoothness constrain which minimises the absolute changes in the model resistivity values was used. Figure 1a shows a test model where the base model has a 50 ohm.m faulted block at the bottom-left side with a surrounding medium of 10 ohm.m. In the time-lapse model, a small rectangular block with a resistivity of 20 ohm.m was added. This is a relatively difficult test model as the apparent resistivity anomaly caused by the faulted block is much large than that caused by the small block (Figures 1b and 1c). Figure 2 shows the differences in the resistivity of the models obtained from the inversion of the initial and the time-lapse data sets. Ideally, the difference section should show a value of 100% where the small block is located and 0% elsewhere. The difference sections for all the inversion methods do show large positive values of over 70% at the location of the small block. However, the section obtained with independent inversions with no cross-model constrains show significant changes in other areas, particularly near the upper-right corner of the faulted block (Figure 2a). The distortions are smaller with the damped least-squares (Figure 2b) and the least-squares smoothness crossmodel constrain (Figure 2c) methods. However, the best result is obtained with the robust smoothness cross-model constrain method (Figure 2d).
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Effects pf dispersive conductivity on shallow em response
Authors W. Frangos, C. Scott and H. F. MorrisonElectromagnetic (EM) geophysical exploration methods, both time and frequency domain, have historically operated in two disparate frequency bands. Methods such as magnetotellurics (MT), CSAMT, loop-loop, and TEM work at frequencies of a millihertz to tens of kilohertz, investigating depths from several tens of meters to tens of kilometers. Wave propagation methods such as Ground Penetrating Radar (GPR) operate at frequencies above a few tens of megahertz, with exploration depths of a few meters. The intervening gap in exploration frequencies is critical to much near-surface geophysics as practiced by the environmental and hydrologic geophysics communities. The transition between the diffusion and wave modes of electromagnetic propagation occurs in a small spectral interval of about one decade of frequency. Below the transition, displacement currents are negligible and conventional diffusive electromagnetic theory provides an adequate description, while above the transition, conduction currents are small and wave propagation theory applies. The rock properties conductivity and dielectric permittivity are frequency dependent, i.e., best described as complex quantities over portions of the spectrum with conductivity increasing with frequency while permittivity decreases. At issue for any work in the transition portion of the spectrum is how the dispersive behavior of the properties may influence the observed data in electromagnetic surveys for delineation of targets associated with the shallow subsurface.
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Enviromt. A new radio/controlled source system
Authors L. B. Pedersen, L. Dynesius, M. Bastani, M. Gharibi and U. MatzanderNovel features of the EnviroMT system Broad band radio-signal reception For the estimation of transfer functions we only make use of the signal from distant manmade sources in the form of VLF and Radio transmitters in the LW and MW band in contrast to standard MT where the whole frequency band is considered as plane wave sources perhaps mixed with non-planar events. In Northern Europe there are generally more than two independent transmitters available in each half-octave/octave band that transfer functions can be estimated starting at 14 kHz. Controlled source with synchronous detection Whereas the standard estimation of transfer functions in the RMT mode is based upon the assumption that they are constant in each sub-band no such assumption is necessary in the controlled source mode. Here, unique transfer functions can be found right at the transmitter frequencies chosen because the transmitter consists of two independent horizontal dipole coils. Even though these transfer functions are most easily interpreted in the plane wave (or far field) case they can still be uniquely defined in the intermediate and near field range. Built in database handling In addition to showing pseudo-sections the system also provides a facility for automatic 1D inversion of selected impedance combinations (off-diagonal elements, determinant, etc.) along selected profiles. In the future even a fast 2-D inversion code will be added to the system to make it more flexible and self-contained. All the above mentioned manipulations can be done in-field. The built-in colour display unit is of excellent quality and with the fast built-in computer, in a few minutes the user can get a complete overview of the data collected along various profiles.
