The Downhole EM Response of an Intersected Massive Sulphide Deposit, South Australia

A downhole EM (DHEM) case history is presented for a massive pyrite-pyrrhotite body discovered during base metal exploration within the Kanmantoo Trough, South Australia. Two transmitter loop locations were used to survey hole WCP2 which intersected two sulphidic intervals totalling a width of 35 m. Changes in the shape of the DHEM profiles with time indicate a complex evolution of the induced eddy current system after transmitter turnoff. At early times, the DHEM response is dominated by currents in the host rock. Superimposed on the host response is the conductor anomaly which changes in sign from early to late time. This sign change is interpreted to result from the use of dB/dt rather than B field measurements. With time, the system of eddy currents within the conductors first expands across the drill hole, then contracts to a late time position above the drillhole. This behaviour is related to the nature of diffusion of electromagnetic fields into a conducting earth and to the strong coupling between the upper parts of the conductor and the transmitter.