Removing the regolith: EM defined structure fails to correct gravity at Elura

Abstract

On the Australian continent, inhomogeneous regolith acts as a shallow source of gravity anomalies, and hence provides a significant interpretational problem for mineral explorationists. Published results from Western Australia, identify the geometry of the basement topography as being reflected in the gravity profiles. Exploration beneath this inhomogeneous regolith, using gravity without the effect of the regolith cover itself, would provide a distinct improvement of gravity?s diagnostic capability. Initial analysis suggested that EM was a feasible method to define basement topography, from which a gravity correction could be attempted. The primary question needed to assess the feasibility was: ?Does a mappable conductivity boundary correspond to each major boundary of density contrast?? Testing of this hypothesis was completed at Elura, New South Wales (N.S.W.) by constructing an EM model and a gravity model for the purpose of interpretation. In surprising contrast to petrophysical evidence and Western Australia experience, gravity profiles at Elura reflected lithology as being a more important source of shallow anomalies than the depth of weathering. An attempt to apply the geometry of the modelled EM basement/regolith contact as a means of creating a model for gravity correction saw no improvement in the residual gravity profile. The regolith structure at Elura was geophysically delineated as having a progressively increasing resistivity with depth, but one where gravity highs are not reflected in basement highs.