Investigating Australia's Under Cover Geology Using the World's Largest Airborne Electomagnetic Survey

The AusAEM survey is the world's largest airborne electromagnetic (AEM) survey flown to date, extending across an area exceeding 3.5 million km2 over Western Australia, the Northern Territory, Queensland, New South Wales Victoria and South Australia. Airborne electromagnetics is a geophysical method at the forefront in addressing the challenge of exploration under cover. In collaboration with the state and territory geological surveys, Geoscience Australia has led a national initiative whose goal is to acquire AEM data at a nominal line spacing of 20 km across Australia. The interpreted AEM conductivity sections were inverted using Geoscience Australia's open source Layered Earth Inversion Sample-By-Sample Time Domain Electromagnetics (GALEISBSTDEM) inversion. Horizontal along-flight line resolution is 12.5 m, and the vertical resolution varies exponentially with depth. Inversion cell sizes increase from 4.0 m at the surface to ~55 m in the bottom cell of the conductivity sections, ~500 m below surface. Consequently, the ability to resolve fine detail varies with depth. Using this dataset, we interpret the depth to chronostratigraphic surfaces, assembled stratigraphic relationship information, and delineated structural and electrically conductive features. Our results improved understanding of upper-crustal geology, led to 3D mapping of palaeovalleys, prompted further investigation of electrical conductors and their relationship to structural features and mineralisation, and helped us continuously connect correlative outcropping units separated by up to hundreds of kilometres. Our interpretation is designed to improve targeting and outcomes for mineral, energy and groundwater exploration, and contributes to our understanding of the chronostratigraphic, structural and upper-crustal evolution of northern Australia. Almost 200,000 regional depth measurements have been collected, each attributed with detailed geological information, are an important step towards a national geological framework, and offer a regional context for more detailed, smaller-scale AEM surveys. The AusAEM programme delivers much more than just reliable depth-to-cover estimates and the location of paleochannels. It can reveal basin architecture and regionally map structures, making it a crucial layer of data for mineral, energy and groundwater and exploration. It has become an essential part of data-driven decision making for conservation and environmental management.