Geoscience Australia (GA) is a leading promoter of airborne electromagnetic (AEM) surveying for regional mapping of cover thickness, under-cover basement geology and sedimentary basin architecture. Geoscience Australia flew three regional AEM surveys during the 2006-2011 Onshore Energy Security Program (OESP): Paterson (Western Australia, 2007-08); Pine Creek-Kombolgie (Northern Territory, 2009); and Frome (South Australia, 2010). Results from these surveys have produced a new understanding of the architecture of critical mineral system elements and mineral prospectivity (for a wide range of commodities) of these regions in the regolith, sedimentary basins and buried basement terrains. The OESP AEM survey data were processed using the National Computational Infrastructure (NCI) at the Australian National University to produce GIS-ready interpretation products and GOCADTM objects. The AEM data link scattered stratigraphic boreholes and seismic lines and allow the extrapolation of these 1D and 2D objects into 3D, often to explorable depths (~ 500 m). These data sets can then be combined with solid geology interpretations to allow researchers in government, industry and academia to build more reliable 3D models of basement geology, unconformities, the depth of weathering, structures, sedimentary facies changes and basin architecture across a wide area. The AEM data can also be used to describe the depth of weathering on unconformity surfaces that affects the geophysical signatures of underlying rocks. A number of 3D models developed at GA interpret the under-cover geology of cratons and mobile zones, the unconformity surfaces between these and the overlying sedimentary basins, and the architecture of those basins. These models are constructed primarily from AEM data using stratigraphic borehole control and show how AEM data can be used to map the cross-over area between surface geological mapping, stratigraphic drilling and seismic reflection mapping. These models can be used by minerals explorers to more confidently explore in areas of shallow to moderate sedimentary basin cover by providing more accurate cover thickness and depth to target information. The impacts of the three OESP AEM surveys are now beginning to be recognised. The success of the Paterson AEM Survey has led to the Geological Survey of Western Australia announcing a series of OESP-style regional AEM surveys for the future, the first of which (the Capricorn Orogen AEM Survey) completed acquisition in January 2014. Several new discoveries have been attributed to the OESP AEM data sets including deposits at Yeneena (copper) and Beadell (copper-lead-zinc) in the Paterson region, Thunderball (uranium) in the Pine Creek region and Farina (copper) in the Frome region. New tenements for uranium, copper and gold have also been announced on the results of these surveys. Regional AEM is now being applied in a joint State and Commonwealth Government initiative between GA, the Geological Survey of Queensland and the Geological Survey of New South Wales to assess the geology and prospectivity of the Southern Thomson Orogen around Hungerford and Eulo. These data will be used to map the depth of the unconformity between the Thomson Orogen rocks and overlying sedimentary basins, interpret the nature of covered basement rocks and provide more reliable cover thickness and depth to target information for explorers in this frontier area.

Airborne electromagnetic (AEM) data are an immensely useful tool for mapping cover thickness and under cover geology in Australia. The regional AEM surveys conducted by Geoscience Australia (GA) are an ideal starting point for integrating legacy AEM datasets across a range of scales with other information, e.g. borehole stratigraphy and shallow seismic data, to add to a national cover thickness map. Geoscience Australia is working towards this end as part of the UNCOVER Initiative.

The GEOPHYS_SURV database describes geophysical surveys (air, land, and marine), the datasets derived from those surveys, and the methods used for delivery of those datasets. The database includes metadata for all surveys conducted or managed by Geoscience Australia and its predecessor agencies, as well as data and surveys from State and Territory geological survey agencies.

Ross C Brodie Murray Richardson AEM system target resolvability analysis using a Monte Carlo inversion algorithm A reversible-jump Markov chain Monte Carlo inversion is used to generate an ensemble of millions of models that fit the forward response of a geoelectric target. Statistical properties of the ensemble are then used to assess the resolving power of the AEM system. Key words: Monte Carlo, AEM, inversion, resolvability.

