Abstract

Transient electromagnetic (TEM) measurements have been made with SIROTEM on four separate surveys over the Teutonic Bore orebody (Western Australia), both before mining operations began and subsequently during different stages of stripping overburden from the mineral deposit. In the late stage of the transient decay the target response was relatively free of the overburden and host-rock response. Beyond approximately 6 ms, the maximum anomalous response was a factor of 8 to 10 greater than the combined overburden and host-rock response.Analog modeling with a copper plate in free space shows that the TEM response of the target consists of a single peak at early delay times, while at delay times beyond approximately 4.2 ms, the response becomes a double-peak anomaly with a low directly over the top of the plate. Mathematical modeling of the TEM response with a free-space infinitely thin plate produces profile characteristics similar to those obtained by analog modeling beyond a delay time of approximately 4.2 ms. Inversion of premining survey profiles in the delay time range 7.0 to 13.2 ms gives values of 82 m for target depth d, and 86 degrees for dip angle theta . These agree well with the values d = 86 m and theta = 82 degrees derived from drilling data. A target conductance value in the range 250 to 320 S is obtained from the TEM data, indicating that the massive sulfide target is highly conductive.Responses calculated for surveys made during overburden stripping are lower than corresponding field values at early delay times because of the absence of overburden response in the model measurements. At delay times beyond 8.5 ms, the model values are consistent with the field values. These results indicate that for a case similar to the Teutonic Bore orebody, where the maximum anomalous late-time response is a factor of 8 to 10 times greater than the background response, important target parameters may be derived from free-space models.

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