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Continuously recording radio-frequency electromagnetic (RF-EM) method (15–300 KHZ) without ground contact: A powerful tool for groundwater vulnerability mapping in fissured rocks
Authors F. P. Bosch, P. Turberg and I. MüllerIntroduction Density and size of open fractures, which connect the water reservoir with the surface, are main features for characterizing the vulnerability of fractured aquifers. The Hydrogeology Centre, University of Neuchâtel has developed a very fast and precise geophysical survey technique to locate and extensively map these types of high-permeability fracture zones: a Radio Frequency Electromagnetic (RF-EM) system. Theoretical background The RF-EM method measures the earth’s electromagnetic response (secondary field) induced by radio signals emitted from terrestrial transmitters (horizontal, primary field). These transmitters are used for navigation and submarine communication in the frequency range of 15-300 kHz. The induced secondary field has components that are oscillating both ‘in phase’ and ‘out of phase’ with the primary field. The intensity of the vertical component of the secondary field depends on the size of the conductive body. Therefore electrically conductive features such as geologic faults and fractures are detectable. In particular using the higher frequencies (up to 300 kHz) enables fracture detection in the epikarstic zone. Our prototype instrumentation continuously measures both fields with two orthogonal induction coils and
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Electromagnetic measurements on the surface of a lake
More LessThe contamination of lakes and rivers by industrial activities constitutes a serious problem for human health, especially when lakes are used for swimming an fishing. The radiomagnetotelluric technique (RMT) has been successfully applied for the exploration on waste sites, the detection of contaminative plumes and for archaeological prospection. (Tezkan et al., 1996, Zacher et al., 1996, Recher et al., 1998) In order to test the applicability of the RMT technique for the geophysical investigation of lakes and rivers, RMT measurements were carried out on a profile crossing from one shore to the other shore of a lake close to Cologne in Germany. Fig. 1 demonstrates the field setup of the RMT survey on a lake schematically. A small boat was used for the survey and civilian and military radio stations were used as transmitters broadcasting in frequency range between 10 kHz and 1 MHz. The magnetic field was measured by a coil in the boat and the electric field by an electric antenna placed on the lake’s surface. Apparent resistivities and phase values were derived from the measured magnetic and electric fields so that a sounding information was available for each station. The data were interpreted by a 2D inversion algorithm (Mackie et al., 1997). Thus a 2D conductivity model was derived for the investigated lake and its surrounding host which is demonstrated in Fig. 2. The lake can be seen as a strong conductive anomaly in the 2D inversion model.
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The TEM arbitrary impulse equipment
Authors M. I. Epov, E. Y. Antonov and I. O. IssaevIntroduction Transient electromagnetic (TEM) sounding is an important method for environmental investigation and engineering geophysics. The similar existing TEM techniques proved to have, along with many advantages, numerous essential limitations, such as: -difficulties in studying the upper part of the section close to the surface; this is because only the late stage of transient process is interpretable; -inapplicability of many statistical methods for noisy data processing because of logarithmic time-step used and only accumulated data registered; -poor spectrum of common used signal (turning on/off of current). In this paper we describe an equipment for measuring and a method for interpreting the nearsurface TEM data. This method is based on using the arbitrary fully controlled current impulse. All soundings are registered separately with arithmetic time step. It is shown, that with technique suggested, one can overcome the above pointed difficulties.
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3-D Electromagnetic research of the structure and state of the rock massive
Authors O. A. Hachay, E. N. Novgorodova and A. Yu. HachayIt is offered a new approach to the research of high complicated geological media, which are rock massives [1-2]. That approach is based on the theory of 3-d interpretation of alternating electromagnetic fields. The effect of it's use for the research of rock massives in mines and the possibility of structure identification and state of it is shown , using natural observations. It is offered a new effective method of the electromagnetic monitoring of inhomogeneous rock massives, which are influenced by intensive technogenic forces. There had been analysed and outlined parameters, which characterise the massive as a whole, and that, which are sensitive to local changes into the massive, which can be a potential dangerous zones and must to be systematically observed by additional geophysical and geomechanical methods. That parameters are: the geometrical distribution of apparent resistivity (fig1.c points - before rock outworking and squares - after rock outworking) and profile distribution of the average parameter of geoelectrical inhomogeneities (fig 1. a – before rock outworking and b – after rock outworking).
