THEORETICAL HORIZONTAL RESISTIVITY PROFILES OVER HEMISPHERICAL SINKS

Abstract

An album of 40 sets (about 200 different curves) of theoretical horizontal resistivity profiles over and near outcropping hemispherical sinks shows the effects of sink diameter, reflection factor k, and distance of the traverse from the center of the sink upon the magnitude and shape of the apparent resistivity curves. Curves for sink diameters of 16a, 8a, 6a, 4a, 2a, a, and 0.5a, and reflection factors k of ±0.8, ±0.6, ±0.4, and ±0.2 are included. The values of apparent resistivity for both the Lee and Wenner configurations were computed using a digital computer. In general, the larger the sink, the larger the magnitude of the apparent resistivity anomaly up to a sink diameter of about 8a, for which the apparent resistivity over the central part of the sink is essentially equal to the resistivity of the sink, irrespective of the reflection factor. For sink diameters equal to the electrode separation a, large apparent resistivity anomalies result from “pseudo‐focusing” effects. The major peaks of the apparent resistivity curves, which lie outside the edges of the sink for negative reflection factors and inside for positive reflection factors, can be used to detect the edges of the sinks. The album of curves provides a general utility for the qualitative interpretation of apparent resistivity anomalies over features other than hemispherical sinks, such as faults (for large sinks), dikes, grabens, and similar features.