Seismic reflection surveys remain a very effective way of imaging the sub-surface crustal architecture of sedimentary basins. They can also often image the base of the basin (or top of basement) which is an important interface for understanding the structural controls imposed on basin fill and the prospectivity of a basin system because the basin base represents the lower boundary of effective pore space. However, imaging basement in places where basin fill is thick can be problematic in seismic data optimised for near-surface resolution, and in vintage seismic data which is noisy and/or does not penetrate deeply. Airborne gravity and gravity gradiometry surveys can be effective and complementary in these scenarios, and also in proximity to the coast where seismic acquisition is problematic (logistics and high ambient noise). 3D geometries of geological structures can be tested quantitatively against geophysical data - specifically airborne gravity and gravity gradiometry in this example - using a 3D forward modelling and inversion approach. Three-dimensional forward modelling is a technique that involves construction of geological 3D computer models, assigning rock property data (such as density) to packages of rocks, and calculating the synthetic gravity response of the theoretical model. The difference between the calculated and observed response (the residual gravity response) indicates a mis-match, where changes to the 3D model need to be made. This study aims to provide a workflow to tackle the problem of superposition of geophysical anomalies. We use the Otway Basin in southeast Australia as a `laboratory' to provide a systematic approach for understanding basin and underlying basement geometry, using airborne gravity data in conjunction with other geophysical and geological datasets. A new regional 3D model of the Otway Basin is presented which is not only consistent with airborne gravity gradiometry data acquired over the on- and near-shore Otway Basin, but which also integrates the current understanding of the crustal architecture in the Otway region with previous studies including seismic interpretation of the basin and of the basement, previous 3D geological framework models of the basement, and also insights gained through surface mapping.

Open-Access Online Publication: May 29, 2023