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3-D Resistivity and IP probability tomography in near-surface geophysics
Authors P. Mauriello and D. PatellaIn a previous paper (Patella and Mauriello, 1998) we extended to resistivity and induced polarization (IP) measurements the principles of probability tomography (Patella, 1997). In this work, we deal with the 3-D case for both types of data and propose a method for a combined tomography, always based on a probabilistic point of view. In the present formulation we start from the tensorial analysis of geoelectric data (Bibby, 1986), now extended to the IP case. In fact, from the apparent resistivity or chargeability tensor it is possible to obtain tensor invariants, which are independent of the direction of the source field and well confined in the neighbourhood of the causative bodies. In particular we focus our attention on the trace tr(A) of the data apparent anomaly tensor. Implementing the mathematical backgrounds reported in the previous paper (Patella and Mauriello, 1998), we write the departure of the measured tensor trace from that calculated on the basis of a reference model as:
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Electrical anisotropy parameters and resistivity sounding interpretation
More LessIn general, resistivity sounding data interpretation assumes a homogeneous, isotropic and horizontally layered earth model. However, in many field situations these conditions are not valid as the complexity of local geology can originate strong orientation effects caused by lateral inhomogeneities and anisotropy. Under these circumstances it is good practice to keep constant the orientation of the electrode array in use and therefore orientational variation of resistivity data is often ignored. If the strike of concealed formations is known it is possible to choose an array orientation perpendicular to the strike and to obtain further measurements perpendicular to that orientation. This procedure allows a better description of the area but in regions where moderate to strong anisotropic effects exist can lead to results such as the ones shown on Fig.1.
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Two-dimensional resistivity inversion using a new topographical correction method
Authors M. Varga, A. Kovács, C. Gáspár 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 inhomogeneities 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 cell-resistivities) taking into account the topographical data, too. Therefore it is very important to use real topography both in the forward and inverse 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 cell-system (computational grid) 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 economic method compared with the traditional solvers. Grid shown in Figure 1 is generated automatically using only some control parameters for a halfspace having rough enough topography and two buried inhomogeneities. This is only an initial grid because by the help of multigrid technique using e.g. 10 levels every cell will be split up into 1024 small cells. On the earth-air interface and near the boundaries of inhomogeneities the size of cells is small already in the starting grid and going away from this regions, which are the most interesting in the description of the resistivity distribution, the size of cells increases gradually. Of course the presented grid will be expanded both on the left and right to fulfill by solution the prescribed boundary conditions. On the upper figure that grid can be seen which was generated automatically on the prescribed rectangle. However during the solution of forward problem those cells are not taken into account which are in full extent in the air so only the grid shown in the lower figure is used in the forward ( and inverse ) modeling. During inversion conductivities of the cells of the initial grid are determined from the apparent resistivities measured by a multi-electrode system using electrodes laid down equidistantly along real topography. It is important that the effect of topography is taken into account not by applying a topographical correction on measured data before inversion using a homogeneous halfspace having the actual topography but the real topography is used in every step of inversion.
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Problems in interpretation of resistivity sounding data, distorted by geological noise
Authors V. A. Shevnin, A. A. Bobachev, I. N. Modin and E. V. PervagoThe traditional resistivity sounding method deals with isolated sounding sites located casually on profile or area of investigation and executed with logarithmic step in distance growth. This technology has very restricted possibilities. Therefore it has begin to exchange practically simultaneously in different countries into new sounding technology, called multielectrodic sounding, or resistivity tomography, electrical imaging, etc. Its main advantage - more dense survey along profile - is clear visible, while the other - overlapping system of measurements is not so evident. Our group in MSU began to apply multi-electrodic resistivity sounding to overcome distorting influence of geological noise. From the end of the 80-th we have been working in areas with high resistivity contrast along both vertical, and horizontal direction. Such sounding data have different features of distortions or differences from response for horizontally layered models. At that period we developed theory of distortions of DC electric field by deep and near-surface inhomogeneities (Electrical..., 1994) and software to establish VES data distortion with the help of various indicators, noticeable on VES curves. We have found, that more than 70 % of all VES curves are distorted. That means, that the distortions are practically constant features of resistivity sounding. Carrying out the large volumes of VES field data and their interpretation (up to 1000 VES sites a year) we analyzed misfit errors of VES curves interpretation. The average value of misfit error (average RMS or D , where D is dispersion) consists 8-12%, that seems rather high value and needs to be explained.
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