3D Gravity Modelling and Interpretation for the 1:250,000 Boulia Map Sheet, Queensland

Abstract

Sedimentary cover thickness strongly affects area selection for exploration in Proterozoic and Archean basement. Depth-to-basement is often inferred from gravity data. Interpretation of gravity is, however, fraught with ambiguity. The uncertainty can be minimised if all available information is exploited. In this paper drill logs, aeromagnetic depth-to-basement and basement geology interpretations, and a compilation of density values, as well as gravity data, are used to constrain a 3D density model of the Boulia 1:250,000 map area, Queensland. Four recent events riggered this work: acquisition of 4km-spaced gravity data by the Queensland Department of Mines and Energy; publication of an aeromagnetic interpretation by AGSO; development of a computer program for constrained 3D inversion of gravity; and availability of sophisticated 3D geological modelling and visualisation software. A 3D starting model was constructed, with basement topography, lithology, and density conforming to drill hole data and aeromagnetic interpretations. Paleozoic cover was represented as a layer of uniform density. The calculated gravity response of the starting model resembled the observed gravity in gross terms, but did not provide a satisfactory fit. Shallow magnetic basement correlated with low gravity. Denser basement units tended to be deeply buried. The basement densities and elevations have been adjusted via inversion, to achieve a close fit to the data. In the areas of shallow basement (less than 1 km), basement depths have changed by up to 300m. In the NW corner and the central south of the map sheet, the interpreted depth of undivided granite basement has in places been reduced to 100m and 200m respectively. The Boulia 3D density model represents a starting point for future refinement as additional holes are drilled and as new gravity and density data are collected. In particular, the gravity contribution from limestones in the cover sequence is not well understood. Additionally, combined magnetic and gravity inversion could further advance geological understanding of the area.