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A two-zone theoretical model, consisting of a cylindrical borehole filled with drilling mud and surrounded by homogeneous, isotropic rock, was studied to aid interpretation of electrical resistivity logs. Apparent resistivities are numerically calculated as a function of the rock and the drilling mud resistivity and the separation of the electrodes on the coaxial measuring sonde. For practical use, the inverse, interpretation problem must be solved. Therefore, graphs for finding the true matrix resistivity -- given the hole diameter, mud resistivity, and electrode spacing’s -- are presented for ranges applicable to Hawaiian conditions. The interpretation may be done with the interpolative digital computer program provided.