Predictive maps of the subsurface can be generated when geophysical datasets are modelled in 2D and 3D using available geological knowledge. Inversion is a process that identifies candidate models which explain an observed dataset. Gravity, magnetic, and electromagnetic datasets can now be inverted routinely to derive plausible density, magnetic susceptibility, or conductivity models of the subsurface. The biggest challenge for such modelling is that any geophysical dataset may result from an infinite number of mathematically-plausible models, however, only a very small number of those models are also geologically plausible. It is critical to include all available geological knowledge in the inversion process to ensure only geologically plausible physical property models are recovered. Once a set of reasonable physical property models are obtained, knowledge of the physical properties of the expected rocks and minerals can be used to classify the recovered physical models into predictive lithological and mineralogical models. These predicted 2D and 3D maps can be generated at any scale, for Government-funded precompetitive mapping or drilling targets delineation for explorers.

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Summary of GA's plans for marine seismic and reconnaissance surveys off southwestern Australia in 2008/09 as part of the Offshore Energy Security Program

The Southwest Margin of Australia includes the Paleozoic to Mesozoic Perth basin. Depth-to-basement and basement structure and composition across this region remain poorly understood due to a limited extent of exposed basement outcrop, few wells intersect basement and the lack of resolvable basement horizon in many of the seismic lines. This study uses the integrated modelling and interpretation of all available geophysical and geological datasets to produce new interpretive maps of basement architecture, composition and structural fabric to better characterise the nature of basement across the region. The basement domain, structure and composition maps have been constructed through the integrated interpretation of all available geological and geophysical datasets, including outcrop, wells, geochronology, seismic, gravity, magnetic and bathymetry datasets. These products are predictive tools for better understanding structural reactivation patterns and associated changes in basin geometry through time, as well as variations in basement derived heat flow. A depth-to-basement model was developed using the Spector and Grant method, implemented using custom software. Depths are measured from straight line segments in the azimuthally averaged power density spectrum of sub-sectioned magnetic grids. This allows additional geological and geophysical data (e.g. wells, surface outcrop, gravity and seismic interpretations) to be integrated into the workflow, resulting in a more geologically plausible model. The model provides a new view of Perth basin geometry, not obtainable from seismic data alone, which highlights the location and geometry of key depocentres and provides additional constraints on the possible thickness of pre- and early syn-rift sediments.

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