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.

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.

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At calculation of rocks resistivity some statistical formulas are frequently used, the best example is the Archie formula. In the present report calculations of rocks' resistivity are made on the basis of the exact physical-chemical theory. The merit of the exact theory application is that this approach allows understanding more directly the mechanism of rocks resistivity forming and the different factors' influence. The demerit is, that the algorithm requires a good knowledge of physics and chemistry and inputting additional information, which a practical geophysicist not always has. The main stages of rock' resistivity calculation are: electrical conductivity account for electroneutral solutions; calculation of electrical conductivity of a liquid in a double electric layer taking into account the electric properties of the solid phase, then calculation of rock's resistivity for definite lithology, porosity, humidity and grain size. On a series of examples the possibilities of such calculation and the influence of the different factors are shown. Most evidently the algorithm's opportunities are uncovered at resistivity calculation of sandy-clayish sediments. It is possible to use the algorithm at the inverse problem solution for resistivity method-for the estimation of rocks hydro-geological properties on their resistivity.

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