Correction for Finite Distance Between Electrodes in Schlumberger Resistivity Soundings

Abstract

Resistivity soundings made with an expanding Schlumberger array usually result in sounding curves comprising several offset segments, each corresponding to a different spacing of potential electrodes. The offsets are partly due to resistivity inhomogeneities near the potential electrodes and partly to the necessity of using finite separations between potential electrodes in order to obtain measurable signals. By expanding the general theoretical expression for the Schlumberger apparent resistivity in terms of MN/AB (ratio of distances between potential and current electrodes) a power-series expression is obtained for the offset caused by finite potential electrode spacing. The power series is even, with the second order correction term being related to both the slope and curvature of the sounding curve. Analysis of the corrections needed for sounding curves typical of geothermal regions allows the seriousness of the measurement problem to be assessed and leads to the development of a stepwise correction process which can be applied to existing field data and will result in more reliable layer interpretations.