EM-coupling removal from time-domain IP data

Abstract

Electromagnetic (EM) coupling of frequency-domain induced polarisation (IP) data has been the subject of many studies, and a number of 'de-coupling' procedures have been devised. However, there has been far less emphasis on coupling in the time domain, the normal approaches being to wait until late times and assume the EM contribution is insignificant or, less frequently, to invoke a Cole-Cole model to account for the EM-coupling response. A fast and simple procedure has been devised for suppression of EM-coupling effects in time-domain IP data. The essence of the approach is to represent the EM-coupling as a half-space decay. The half-space resistivity (EM apparent resistivity) is adjusted via inversion until the fit to the observed transient voltage decay is optimal in the least squares sense. The EM voltages associated with this best-fitting EM half-space decay are then subtracted from the measured voltages to yield a de-coupled 'IP transient'. Transients that are well represented by an EM half-space decay are deemed 'non-responsive' in the context of IP. Transients that deviate markedly from an EM half-space decay are indicative of high apparent chargeability. The application of the new procedure is illustrated on dipole-dipole IP data from the Yandal greenstone belt of Western Australia.