Some case histories of source discrimination using time‐domain spectral IP

Abstract

It has long been known that induced polarization (IP) transient voltages decay with time at a rate that is dependent strongly on the mean grain size of the metallic conducting particles in the IP source. The Cole‐Cole model provides a three‐parameter representation (M, τ and C) for IP responses. The time‐constant (τ), in particular, has been found to be very useful in resolving IP sources with differing mean particle size. The characterization of IP responses in terms of these parameters has been termed “spectral IP.” These parameters may be determined either through the analysis of the response of the earth to sequential transmission of ac currents of different frequencies (i.e., frequency‐domain IP), or through the analysis of the transient decays resulting from the transmission of interrupted square‐wave currents (i.e., time‐domain IP). The latter approach offers the major convenience of being applicable to data obtained in the course of routine production surveys, with no increase in survey time. In practice, spectral IP parameters are determined most readily from time‐domain transients through the computer matching of the observed data to the best fit in a family of precalculated Cole‐Cole curves. This may be done, off‐line, using a PC, or, in a recent receiver, essentially on‐line, using software imbedded in the receiver. Field case histories from Canada, Finland, and Australia are given to illustrate the application of spectral IP to the resolution of IP sources in the time domain. It is recommended that this approach to the processing and presentation of time‐domain IP data should be applied routinely, as a very cost‐effective enhancement to the exploration value of such data. An additional benefit from this presentation is that it will facilitate a sharing of experimental results with workers in the frequency domain who may also use the spectral IP approach.