Exploring through cover ? the integrated interpretation of high resolution aeromagnetic, airborne electromagnetic and ground gravity data from the Grant?s Patch area, Eastern Goldfields Province, Archaean Yilgarn Craton. Part B: Gravity inversion as a bed

Abstract

The cost and risk associated with mineral exploration in Australia increases significantly as companies move into deeper regolith covered terrain. The ability to map the bedrock and the depth of weathering within an area has the potential to decrease this risk and increase the effectiveness of exploration programs. This paper is the second in a trilogy concerning the Grant?s Patch area of the Eastern Goldfields. The recent development of the graVP potential field inversion program in conjunction with the acquisition of high-resolution gravity data over an area with extensive drilling provided an opportunity to evaluate three-dimensional gravity inversion as a bedrock and regolith mapping tool. An apparent density model of the study area was constructed, with the ground represented as adjoining 200 m by 200 m vertical rectangular prisms. During inversion graVP incrementally adjusted the density of each prism until the free-air gravity response of the model replicated the observed data. For the Grant?s Patch study area, this model proved easier to interpret than the Bouguer gravity. A regolith layer was introduced into the apparent density model and realistic fresh-rock densities assigned to each basement prism according to its interpreted lithology. With the basement and regolith densities fixed, the graVP inversion algorithm adjusted the depth to fresh basement until the misfit between the calculated and observed gravity response was minimised. The resulting geometry of the bedrock/regolith contact largely replicated the base of weathering indicated by drilling with predicted depth of weathering values from gravity inversion typically within 30% of those logged during RAB and RC drilling.