Geoscience Australia flew three regional airborne electromagnetic (AEM) surveys as part of the Australian Government's 5-year Onshore Energy Security Program in 2007-08 (Paterson, WA), 2009 (Pine Creek, NT) and 2010 (Frome, SA). The aims of the surveys were to reduce risk and stimulate exploration investment for uranium by providing reliable pre-competitive data. When the data and interpretations of the surveys were released, there was a measurable upswing in industry investment in and around the survey areas and a number of new discoveries were made using the new data. Geoscience Australia is committed to the Australian Academy of Science's Searching the Deep Earth (UNCOVER) initiative, which has been adopted by Geoscience Australia as part of its long-term strategic planning. To assist this initiative, we are assessing the potential of AEM to characterise areas that are prospective for a range of commodities including gold, copper, lead, zinc, nickel, platinum group elements and rare earth elements, as well as uranium. The assessment will also extend to the potential for mapping geology under cover to explorable depths (< 400 m), mapping cover thickness around the flanks of major outcrop areas and providing new information on groundwater resources. Potential new areas for regional AEM surveying could include (in no particular order of priority): the Westmoreland region; the Georgetown Inlier; the Mt Isa region; the Broken Hill region, the Peake and Denison Ranges; the Eyre Peninsula (Gawler Craton); the Ngalia-Amadeus region; the Musgrave Province; the Windimurra Igneous Complex; the Capricorn-Ashburton area; the Lachlan-Thomson orogens; the Stawell and Ballarat areas; the southeast Yilgarn region (Yilgarn Craton flanks); and, the Tanami area.

Orogenic gold deposits provide a significant source of the world’s gold, but their depth of formation is contentious. One hypothesis is that orogenic gold deposits formed along crustal faults over a wide range of depths spanning sub-greenschist to granulite facies. Other authors suggest that the source is restricted to a smaller range of crustal depths (~20-30 km) and temperatures (~550⁰C) that correspond to the transition from greenschist to amphibolite metamorphic facies. Rapid burial of C and S-rich oceanic sediments and amphibolite-grade metamorphism leads to the production of large amounts of fluid in a short amount of time. In order to help discriminate between these competing hypotheses, we compiled thirty years of magnetotelluric (MT) and geomagnetic depth sounding (GDS) data across western Victoria and south-eastern South Australia. This region contains one of the world’s foremost and largest orogenic gold regions that has produced 2% of historic worldwide gold production. Three-dimensional inversion of the MT and GDS data shows a remarkable correlation between orogenic gold deposits with >1 t production and a <20 ohm.m low-resistivity region at crustal depths >20 km. Such depths are at the pressures and temperatures of greenschist to amphibolite-grade metamorphism that releases HS- ligands for Au from C and pyrite (FeS2) rich sediment interbedded with mafic oceanic rocks. Carbon is then precipitated through retrograde hydration reactions with CO2 precipitating as conductive flake graphite. Thus, our model indicates that orogenic gold in western Victoria is most likely sourced from C and FeS2 rich oceanic sediments at amphibolite-grade facies. Citation: Heinson, G., Duan, J., Kirkby, A. et al. Lower crustal resistivity signature of an orogenic gold system. Sci Rep 11, 15807 (2021). https://doi.org/10.1038/s41598-021-94531-8

The Southern Thomson Orogen VTEM-plus® Airborne Electromagnetic Survey was conducted by Geoscience Australia as part of a collaborative project with its partners the Geological Survey of New South Wales and the Geological Survey of Queensland. The Survey contributes to the Australian Academy of Science's UNCOVER Initiative and Geoscience Australia's response to this as part of the National Mineral Exploration Strategy. Geoscience Australia contracted Geotech Airborne Ltd to acquire VTEM-plus® airborne electromagnetic (AEM) data over part of the Southern Thomson Orogen in Queensland and New South Wales in April and May 2014. The data were also processed by Geotech Airborne Ltd using its FullWaveForm® processing techniques. The survey is designed to assess the under-cover geology and prospectivity of the Southern Thomson Orogen around Hungerford and Eulo and straddles the New South Wales-Queensland border. The survey comprises two parts: 1. A regular regional survey on 5000 m spaced East-West lines totalling 3352 line km and covering an area of 16 261 km2. 2. Two regional traverses adjacent various roads totalling 915 line km. The Southern Thomson Orogen is a priority area for mineral systems research. Much of the area lies underneath cover of sedimentary basins and is a poorly-understood element of Australia's geology. The Orogen contains Cambro-Ordovician rocks that have potential for Iron Oxide Copper-Gold (IOCG) resources, porphyry copper-gold and Volcanic-Hosted Massive Sulphide (VHMS) deposits. Survey data will add to knowledge of cover thickness and character and will inform future geological mapping in the region. The Southern Thomson Orogen VTEM-plus® AEM Survey data release includes the final contractor supplied (Phase 1) datasets AEM survey. The data will be available from Geoscience Australia's web site free of charge: http://www.ga.gov.au/about/what-we-do/projects/minerals/current/continental-geophysics/airborne-electromagnetics The data release package includes: 1. 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: a. Main survey block Part 1 (flight lines 1000-1171) b. Main survey block Part 2 (flight lines 1180-1360) c. Traverse lines (flight lines 3000-3006 and 4000-4007) d. Repeat lines e. High altitude lines. 2. Waveform files for every flight containing the 192 kHz sampling of the transmitter current and receiver waveforms. 3. 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. These data are in ASCII column format with associated README and ASEG-GDF2 header files. All regular survey, traverses and repeat lines are included in a single download file. 4. 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. 5. 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. 6. Operations Report. 7. ESRI shapefiles and KML files of flight lines. 8. Metadata and License files.

