Laboratory modelling of self-potential anomalies due to spherical bodies

Abstract

The relationship between the self-potential (SP) produced by a polarised sphere and its depth was studied in a laboratory experiment. This was carried out by using a sphere made up of two hemispheres of different materials: one copper and the other zinc. Self-potentials were measured by placing the sphere at a given depth in a rectangular glass tank filled with water. The surface of the water was covered by a sheet with 684 brass electrodes. A sensitive, high impedance digital voltmeter was used to measure potentials from each electrode to a ‘base’. We have measured the SP response of the metallic body and our work shows that SP signals of several millivolts are generated due to the sphere placed within water. The gridded SP data show a clear anomaly over the sphere at shallow depths, and as the depth of the sphere increases, the measured SP signal due to the sphere decreases. An analytical formula is given to determine the maximum depth of the sphere at which the presence of the anomaly can be detected. Responses from other geometries are examined as well.