The AusAEM1 airborne electromagnetic survey extends across an area exceeding 1.1 million km2 over Queensland and the Northern Territory. Approximately 60,000 line kilometres of data were acquired at a nominal line spacing of 20 km (Ley-Cooper et al., 2020). To improve targeting and outcomes for mineral, energy and groundwater exploration, we conducted a regional interpretation of this dataset to characterise the subsurface geology of northern Australia. The interpretation includes the depth to chronostratigraphic surfaces, compilation of stratigraphic relationship information, and delineation of structural and electrically conductive features. In addition to help connecting correlative outcropping units separated by up to hundreds of kilometres, the results led to 3D mapping of palaeovalleys and prompted further investigation of electrical conductors and their relationship to structural features and mineralisation. Approximately 200,000 regional depth point measurements, 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. Refer to Wong et al., (2020) for more details on the AusAEM1 interpretation.
Maintenance and Update Frequency: asNeeded
Statement: The interpreted AEM conductivity sections were inverted using Geoscience Australia’s (GA’s) Layered Earth Inversion Sample-By-Sample Time Domain Electromagnetics (GALEISBSTDEM) inversion (Ley-Cooper and Brodie, 2020). Horizontal along-flight line resolution of the interpreted data 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. The depth of investigation (Hutchinson et al., 2010) of the dataset varies depending on the bulk electrical conductivity of the subsurface, and averages ~250 m across the entire survey area. The total depth of signal penetration is estimated to be >500 m in electrically resistive terrain. Refer to Ley-Cooper and Brodie (2020) for more details on the AusAEM1 survey. A new interpretation workflow was developed to ensure compatibility of the interpretation with a range of 3D software and to accommodate the requirements for the intended storage location, the Estimates of Geological and Geophysical Surfaces (EGGS) database, which is available as a web service from GA (Mathews et al., 2020). This workflow uses a combination of 2D geographic information systems (GIS) and 3D visualisation software. The GIS provided the functionality for simple attribution of metadata for each interpreted feature. The 3D software, including GA’s EarthSci and commercial 3D software, was used for data integration, to display AusAEM1 electrical conductivity sections, drillholes, geological maps, potential fields, seismic data, and so on. Scripts were developed to convert the 2D interpretation lines to 3D space. This interpretation took place in 2019-21, with this data package representing the most up-to-date version of the interpretation.
Our interpretation focused on delineating major chronostratigraphic boundaries, specifically the bases of Cenozoic, Mesozoic, Paleozoic and Neoproterozoic units. These boundaries are stored in, and are retrievable from, the Estimates of Geological and Geophysical Surfaces database. Attributed metadata provide additional information on chronostratigraphic relationships, structural features and the data that support the interpretation. These metadata are not only important for cover thickness modelling, but are also intended for use in many other applications, such as exploration targeting or understanding the upper-crustal structural evolution of northern Australia.