A B S T R A C TThe determination of clay content in near-surface formations is crucial for geotechnical, hydrogeological and oil-contamination studies. We have developed a technique for estimating clay content that consists of the minimization of the difference between the theoretically calculated and measured soil resistivities as a function of water salinity. To calculate the resistivity, we used a model that takes into account the electrochemical processes in the clay micropores. The experimental measurements of soil resistivity were performed on soil samples, completely saturated by brines at different concentrations of NaCl salt in the range 0.6-100 g/l, to obtain the resistivity versus salinity curve. The parameters obtained with this curve inversion are the clay content, the total porosity and the cation exchange capacity. To verify the new technique, we determined clay concentrations of artificial mixtures of calibrated sand and clay. The relative mean error in the clay content does not exceed 20% for a 5% fitting error of the resistivity versus salinity curves. Such evaluations allow the correct separation of the main lithological groups (sand, sandy loam, loam, and light, medium and heavy clay).We applied this technique to estimate the petrophysical parameters of soils (clay content, porosity and cation exchange capacity) at various sites in Mexico. The results improved the interpretation of the vertical electrical soundings, the lithological soil characterization and the delineation of oil-contaminated areas.

The most widespread source of pollution in Russia is the oil pollution. It occurs at all stages of oil production, transportation and processing. Outflows, proceeding during decades result to formation of oil secondary deposits. In a near-surface zone, oil pollution becomes especially chemically active and reacts with geological environment, that results in the anomalies of various geophysical methods: SP, IP, GPR and resistivity. The oil pollution is an unusual object due to its ability to oxidation and mobility. The pollution causes processes, occurring with the speed, differed from natural geological processes. Changes of rock properties, caused by oil pollution are inconsistent. A priory, for example, the oil is an isolator, but frequently it causes anomalies of lowered resistivity. For engineering and environmental studies with electrical methods in urban areas (including pollution studies) the main problem is the influence of geological noise. Upper part of cross-section includes many near-surface inhomogeneities (NSI), caused with artificial ground, asphalt cover, trenches, cables, tubes, etc. These inhomogeneities create strong distortions and influence like "the broken glass", preventing from clear seeing deep objects. NSI distorting influence can be canceled by the application of total electric sounding (TES) technology, developed in MSU. In TES technology we apply two-sided pole-dipole array with step of distance growth equal to step between VES sites along profile. NSI creates distortions of four different types (fig.1C), when movable (single or dipole) element crosses the NSI, or unmovable (single or dipole) element is placed in NSI limits (fig.1B). Some distortions of sounding curves are conformable (fig.1C, 1-2), whereas others are non-conformable (fig. 1C, 3-4). On apparent resistivity cross-section distortions display as vertical and inclined strip zones (fig.2,3). For different types of array (Pole-dipole, polepole, Schlumberger, Wenner, dipole-dipole, etc.) distortions are different (fig.3). For several NSI (fig.3, 3) interference of distortions appears. We developed software for canceling such distortions. The algorithm of median polishing was offered by J.W.Tukey (1981), and after modification made by E.V.Pervago was applied for processing of Total Electrical Sounding (TES) data. The algorithm's operation is based on regularity of distortion effects and it allows to cancel effectively these effects on apparent resistivity pseudocross-section. J.W.Tukey described processing algorithm for data, given in the table as following: at the first step-counting and subtracting median value for each column; at the second stepcounting and subtracting median value for every row. Then the 1st and 2nd steps repeat several times.

We considered clay content in loose soil as the factor mostly influencing on hydraulic conductivity (filtration coefficient). We collected and analyzed some published experimental data about hydraulic conductivity relation with soil lithology and clay content in the form of grain size. Also we performed some theoretical modeling modifying well-known formulas to include clay content in them. Experimental and calculated data showed quite good coincidence. Correlation between hydraulic conductivity and clay content seemed better, than correlation between hydraulic conductivity and resistivity. We created some approximation formulas relating filtration coefficient with clay content. Clay content in soil can be estimated on soil resistivity obtained from VES data interpretation and from groundwater salinity found from its resistivity. Then filtration coefficient is determined on clay content. Some examples of this method practical application at clean and oil contaminated areas are presented. We considered anomalies of decreasing filtration coefficient in contaminated zones not as a real effect, but as a good indicator of contamination, though in several publications there were some indications of hydro geological changes in soil properties due to oil contamination.

The firsi aim of resistivitv survey is a study of any inhomogeneities. Frequently in practice the effect of anisotropy is displayed together with that of layenng or inhomogeneities. It coinplicates data mterpretation witlun the frainework of anisotropic models, and distorts resuits of interpretation in the framework of layered or inliomogeneous media. Ignoring of anisotropy resuits in wrong data interpretation. At the saine time anisotropy studying can give valuable geological information. That means that anisotropy itself and the mutual infinence of anisotropy and inhomogeneities needs to be studied. We have developed the efi'ective spectraJ algorithni for azimuthal resistivity survey (ARS) modeling over two-layercd (and multilayered) media witli the amsotropic basement. Its advantage is in the fact, that the most tabor-consuming part of the decision is carried Out in speetral domain, and then transformation of the decision info real doniain is fuifliled easily and fast for any azimuths. For two-layered model that approach gives the decision as series, when calculation goes fast and with controllable accuracy The spectral approach heips to fulliti more deep and full analysis of the decision. The ARS diagram is a periodic function, and has a discrete spectrum. With the help of ARS spectra the important fact was established: for any array above the anisotropic halfspace or horizontaily layered medium the odd harmonics are absent in the spectrum. These appear only when the current source is lowered on a depth or the array is placed near some inhomogeneities.The spectral approach has atlowed to explain many facts, established earlier for anisotropic media 1 2

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