Laboratory Confirmation of Non-Linear Electrical Effects in Mineralised Rocks

Abstract

Induced Polarisation (IP) and Transient Electromagnetism (TEM) are among the most widely used ground electrical geophysics methods in Australia, being specialised in detecting conductive and chargeable targets such as sulphide deposits. A key challenge of IP &TEM methods is that mineralogy of a chargeable target is not able to be determined; often a chargeable body is not a sulphide deposit but a graphite-rich formation of black shale or clays. As these shale deposits are undesirable targets, distinguishing by physical methods such as drilling is exhaustive on exploration resources. Graphite-rich shales, which plague induced polarisation surveys, cannot be distinguished from sulphide mineralisatio by existing electrical methods. The effect of disseminated sulphide grains acting as a diode in the presence of two electrical signals of nonharmonically related frequencies was originally studied in the 1970s. There is some evidence that shales produce a strictly linear response. This contrasts to the non-linear response of sulphide-rich rocks which generates additional signals in the frequency domain. A laboratory-based reinvestigation into non-linear properties of sulphides has been undertaken with a view towards field application. Variable frequencies and signal strengths were used to validate the method, based initially on the processes described in White (1974). The intermodulation mixing terms produced as a response by sulphide mineralised deposits in this laboratory-based investigation may allow for electrical distinction of sulphides from non-mineralised shales and clays. The application of a direct current (DC) bias to either signal increased the detectability of non-linear responses, allowing for a lower current density threshold for measurement. The magnitude of the non-linear signal detected provides a basis for field testing of this previously unutilised electrical property, with the eventual aim to providing a method for greatly reducing the number of drill targets barren of mineralisation.