Abstract

Using a finite-element algorithm which allows for subsurface current and potential electrodes in dc resistivity, we have analyzed the detection of a thin, 2-D, conductive inhomogeneity in the presence of several sources of geologic noise. The pole-pole array with the current electrode fixed in one borehole and the potential electrode movable in adjacent boreholes is the main array of concern. The sources of noise are surface topography, buried topography, random geologic noise, quasi-random geologic noise (nontarget inhomogeneities), layering, and a vertical contact. For several positions of a downhole source electrode, normalized apparent resistivities have been computed. These resistivities have been contoured in section view as appropriate to cross-borehole investigations.For the models studied here, surface topography, buried topography, random and quasi-random geologic noise do not obscure the anomaly due to the thin conductive inhomogeneity. In a vertically or horizontally layered earth, the anomaly due to the inhomogeneity is almost totally obscured. Normalization of the apparent resistivity by the variable apparent resistivity of a layered half-space can help alleviate the problem. However, it may not be possible to apply such normalization in field studies. Other arrays such as the cross-borehole dipole-dipole array, with the dipoles moved simultaneously, may then be required.

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