Magnetometric resistivity and induced polarisation response of the Elura deposit, Cobar, NSW

Abstract

Magnetic sensors are being used to measure magnetic fields associated with galvanic current flow in a number of countries because of claims that they can detect conductive features beneath conductive overburden. An experimental magnetometric resistivity (MMR) survey was conducted in an area of deep weathering around the Elura lead?zinc deposit near Cobar, Australia. The survey did not appear to detect the Elura deposit. However, anomalies related to shallow conductive features and possible bedrock boundaries were observed. It is shown that the optimum array size for locating a conductive body under conductive overburden, is a compromise between trying to inject more current into the bedrock and not making the array too large to resolve the body being sought. The results from Elura, supported by theoretical calculations, indicate that this compromise will not be met easily with the weathering conditions in Australia, where the thickness and the conductivity of the overburden will frequently attenuate the maximum possible MMR anomaly by about 85 percent. By extending the accepted formulation for magnetic induced polarisation (MIP) response, it is shown that the ability of the MIP method to detect a polarisable body can be predicted from the results of MMR measurements if the body has a conductivity and polarisability contrast with the surrounding medium. This concept indicates that the absence of an MMR anomaly over Elura precludes the MIP method being a successful exploration method for Elura type deposits in this area. However, the results do indicate that MMR can be used as a mapping tool and that it is possible to discriminate between long striking conductors and localised ones by setting up electrode arrays at right angles to the supposed strike of the feature.