As part of the Federal Government's Exploring for the Future program, whole-of-crust 3D gravity and magnetic inversion models have been produced for an area encompassing the North Australia Craton (NAC). These models were created to aid 3D geological mapping and identification of large-scale mineral systems such as iron oxide copper-gold (IOCG) systems. The inversion models were derived using the University of British Columbia - Geophysical Inversion Facility (UBC-GIF) MAG3D and GRAV3D programs. We used reference models that had layers for Phanerozoic sediments, Proterozoic sediments, undifferentiated crust and the mantle. The reference model for the magnetic inversion incorporated a Curie depth surface below which magnetic susceptibility was set to zero. To facilitate cross-referencing of the density and magnetic susceptibility models, we used identical meshes for the two inversions. The spacing of the available gravity data dictated a horizontal cell size of 1 km. We used 61 vertical layers of thickness increasing with depth. The area of interest was 2450 km by 1600 km, which meant that the mesh for the NAC models had ~240 million cells. It was not possible to invert a model of this size. Instead, we broke the problem down into a grid of overlapping 'tiles' with 8 rows and 10 columns. Each tile was independently inverted. When the overall model was reconstructed using the core region of each tile, some low-level edge effects were observed, increasing in significance with depth. These effects were satisfactorily attenuated by applying cosine weighting from the centre of each tile out to the edge of the data-padding zone during reconstruction. The success of the NAC modelling exercise has given us confidence that we can expand the coverage to produce coincident gravity and magnetic inversion models for the entire Australian region.

Open-Access Online Publication: March 01, 2023