We develop a new scheme for the interpretation of<br>geophysical resistivity logs tapping fresh water<br>aquifers. The procedure is based on a volumetric<br>matrix-conductivity concept and on Bruggeman’s<br>equation for a concentrated mixture of “solid<br>grains” in a continuous electrolyte. The bulk<br>conductivity of this fully saturated sandstone is<br>controlled by its effective porosity and pore<br>tortuosities, by an equivalent grain conductivity<br>resulting from the electrical double layers<br>developed at clay-electrolyte interfaces, and by the<br>conductivity of the electrolyte. The scheme<br>assumes that the resistivity readings of a induction<br>log or a laterolog, are the expressions of can be<br>converted into the electrical characterization of the<br>invaded and of the virgin zones of the aquifer. It<br>also assumes that the effective porosity, and the<br>native water and mud filtrate resistivities are<br>known independently. The scheme has been<br>satisfactorily tested with laboratory data on shaly<br>sandstone samples and applied in the electrical log<br>interpretation of wells drilled in the Recôncavo-<br>Tucano basin of Bahia, NE Brazil.


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