<div> A key issue for explorers in Australia is the abundant sedimentary and regolith cover obscuring access to underlying potentially prospective rocks. &nbsp;Multilayered chronostratigraphic interpretation of regional broad line-spaced (~20&nbsp;km) airborne electromagnetic (AEM) conductivity sections have led to breakthroughs in Australia’s near-surface geoscience. &nbsp;A dedicated/systematic workflow has been developed to characterise the thickness of cover and the depth to basement rocks, by delineating contact geometries, and by capturing stratigraphic units, their ages and relationships. &nbsp;Results provide a fundamental geological framework, currently covering 27% of the Australian continent, or approximately 2,085,000&nbsp;km2. &nbsp;Delivery as precompetitive data in various non-proprietary formats and on various platforms ensures that these interpretations represent an enduring and meaningful contribution to academia, government and industry.&nbsp;The outputs support resource exploration, hazard mapping, environmental management, and uncertainty attribution.&nbsp;This work encourages exploration investment, can reduce exploration risks and costs, helps expand search area whilst aiding target identification, and allows users to make well-informed decisions. Presented herein are some key findings from interpretations in potentially prospective, yet in some cases, underexplored regions from around Australia.&nbsp;</div> This abstract was submitted & presented to the 8th International Airborne Electromagnetics Workshop (AEM2023) (https://www.aseg.org.au/news/aem-2023)

Effective mineral, energy and groundwater resource management and exploration rely on accurate geological maps. While geological maps of the surface exist and increase in resolution, maps of the subsurface are sparse, and the underpinning geological and geophysical constraints are disordered or non-existent. The Estimates of Geological and Geophysical Surfaces (EGGS) database seeks to enable robust subsurface geological mapping by establishing an ordered collection of precious geological and geophysical interpretations of the subsurface. EGGS stores the depth to geological boundaries derived from boreholes as well as interpretations of depth to magnetic top assessments, airborne electromagnetics inversions and reflection seismic profiles. Since geological interpretation is iterative, links to geophysical datasets and processing streams used to image the subsurface are stored. These metadata allow interpretations to be readily associated with the datasets from which they are derived and re-examined. The geological basis for the interpretation is also recorded. Stratigraphic consistency is maintained by linking each interpretation to the Australian Stratigraphic Units Database. As part of the Exploring for the Future program, >170 000 points were entered into the EGGS database. These points underpin construction of cover thickness models and economic fairway assessments. <b>Citation:</b> Mathews, E.J., Czarnota, K., Meixner, A.J., Bonnardot, M.-A., Curtis, C., Wilford, J., Nicoll, M.G., Wong, S.C.T., Thorose, M. and Ley-Cooper, Y., 2020. Putting all your EGGS in one basket: the Estimates of Geological and Geophysical Surfaces database. In: Czarnota, K., Roach, I., Abbott, S., Haynes, M., Kositcin, N., Ray, A. and Slatter, E. (eds.) Exploring for the Future: Extended Abstracts, Geoscience Australia, Canberra, 1–4.

<div>As part of the Australia's Resources Framework Project, in the Exploring for the Future Program, Geoscience Australia and CSIRO have undertaken a magnetic source depth study across four areas. These are: 1) the western part of Tasmania that is the southernmost extension of the Darling-Curnamona-Delamerian (DCD) project area; 2) northeastern Queensland; 3) the Officer Basin area of western South Australia and southeastern West Australia; and 4) the 'Eastern Resources Corridor' (ERC) covering eastern South Australia, southwest Queensland, western New South Wales and western Victoria. This study has produced 2005 magnetic estimates of depth to the top of magnetization. The solutions are derived by a consistent methodology (targeted magnetic inversion modelling, or TMIM; also known as ‘sweet-spot’ modelling). </div><div><br></div><div>The magnetic depth estimates produced as part of this study provide depth constraints in data-poor areas. They help to construct a better understanding of the 3D geometry of the Australian continent, and aid cover thickness modelling activities. </div><div><br></div><div>A supplementary interpretation data release is also available through Geoscience Australia's enterprise catalogue (ecat) at https://pid.geoscience.gov.au/dataset/ga/149499.</div><div><br></div><div>Geoscience Australia’s Exploring for the Future program provides precompetitive information to inform decision-making by government, community and industry on the sustainable development of Australia's mineral, energy and groundwater resources. By gathering, analysing and interpreting new and existing precompetitive geoscience data and knowledge, we are building a national picture of Australia’s geology and resource potential. This leads to a strong economy, resilient society and sustainable environment for the benefit of all Australians. This includes supporting Australia’s transition to net zero emissions, strong, sustainable resources and agriculture sectors, and economic opportunities and social benefits for Australia’s regional and remote communities. The Exploring for the Future program, which commenced in 2016, is an eight year, $225m investment by the Australian Government. This work contributes to building a better understanding of the Australian continent, whilst giving the Australian public the tools they need to help them make informed decisions in their areas of interest.</div>

We have used Audio-frequency Magnetotelluric (AMT) data to characterise cover and to estimate depth to basement for a number of regional drilling programs in geologically different regions across Australia. We applied deterministic and probabilistic inversion methods to derive 2D and 1D resistivity models. We have also used borehole results to ground-truth and validate the resistivity models and to improve geophysical interpretations. In the East Tennant region, borehole lithology and wireline logging demonstrates that the modelled AMT response is due to bulk conductivity/resistivity of the cover and basement rocks. The groundwater in the region is suitable for cattle drinking water, thus is of low overall salinity and is regarded as having little effect on bulk conductivity. Therefore the bulk conductivity/resistivity is due primarily to bulk mineralogy and the success of using the AMT models to predict cover thickness is shown to be dependent on whether the bulk mineralogy of cover and basement rocks are sufficiently different to provide a detectable conductivity contrast, and the sensitivity of the AMT response with increasing depth. In areas where there is sufficient difference in bulk mineralogy and where the stratigraphy is simple, AMT models predict the cover thickness with great certainty, particularly closer to the Earth’s surface. However, the geological system is not always simple, and we have provided examples where the AMT models provide an ambiguous response that needs to be interpreted with other data (e.g. drilling, wireline logging, potential field modelling) to validate the AMT model result. Overall, we conclude that the application of the method has been validated and the results can compare favourably with borehole stratigraphy logs once geological (i.e. bulk mineralogical) complexity is understood. This demonstrates that the method is capable of identifying major stratigraphic structures with resistivity contrasts. Our results have assisted with the planning of regional drilling programs and have helped to reduce the uncertainty and risk associated with intersecting targeted stratigraphic units in covered terrains. <b>Citation:</b> Jiang, W., Roach, I. C., Doublier, M. P., Duan, J., Schofield, A., Clark, A., & Brodie, R. C. Application of audio-frequency magnetotelluric data to cover characterisation – validation against borehole petrophysics in the East Tennant region, Northern Australia. <i>Exploration Geophysics</i>, 1-20, DOI: 10.1080/08123985.2023.2246492