A computer model evaluation of the resistivity and IP response of the Elura deposit

Abstract

The results of a time domain IP survey using a 100 metre dipole?dipole array over the Elura orebody on lines 50800N and 20100E are presented together with an orthogonal Schlumberger sounding centred on the orebody. The IP data show a well-defined, strong chargeability anomaly directly over the orebody. Low background chargeability implies that layered earth EM coupling is a negligible component of the anomaly. The IP anomaly exhibits characteristics which can only be attributed to a buried three-dimensional polarisable conductor in electrical contact with the weathered layer. This is confirmed by two-dimensional and three-dimensional computer model simulations of the Elura orebody, which illustrate the important role that current-gathering plays in producing the strong IP response of a polarisable conductor in electrical contact with a conductive overburden. Insulation of this type of body from a 100 m thick conductive overburden by as little as 25 m of host rock results in a substantial reduction in IP response to the point where it may only be marginally detectable. An interpretation of electrical field anomalies arising from short strike length conductors using two-dimensional interpretative models yields intrinsic IP parameters which are underestimates and intrinsic resistivities which are overestimates. The field IP data and computer model results suggest that polarisable bodies like Elura may be readily detected by modern IP techniques even beneath deeply weathered terrains as long as they intersect the overlying conductive weathered layers. It is unlikely that short strike length, very conductive, blind orebodies that are well insulated from a thick cover of conductive overburden, will be detected by any sort of modern surface IP system.