Modelling of drill-hole TEM responses from multiple targets covered by a conductive overburden

Abstract

A number of transient electromagnetic (TEM) analogue modelling projects have been carried out at CSIRO to compile suites of model responses for surface, drill-hole and airborne surveys. The drill-hole modelling described in this paper has been carried out to determine the response of targets in free space and covered by overburden. The objective of this modelling was specifically to investigate the effect of inductive interaction between a conductive overburden and electrically-isolated targets beneath it, and to examine the effect of current channelled from the overburden into a target in electrical contact with it. For the experiments discussed here, a model consisting of two parallel plate targets with a dip angle of 45° and covered by a conductive overburden was employed. Measurements were made both with and without the overburden present. The modelling results show that for cases of simple conductors without overburden, the characteristics of the drill-hole TEM responses can be understood in terms of the geometrical relationship between the transmitter loop, the drill-hole receiver probe, and the conductive targets. However, the modelling experiments demonstrate that the presence of a conductive overburden can significantly alter the drill hole response. Inductive interaction between the overburden and targets can be large enough to cause polarity reversals of the drill-hole response of the targets. Additionally, when the upper target is in electrical contact with the overburden, current channelling produces a galvanic current that may suppress or enhance peaks in the drill-hole profiles. When interpreting field data with the use of superposition of responses from various components of a given model, unexpected polarity reversals and enhanced or suppressed peak responses in the field data could be explained by these two effects.