One-loop and two-loop TEM responses of the Elura deposit, Cobar, NSW

Abstract

The Bureau of Mineral Resources (BMR) carried out a number of TEM surveys over the Elura deposit in the period 1974?1979. The one-loop (coincident-loop) TEM method gives an anomaly of about twice background with both MPPO-1 and SIROTEM instruments over the deposit, at times greater than 2 ms. At early times the response is due almost entirely to overburden effects. Grid coverage of an area 600 m ´ 700 m, using 50-m loops, delineated a conductive feature which strikes approximately north?south within the overburden. This feature is located over the Elura deposit, and could be the cause of anomalies found with other electrical and electromagnetic methods. At later times three anomalies were detected in the area. Although the anomaly over Elura was marginally stronger, the other two would, in the course of routine exploration, also be regarded as prospective targets. The most promising method of quantitative interpretation appears to be analysis of the time constant of the decay. This was not possible for the Elura data because of limitations of both the MPPO-1 and SIROTEM. However, semi-quantitative interpretation using the time constant at a 10 mV/A level allows ranking of anomalies and was successful in defining the Elura anomaly as the most promising of the three anomalies recorded at late times. Large loop sizes result in smoother profiles because of the averaging effect on lateral inhomogeneities. Large loop sizes result in smoother profiles because of the averaging effect on lateral inhomogeneities. The two-loop configuration results in an anomaly over the Elura deposit which is opposite in sign to that expected from model studies. The two-loop configuration is optimally coupled to conductors with long strike length, and the near-surface conductive zone is interpreted as contributing in part to the two-loop anomaly.