Applied potential modelling of simple orebody structures

Abstract

In applied potential or mise à la masse surveys current is injected into an orebody in one drillhole and the resulting electric potential is measured as a function of depth in another drillhole, or as a function of horizontal position over the ground surface. The crosshole measurements are intended to establish the continuity or otherwise of the conducting ore. One can imagine the situation where metallic ore is encountered in both holes, but it is not known whether the holes intersect the same continuous orebody, or whether it is broken between the holes, or whether the intersections are of different ore surfaces. Alternatively, ore is intersected in only one hole and the question is what can one infer from DC electrical measurements about the extent of the conductor between the two holes. In this paper we present a formulation for calculating the electrical potential distribution in an inhomogeneous 2-D or 3-D earth for any number of current electrode sources or sinks. Numerical modelling has been carried out for various classes of ore body structure, to understand the effects of conductor continuity, depth, thickness, dip and irregularity on the applied potential response. The effect of current electrode placement inside and outside the conductor, was also studied. It is possible from the shape and amplitude of the potential profiles to partially discriminate between continuous, terminating and faulted conductors. The modelling is especially useful as an aid to interpretation of field measurements and in the design of applied potential surveys.