Interpretation and design of time domain EM surveys in areas of conductive overburden

Abstract

The TEM method is being increasingly used for mineral exploration and geological mapping in Australia. The techniques can be divided into one-loop and coincident-loop systems (MPPO-1, MPP-3, SIROTEM) and two-loop systems (Pulse EM, Newmont EMP, SIROTEM, UTEM) which exhibit markedly different responses over the conductive surface layers that cover much of Australia. Successful exploration in the 1980s will depend to a large extent on developing methods of quantitative interpretation of anomalies caused by variations in the surface layer. The response of both one-loop and two-loop systems to variations in the conductivity and thickness of a conductive surface layer or host rock has been investigated. The response of a homogeneous ground is relatively simple for a one-loop configuration, but becomes much more complex as the transmitting and receiving loops are separated and form a two-loop configuration. For a two-loop configuration the response changes sign at a time which depends on the loop separation and ground conductivity. In resistive terrains the sign reversal is usually not observed, but in areas with conductive surface layers the sign reversal can occur at quite late times. Variations in the conductivity of the overburden can cause complex anomalies which are difficult to interpret intuitively and to distinguish from anomalies caused by conductive bedrock sources. This problem holds for both one-loop and two-loop configurations, but is further complicated by the sign changes in a two-loop configuration. These features are seen in Pulse-EM data from Cobar. Interpretation of two-loop data is further complicated by the fact that the response of a finite conductor in a resistive host rock will also exhibit sign changes which depend on depth of burial or conductivity. Thus for ease of interpretation, a one-loop configuration is recommended. This can be approximated in a two-loop system by locating the receiver in the centre of the transmitter loop (sometimes called "in-loop" or "frame-loop" configuration). Limits on the detection of mineral deposits in conductive terrains have been investigated by studying the modelled response of the Elura and Roxby Downs deposits using an interactive mini-computer analogue model system. The TEM response is determined for a variety of loop sizes, depths of burial, and conductivities of the deposits and host rocks. The limits of detection depend on all of these factors. In general, the optimum loop size is larger than the body in resistive terrain but should be smaller than the body in a conductive terrain to minimise coupling with the host rock. For both the Elura and Roxby Downs model studies, it was found that the optimum time range for measurement was between 1 and 50 ms. Measurements at earlier and later times, however, give useful information on conductive host rock or overburden.