EVALUATION OF THE MEASUREMENT OF INDUCED ELECTRICAL POLARIZATION WITH AN INDUCTIVE SYSTEM

Abstract

The induced polarization (IP) technique is based on the observation that variations of earth conductivity with frequency may be indicative of buried metallic mineralization. Conventional IP involves the use of grounded electrodes, the installation of which can be quite time consuming. However, the electromagnetic fields about an oscillating magnetic dipole also depend on ground conductivity. Thus, it is conceivable that we might be able to detect IP anomalies with an inductive system, thereby eliminating the need for grounded electrodes. An airborne induced polarization method is a theoretical possibility. Theoretical calculations based on a conductivity model determined experimentally at frequencies less than 30 hz suggest that the effect of polarizable material on electromagnetic response is quite small. In order to check the theory and to determine experimentally whether inductive IP is feasible, field tests were conducted in two areas in Nevada which exhibit strong conventional IP anomalies. The field tests consisted of measurements of the amplitudes of the electric and magnetic fields about a horizontal loop of wire carrying current at frequencies ranging from 15 hz to 1500 hz. The presence of the polarizable material is not evident in the inductive data; in fact, the observations can be fitted to theoretical curves for nonpolarizable models. Hence, on the basis of both theory and field tests, it is concluded that inductive IP based on amplitude measurements is not a practical exploration tool for environments such as that of the southwestern United States.