Augmented Mise-à-la-Masse interpretation using rapid numerical methods

Abstract

The applied potential, mise-à-la-masse (MALM) method, is used in the exploration of base metal deposits (Golden Grove, Western Australia and Prominent Hill, South Australia), where MALM data was collected. The method maps surface potentials (or voltages) associated with resistivity contrasts intrinsically linked with the geology. lnformation pertaining to the structural geometry and electrical connectivity of the conductive targets can be deduced from the surface potentials with the traditional placement of the source electrode in contact with the conductive body. However, in many cases, boreholes do not intersect the mineralisation. Such boreholes can still be used if the mineralisation is close and the surface potentials have similar anomaly patterns to the in-mineralisation responses. For electrodes in these "near-miss" positions, at shallow depths, the sudace potentials are, however, dominated by potentials associated with the proximity of the source elechode. Two new numerical methods have been developed to analyse surface potentiais of MALM data from a VMS deposit at Golden Grove, WA and a Cu-Au deposit at Prominent Hill, SA. A finite element method modelling program developed by Zhou and Greenhalgh (2001) modelled the physical characteristics of a conductive body and in a resistive half-space for various locations of the down-hole electrodes. Parameters including varying resistivity contrasts, volume and dip of the conductor were investigated. Other models studied the effects of near-miss electrode positions and associated surface responses. The methods developed here aid in the interpretation of MALM data by firstly separating the electrode response from surface potentials, leaving the conductive body response and secondly by developing a three-dimensional image approximation of the resistivity contrasts to assist in geological interpretation. To enhance the potentials attributed to resistivity contrasts, a method of numerically calculating the potentials produced by the source electrode and subtracting it from MALM surface potentials yields residual potentials that better represent the subsurface geology. This method was most effective with surveys that had surface potentials distorted by the electrode response, particularly for shallow near-miss electrode positions. To image the subsurface in three dimensions, a modified version of Hämmann et al.'s (1997) image reconstruction algorithm was adapted to use the MALM sudace potentials. The method represents the surface potentials by the super-positioning and correlation of elementary point sources of electrical potential. The imaging proved to be sensitive to changes in the distribution of the surface potentials and aided in MALM analysis, These two methods used in combination with a priorigeologic/geophysical of the field sites information aided in the interpretation of MALM field data sets, The outcomes of the method showed that the combined use of the electrode effect removal and subsurface approximation better approximated the geology than the unprocessed MALM data.