Frequency and current analysis of non-linear electrical effects in mineralised rocks

Available electrical geophysical methods are specialised in detecting conductive and chargeable targets, particularly sulphide deposits. A key challenge facing Induced Polarisation (IP) and Transient Electromagnetism (TEM) methods is that often a chargeable target is not a sulphide deposit but a graphitic shale or clay formation. These shale deposits are undesirable targets, as discrimination is not able to be determined by geophysical methods but rather requires expensive physical methods such as drilling. There is some evidence that shales produce a strictly linear electrical response, with respect to current density. This contrasts to the non-linear response of sulphide-rich rocks which generates additional signals in the frequency domain. 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. A laboratory-based reinvestigation into nonlinear electrical properties of sulphide minerals, as well as initial field trial results was undertaken. The early results of this work prompted this further study into the current and frequency dependences, in order to determine the impact these factors have on the measurement of mixing terms. Further work on the application of a direct current (DC) bias in order to increase the detectability of non-linear responses has also been undertaken. The encouraging results will direct further field testing, with the eventual aim of providing a new method for mineral discrimination, greatly reducing the number of drill targets barren of mineralisation.