<p>This package contains Airborne Electromagnetic (AEM) data from the “SkyTEM helicopter EM Ord-Keep rivers region” survey which was flown over the Ord-Keep Rivers Region, Western Australia/Northern Territory, Australia during May - June 2017. High resolution magnetics were also acquired during the flights. As shown in Figure 1, the area is located in the 1:250000 map sheets of SD52-14 (Cambridge Gulf), SD52-11 (Port Keats) and SD 52-15 (Auvergne) near the town of Kununurra. 8100 line km of TEM and magnetic data were acquired. The projected grid coordinates have been supplied in GDA94 MGA Zone 52. <p>The aim of the survey is to provide geophysical information to support investigations of the regional groundwater system and identify regional groundwater sources. It will provide data to allow for the modelling of the following at a reconnaissance scale: <p>a) trends in regolith thickness and variability <p>b) variations in bedrock conductivity <p>c) conductivity of key bedrock (lithology related) conductive units under cover <p>d) the groundwater resource potential of the region <p>e) palaeovalley systems known to exist in the region. <p>This report lists the SkyTEM system information and specifications relevant for this survey, and describes the processing carried out on the data. <p>Geoscience Australia commissioned the survey as part of the Exploring for the Future (EFTF) program. The EFTF program is led by Geoscience Australia (GA), in collaboration with the Geological Surveys of the Northern Territory, Queensland, South Australia and Western Australia, and is investigating the potential mineral, energy and groundwater resources in northern Australia and South Australia. The EFTF is a four-year $100.5 million investment by the Australian Government in driving the next generation of resource discoveries in northern Australia, boosting economic development across this region (https://www.ga.gov.au/eftf).

Ross C Brodie James Reid Monte Carlo Inversion of SkyTEM AEM data from Lake Thetis, Western Australia A SkyTEM airborne electromagnetic dataset was inverted using a 1D reversible jump Markov chain Monte Carlo algorithm. The inversion of each dual-moment sounding generates an ensemble of 300,000 models that fit the data. The algorithm automatically varies the number of layers in the large range of models that are tested. Analysis of the statistical properties of the ensemble yields a wealth of information on the probable conductivity distribution plus the mean, mode, median and most likely summary models. Robust information on the non-uniqueness and uncertainty of the results is also afforded by the ensemble. These are conveyed on conductivity map and section products. Estimates of the probable depths to interfaces are a further outcome. These depth estimates show great potential as an aid for mapping geological surfaces. The resulting conductivity maps and sections are coherent and appear to be geologically realistic on face value. However it is demonstrated with 3D modelling that a plausible hydrogeological interpretation on the sections is likely to be an artefact of 1D inversion of a 3D geological scenario. Key words: Electromagnetic, airborne, inversion, Monte Carlo, uncertainty, 3D.

Alan Yusen Ley-Cooper Ross C. Brodie Inversion of SPECTREM AEM data for conductivity and system geometry We evaluate the use of airborne electromagnetic data from the SPECTREM2000 system flown for ore body detection, regolith mapping and assessment of aquifers. Since the position and orientation of the receiver bird are not measured, the primary field at the bird cannot be known and removed precisely. In order to successfully invert the AEM data, and produce conductivity-depth models, we first reinstate the removed primary field estimate and convert the data from ppm units to Teslas. We then simultaneously inverted the X and Z component data, to solve for a 1D layered conductivity model and receiver position. The SPECTREM system has flown many line kilometres in other parts of the world but substantially less in Australia. Through further processing and inversions we have resolved conductivity-depth structures very similar to those previously obtained from other well-established AEM systems flown under Australian conditions. We also present a section of AEM data with logged drilling core data as a means of assessment of our inversion models against an independent data set. Key words: Airborne EM, inversion, geometry, SPECTREM, electrical conductivity.

