A finite difference model for mineral self potentials in complex environments

Abstract

Popular techniques for the quantitative interpretation of spontaneous mineralization potentials assume an electrically uniform conducting environment. In reality this is seldom the case. The mineralized environment typically displays a range of conductivities due to the effects of weathering, lithology changes, geological structure and the mineralization itself. Integral equation modelling of the self potential (SP) field in horizontally and vertically layered earth models suggested by these influences demonstrate the significant effect that the conductivity structure can have on self potentials. Any quantitative interpretation of SP data must therefore allow for the conductivity of both the source body and its environment. This suggests that SP surveys should in general be augmented by additional electrical measurements (e.g. resistivity or electromagnetic) to define the conductivity structure. It also highlights the need for a numerical technique capable of accomodating the complex conductivity variations typically associated with the environment of sulphide and other conductive mineralization. Accordingly a finite difference model for mineral self potentials is developed. It is first demonstrated that the double layer source of recent SP models is simply related to a volume density of current dipole moment (current polarization). The potentials arising from this latter source are easily modelled by means of a finite difference formulation based on a local integral form of the continuity relation for stationary current flow. The method is evaluated by a comparison of the predicted potentials against those computed by the origional integral equation technique.