Induced‐polarization effects in time‐domain electromagnetic measurements

Abstract

Sign reversals in the coincident‐loop transient response can be produced by employing a Cole‐Cole model in numerical TEM modeling of polarizable conductors. These reversals may be thought of in terms of a polarization current which changes sign during the transient, passing from a charging current at early times to a discharging current at late times. In a layered earth, the relative strength of this current compared to the normally induced vortex current dictates whether or not a reversal is seen. If the earth is conductive, the effects of the polarization current may never be seen. If, however, the earth is only moderately conductive, the polarization current may dominate. In the case of a 3-D polarizable conductor in a conductive host, the addition of a host response serves to delay the time of any sign reversal in the transient. Reducing the host rock response by increasing its resistivity enables the polarization current to dominate earlier. By bringing the conductor closer to the surface, the amplitude of the negative response can be made greater and hence the sign reversal brought earlier in time. In such cases, moderate polarization parameters may cause substantial negative responses. It is possible to interpret TEM anomalies exhibiting sign reversals. The location and geometry of a discrete polarizable conductor can be correctly assessed, and a valid but approximate TEM time constant can be measured, from the positive part of the transient before the sign reversal.