<p>The Southern Thomson Orogen VTEMplus® Airborne Electromagnetic Survey was conducted by Geoscience Australia as part of a collaborative project with its partners the Geological Survey of New South Wales and the Geological Survey of Queensland. The Survey contributes to the Australian Academy of Science's UNCOVER Initiative and Geoscience Australia's response to this as part of the National Mineral Exploration Strategy. Geoscience Australia contracted Geotech Airborne Ltd to acquire VTEMplus® airborne electromagnetic (AEM) data over part of the Southern Thomson Orogen in Queensland and New South Wales in April and May 2014. The data were also processed by Geotech Airborne Ltd using its FullWaveForm® processing techniques. <p>The survey is designed to assess the under-cover geology and prospectivity of the Southern Thomson Orogen around Hungerford and Eulo and straddles the New South Wales-Queensland border. The survey comprises two parts: <p>1. A regular regional survey on 5000 m spaced east-west flight lines totalling 3352 line km and covering an area of 16 261 km2. <p>2. Two regional traverses adjacent to roads totalling 915 line km. <p>The Southern Thomson Orogen is a priority area for mineral systems research. Much of the area lies underneath cover of sedimentary basins and is a poorly-understood element of Australia's geology. The Orogen contains Cambro-Ordovician rocks that have potential for Iron Oxide Copper-Gold (IOCG) resources, porphyry copper-gold and Volcanic-Hosted Massive Sulphide (VHMS) deposits. Survey data will add to knowledge of cover thickness and character and will inform future geological mapping in the region. <p>Previously Released Data (Phase 1) <p>In August 2014 the processed data from the Southern Thomson Orogen VTEMplus® AEM Survey were released in the Phase 1 data release package that is available free of charge from Geoscience Australia's web site (see https://pid.geoscience.gov.au/dataset/ga/81852). The Phase 1 package includes the final processed electromagnetic data, waveform files, multiplots, conductivity estimates from the EM Flow® conductivity depth imaging algorithm, and an operations and processing report, all produced by the contractor Geotech Airborne Ltd. <p>Current Release (Phase 2) <p>This Phase 2 data release package contains results from the electromagnetic inversion of the data in the Phase 1 release. The inversion results were generated using two different types of algorithm, a deterministic regularized gradient based algorithm, which we call GALEISBS (Roach, 2010), and a reversible-jump Markov chain Monte Carlo algorithm, which we call GArjMcMC (Brodie and Sambridge, 2012; Brodie and Reid, 2013). Both algorithms assume a layered earth or 1D conductivity structure. Each airborne electromagnetic sounding is inverted independently and the results are then stitched into combined sections. <p>The deterministic GALEISBS inversion products are available for download in four logical parts based on the type of derived product. These are zipped into the following four files: <p>1. galeisbs_point_located_data.zip <p>2. galeisbs_sections.zip <p>3. galeisbs_georeferenced_sections.zip <p>4. galeisbs_gocad_sgrids.zip <p>The stochastic GArjMcMC inversion products are available for download in five logical parts based on the type of derived product. These are zipped into the following five files; <p>1. rjmcmc_point_located_data.zip <p>2. rjmcmc_sections.zip <p>3. rjmcmc_georeferenced_sections.zip <p>4. rjmcmc_gocad_sgrids.zip <p>5. rjmcmc_probability_map_plots.zip

<p>The Broken Hill Managed Aquifer Recharge (BHMAR) Skytem® Airborne Electromagnetic (AEM) Survey was commissioned by Geoscience Australia to support the BHMAR groundwater project. The survey was funded by the Australian Government through the Department of Environment. <p>The data package includes 32,659 line kilometres of heliborne time-domain SKYTEM® AEM data. Geoscience Australia contracted Geoforce Pty Ltd to acquire the Skytem® airborne electromagnetic data in western New South Wales from 28 June to 3 September 2009. The data were also processed by Geoforce Pty Ltd.. The data were acquired with a nominal transmitter loop height of 30 m on predominantly northwest-southeast oriented flight lines at 200 m spacing. There were some additional flight lines flow along various river traverses and an extension of the survey on northeast-southwest oriented flight lines.

This package contains Airborne Electromagnetic (AEM) data from the regional survey flown over the Great Artesian Basin intake beds in Queensland, Australia in May-June 2021. A total of 4,612.3 line km of transient EM and magnetic data were acquired. The projected grid coordinates have been supplied in GD2020 / MGA Zone 55. The aim of the survey is to provide geophysical information to support investigations of the regional groundwater system and to better understand the architecture of the aquifers within the upper few hundred metres of the GAB intake beds. It will provide data to allow for the study of the following at a reconnaissance scale: a) trends in regolith thickness and variability b) variations in bedrock conductivity c) conductivity of key bedrock (lithology related) conductive units under cover d) the groundwater resource potential of the region Geoscience Australia (GA) flew the survey as part of the Great Artesian Basin (GAB) Project. The Australian Government, through the National Water Infrastructure Fund – Expansion, commissioned GA to undertake the GAB project. The project is led by GA, in collaboration with the Department of Agriculture, Water and the Environment; Department of Infrastructure, Transport, Regional Development and Communications; the Bureau of Meteorology; and state and territory government water agencies across Queensland, South Australia, New South Wales and the Northern Territory.

ASEG 2016 Conference abstract. AMT and BBMT data have been applied by Geoscience Australia to determine the nature and thickness of cover, plus the basement architecture in regions around Australia. The depth of cover assessment produced by MT agrees with depth of cover assessments made by other geophysical techniques and will be tested by an ongoing program of stratigraphic drilling.