AusLAMP is a collaborative national project to cover Australia with long-period magnetotelluric (MT) data in an approximately 55 km spaced array. Signatures from past tectonothermal events can be retained in the lithosphere for hundreds of millions of years when these events deposit conductive mineralogy that is imaged by MT as electrically conductive pathways. MT also images regions of different bulk conductivity and can help to understand the continuation of crustal domains down into the mantle, and address questions on the tectonic evolution of Australia. The AusLAMP data presented here were collected as part of three separate collaborative projects involving several organisations. Geoscience Australia (GA), the Geological Survey of South Australia, the Geological Survey of New South Wales, the Geological Survey of Victoria, and the University of Adelaide all contributed staff and/or funding to collection of AusLAMP data; GA and AuScope contributed instrumentation. The data cover the Paleo-Mesoproterozoic Curnamona Province, the Neoproterozoic Flinders Ranges, and the Cambrian Delamerian Orogen, encompassing eastern South Australia and western New South Wales and western Victoria. This project represents the first electrical resistivity model to image the entire Curnamona Province and most of the onshore extent of the Delamerian Orogen, crossing the geographical state borders between South Australia, New South Wales and Victoria.

The Frome airborne electromagnetic (AEM) survey was designed to provide reliable pre-competitive AEM data to aid the search for energy and mineral resources around the Lake Frome region of South Australia. Flown in 2010, a total of 32,317 line kilometres of high quality airborne geophysical data were collected over an area of 95,450 km2 at a flight line spacing mostly of 2.5 km, opening to 5 km spaced lines in the Marree-Strzelecki Desert area to the north. The Lake Frome region hosts a large number of sandstone-hosted uranium deposits with known resources of ~60,000 tonnes of U3O8 including the working In Situ Recovery (ISR) operations at Beverley, Pepegoona, Pannikin and Honeymoon, and deposits at Four Mile East, Four Mile West, Yagdlin, Goulds Dam, Oban and Junction Dam. The aims of the Frome AEM Survey were to map critical elements of sandstone-hosted uranium mineral systems including basin architecture, palaeovalley morphology, sedimentary facies changes, hydrological connections between uranium sources and uranium sinks and structures that may control uranium mineralisation. Interpretations of the data show the utility of regional AEM surveying for mapping sandstone-hosted uranium mineral systems as well as for mapping geological surfaces and depth of cover over a wide area. Data from the Frome AEM Survey allow mineral explorers to put their own high-resolution AEM surveys into a regional context. Survey data were used to map a range of geological features that are associated with, or control the location of, sandstone-hosted uranium mineral systems and have been used to map and assess the prospectivity of new areas to the north of the Flinders Ranges.

The 2016 Southern Thomson Orogen VTEM™Plus AEM Survey was conducted by Geoscience Australia as part of a collaborative investigation between the Commonwealth of Australia (Geoscience Australia) and its partners the State of New South Wales (Department of Trade and Investment, Geological Survey of New South Wales) and the State of Queensland (Department of Natural Resources and Mines, Geological Survey of Queensland). The Project aims to better understand the geological character and mineral potential of the southern Thomson Orogen region, focusing on the border between New South Wales and Queensland, by acquiring and interpreting multi-disciplinary geophysical, geochemical and geological data. The primary intended impact of this work is to provide the mineral exploration industry with pre-competitive data and knowledge that reduces risk and encourages mineral exploration in the region. Geoscience Australia contracted Geotech Airborne Pty Ltd to acquire VTEM™Plus AEM data over part of the Southern Thomson Orogen in Queensland and New South Wales in May and June 2016.The data were also processed by Geotech Airborne Ltd using its FullWaveForm® processing techniques. The survey area consists of 2415 line km of time-domain AEM geophysical data acquired in five survey blocks. The majority of traverse lines were spaced at 5000 m in an east-west direction, further details about each blocks flight line specifications can be found in Table 1. The original data supplied by Geotech Airborne Pty Ltd has been modified to contain the final data fields of principal interest, enabling a manageable data file size. This data is available from Geoscience Australia's website free of charge. The comprehensive dataset is available from Geoscience Australia by emailing mineralgeophysics@ga.gov.au. The data release package includes: - Point-located electromagnetic dB/dt and derived B-field data with associated position, altimeter, orientation, magnetic gradiometer, and derived ground elevation data. These data are in ASCII column format with associated README and ASEG-GDF2 header files. The dataset consists of a separate download file for the: - Survey Lines - Repeat lines - Waveform files for every flight containing the 192 kHz sampling of the transmitter current and receiver waveforms. - Point-located conductivity estimates derived using the EM Flow® conductivity depth imaging (CDI) algorithm with associated position, altimeter, orientation, magnetic gradiometer, and derived ground elevation data. Data include the conductivity estimate for each 5 m interval and selected depth slices. - Gridded data, at 1 km cell size in, for the conductivity depth slices derived from the EM Flow® CDI data, magnetics and elevation data in ER Mapper® binary raster grid format with associated header files. - Graphical multiplots, in PDF format, for each flight line showing EM Flow® CDI sections and profiles of Z-component dB/dt data, magnetics, powerline monitor, height and orientation data. - Contractor supplied Operations Report. - ESRI shapefiles and KML files of flight lines. - Metadata and License files.

