UNIPOLE METHOD OF ELECTRICAL PROFILING

Abstract

In the unipole method of resistivity prospecting, an electrode arrangement AMNA′ with a third common electrode at infinity, is used as against the arrangement AMNB in the conventional bipole method. The electrodes A and A′ are connected to one and the same pole of electric current. In the case of the unipole method, field intensity at the center of the spread is zero for a homogeneous ground. This results in a pure‐anomaly method, increasing the depth of investigation and the sensitivity of field measurements considerably. The percentage of total current entering the deeper regions is larger in the unipole method than in the bipole method. The difference is further increased if a less resistive layer is present near the surface of the earth. Sensitivity of the unipole method to the near‐surface inhomogeneities is less than that of the bipole method. But the unipole method is more sensitive to the deeper inhomogeneities of interest. The unipole arrangement introduces some focusing of the current within a particular range of the depth which can be controlled by altering the spacing between the current electrodes. In general, response curves for the unipole method are less complicated and admit more controls for interpretation than those for the bipole method. A theoretical and experimental comparison of the unipole method with the conventional bipole method shows that the former method is more effective than the latter in certain applications. The field instruments used at present for the bipolar measurements can be very easily adapted to the unipole method.