Field results for direct-current resistivity profiling with two-electrode array

Abstract

Encouraged by earlier model experimental and theoretical results of resistivity profiling over conducting veins by the two-electrode vis-a-vis other configurations, field tests have been carried out in four areas. Three of these areas concern conducting vein-like targets of altered volcanic neck, sulphides and graphite. The remaining area relates to a resistive target — a quartz vein intrusive into granite. As far as conducting vein-shaped targets are concerned, the simplest unfocused two-electrode array has overwhelming advantages over the Wenner, Schlumberger, modified unipole, and focused surface laterolog systems in shape and amplitude of anomalies, in depth of detection, and in cost of operations. With an electrode spacing only about one-fourth (or less) that of Wenner, the two-electrode anomaly has a comparable or larger magnitude and consists of only a simple centered trough as against a trough-peak-trough combination, for instance, of the Wenner configuration over a thin conducting vein. In addition, with the two-electrode array, the depth of detection seems to be about 75% higher and the cost of operation about 20% lower than those with Wenner. From the limited data concerning resistive veins, no one-electrode system seems distinctly better than the others, except for cost of operations which would be lowest for the two-electrode configuration. The response of the focused surface laterolog system using seven electrodes, for conducting as well as resistive targets, is equivalent or nearly equivalent to that of the modified unipole having only three electrodes, which in turn can be computed from two-electrode measurements. Focusing the current down towards the target does not necessarily improve the response measured on the ground surface.