Scale model investigations of the Applied Potential method for crosshole delineation of a conductor

Abstract

The Applied Potential method finds important application in mineral exploration. To better understand the detection and delineation capabilities of the technique, especially where (conductive) mineralisation is discontinuous, we have carried out a series of laboratory experiments using a 3D tank model system. Crosshole electric measurements were made with a bipole-bipole array, with one current electrode located in each simulated borehole. Both scanning (both bipoles moved together) and profiling measurements (using one fixed bipole and one mobile bipole) were conducted rather than full tomographic imaging. Five different classes of cylindrical model were studied, which were distinguished on the basis of (1) cylinder orientation relative to the plane of the boreholes, (2) whether the target was continuous, terminating, or broken between the points of measurement, and (3) whether the conductor was intersected by one or both boreholes. Each class of model produces a characteristic albeit complicated response, which can be used to identify the conductor and roughly locate its upper and lower boundaries. Data were collected for different bipole separations. Pronounced anomalies are produced for the scanning arrangement for models involving conductor intersection, although it is difficult to always distinguish the models and interpret the data. The character is very dependent on the electrode configuration relative to the target. Electric profiling produces a subtler and less diagnostic pattern, which is hard to discriminate from a background variation. The examples presented provide useful guidance when designing mise- -la-masse or Applied Potential surveys. Moreover, they indicate that full tomographic imaging is required for all but the simplest cases.