A systematic geophysical procedure has been developed and applied to image groundwater contamination caused by industrial activities in Bahia, Brazil. The procedure combines the use of conventional Schlumberger sounding with a regular scheme of sampling the resistivity stratification in depth. This is achieved by traversing an area with multiple profiles measured at selected electrode spacing. By multiple profiles we mean closely spaced partial soundings made using only six electrode spacings. Partial and complete soundings are correlated and inverted assuming horizontally stratified models within the limits covered by each electrode array. Pseudo-resistivity sections constructed from these data are inverted and adjusted using a two-dimensional finite difference algorithm. Electrical and lithological well logs are used to constrain this interpretation. The procedure was successfully applied to investigate the groundwater conditions and to outline contaminant plumes within industrial areas of the Camaqari Petrochemical Center, Rec6ncavo basin, Bahia. The study includes cases of electrically conductive plumes generated by infiltration of inorganic aqueous effluents and a resistive plume containing hydrocarbon contaminants.

In this paper, I evaluate the potential field due to a dc current source located anywhere within a horizontally layered space, all layers possessing exponentially varying resistivities. The solution takes the form of Hankel transforms with their kernel expressions containing functions defined by recursion formulas. The resulting expressions can be used to model any kind of resistivity array. Specializing the general solution to full‐ or half‐space models possessing exponential dependence, I find terms I interpret as primary and secondary contributions. Analytic expressions for the secondary electrical field revealed an error in the Stoyer and Wait solution for a half‐space. To test the derived n-layer solution based on recursion formulas, I modeled resistivity logs for both homogeneous layers and intercalated homogeneous and heterogeneous layers. Curves from the theory reproduced asymmetries and ripples observed in real resistivity logs.

In this work we discuss the results of an experimental study performed with a multi-frequency electromagnetic method over a mature oil field in Recôncavo basin, Bahia-Brazil. Five 1.8 km transects 200m apart and extending over a block of the oil reservoir were surveyed. The processed EM data are represented as cross-sections of apparent resistivity and induced polarization parameter, using a consistent plotting procedure developed by Dias and Sato (1981). All the sections, controlled by seismic and well log data, although showing some distortions in the IP-resistivity configurations, allow to recognize the following geological features: (i) the oil sandstone horizons and their trapping shales; (ii) the oil-water interface and some zones of steam invasion; and (iii) lateral electric contrasts representing fault zones. These results suggest the real possibility of the use of the spectral EM method in the direct detection of hydrocarbons, as well as for monitoring the efficiency of the artificial fluid injection used for secondary recovery.

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