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.

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.

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.

The 2016 Lawn Hill VTEM™Plus airborne electromagnetic (AEM) survey was funded under the Queensland Government’s Future Resources (Mount Isa Geophysics) Initiative and managed by Geoscience Australia on behalf of the Geological Survey of Queensland. The survey covers an area of 3215 km2 which aims to attract explorers into ‘greenfield’ terranes and contribute to the discovery of the next generation of major mineral and energy deposits under shallow sedimentary cover. The survey is an extension to the 2016 East Isa VTEM™Plus Survey (eCAT:104700)

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.

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.

Unconformity-type uranium deposits are high-grade and constitute over a third of the world's uranium resources. The Cariewerloo Basin, South Australia, is a region of high prospectivity for unconformity-related uranium as it contains many similarities to an Athabasca-style unconformity deposit. These include features such as Mesoproterozoic red-bed sediments, Paleoproterozoic reduced crystalline basement enriched in uranium (~15-20 ppm) and reactivated basement faults. An airborne electromagnetic (AEM) survey was flown in 2010 using the Fugro TEMPEST system to delineate the unconformity surface at the base of the Pandurra Formation. However highly-conductive regolith attenuated the signal in the northern and eastern regions, requiring application of deeper geophysical methods. In 2012 a magnetotelluric (MT) survey was conducted along a 110 km transect of the north-south trending AEM line. MT data were collected at 29 stations and successfully imaged the depth to basement, and additionally providing evidence for deeper fluid pathways. The AEM data were integrated into the regularisation mesh as a-priori information generating an AEM constrained resistivity model and also correcting for static shift. The AEM constrained resistivity model best resolved resistive structures, allowing strong contrast with conductive zones.

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.

Geoscience Australia is releasing into the public domain software for the inversion of airborne electromagnetic (AEM) data to a 1D conductivity depth structure. The software includes two different algorithms for 1D inversion of AEM data. The first is a gradient based deterministic inversion code for multi-layer (smooth model) and few-layered (blocky-model) inversions. The second is a reversible-jump Markov chain Monte Carlo stochastic inversion algorithm suitable for assessing model uncertainty. A forward modelling program and some other ancillary programs are also included. The code is capable of inverting data from all of the commercial time-domain systems available in Australia today, including dual moment systems. The software is accessible in three forms. As C++ source code, as binary executables for 64 bit Windows® PCs, and as a service on the Virtual Geophysics Laboratory (VGL). The code is fully parallelized for execution on a high performance cluster computer system or on a multi-core shared memory workstation via either the MPI or the OpenMP programming models.

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.