LIGHTING UP THE REGOLITH WITH ELECTRICAL GEOPHYSICS

Downhole-to-surface electrical geophysics provides a rapid and cheap approach to determine the spatial extent of mineralisation beneath cover. In this method, sometimes known as mise-à-la-masse, a downhole current electrode provides a source, and a roving surface electrode measures the resulting electrical potential. The method is particularly applicable at brown-fields exploration sites where mineralisation or alteration has been intersected by one or more drill holes, as it indicates continuity of electrical interconnection of mineralised regions, extending exploration targets away from drill holes. To date, interpretation methods have been mostly qualitative, involving plots of gridded surface electrical potential data, with little or no processing. This CRC LEME funded study provides a quantitative means of interpreting data, through (a) a numerical approach to separate the surface potential into contributions from the source current electrode and sub-surfaces charges at resistivity boundaries in the Earth, and (b) a simple three-dimensional (3D) imaging method to determine the lateral and vertical extent of the conductive mineralisation. The methods described work in both the case where a drill hole intersects mineralisation and the source current electrode is directly connected to the conductive ore, and in the near-miss scenario in which the drill hole does not intersect mineralisation. Data from the Golden Grove VMS deposit in Western Australia are shown for example