As part of the Australian Government's Onshore Energy Security Program (2006-2011) Geoscience Australia in collaboration with Geological Survey of Western Australia acquired magnetotelluric (MT) data along the deep crustal seismic reflection transect across the Yilgarn Craton, Officer Basin and Musgrave Province in Central Western Australia. The aim of the MT survey is to map the electrical resistivity distribution and improve scientific understanding of the crustal and upper mantle structure in this region. This information is complementary to that obtained from deep crustal seismic reflection, seismic refraction, potential field and geological data, which together provide new knowledge of the crustal architecture and geodynamics of the region. It is important for helping to determine the potential for both mineral and energy resources. Data are supplied as EDI files with support information.

Geoscience Australia first sought feedback on a metadata standard for magnetotelluric (MT) time-series data in 2018 with the publication of a Preview article (Kirkby, 2019) outlining suggestions for metadata fields that should be collected when running an MT survey. This was the first step in standardising the MT formats used by the Australian MT community to ensure a cohesive community approach moving forward. Intrepid Geophysics was subsequently contracted by Geoscience Australia to investigate the current community sentiment around a metadata standard and report on the community’s requirements for a standardised data format. Intrepid Geophysics was chosen as an independent party that had no significant stake in the magnetotellurics discussion. This report is the third made to Geoscience Australia in a series investigating the needs of the Australian magnetotelluric community, with a focus on the definition of the metadata that should be collected along with the raw data of an MT survey. The findings were collated from interviews conducted in the preliminary stage of the project as well as an online questionnaire that was sent to those who had agreed to be contacted. Feedback was constructive, centring on standardisation of parameter naming schemes, adding parameters that were missing and could add value, and misclassification of parameters. Future work should focus on a more widespread community engagement program that involves system manufacturers as well as building the metadata structure around the chosen data format.

The magnetotelluric (MT) method is increasingly being applied to mineral exploration under cover with several case studies showing that mineral systems can be imaged from the lower crust to the near surface. Driven by this success, the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is delivering long-period data on a 0.5° grid across Australia, and derived continental scale resistivity models that are helping to drive investment in mineral exploration in frontier areas. Part of this investment includes higher-resolution broadband MT surveys to enhance resolution of features of interest and improve targeting. To help gain best value for this investment it is important to have an understanding of the ability and limitations of MT to resolve features on different scales. Here we present synthetic modelling of conductive, narrow, near-vertical faults 500 m to 1500 m wide, and show that a station spacing of around 14 km across strike is sufficient to resolve these into the upper crust. However, the vertical extent of these features is not well constrained, with near-vertical planar features commonly resolved as two separate features. This highlights the need for careful interpretation of anomalies in MT inversion. In particular, in an exploration scenario, it is important to consider that a lack of interconnectivity between a lower crustal/upper mantle conductor and conductors higher up in the crust and the surface might be apparent only, and may not reflect reduced mineral prospectivity. Appeared in Exploration Geophysics Journal 05 Dec 2022