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

Mineral exploration in Australia faces the challenge of declining discovery rates despite continued exploration investment. The UNCOVER roadmap, developed by stakeholders from industry, government and academia, has highlighted the need for discovering mineral resources in areas of cover. In these areas, potentially prospective basement is covered by regolith, including transported sediment, challenging many traditional exploration methods designed to probe outcrop or shallow subcrop. Groundwater-mineral interaction in the subsurface has the potential to give the water geochemical and isotopic characteristics that may persist over time and space. Geoscience Australia’s hydrogeochemistry for mineral exploration project, part of the Exploring for the Future Programme, aims to use groundwater chemistry to better understand the bedrock-regolith system and develop new methods for recognising mineral system footprints within and below cover. During the 2017 dry season (May to September), ~150 groundwater samples (including QC samples) were collected from pastoral and water supply bores in the regions of Tennant Creek and McArthur River, Northern Territory. The Tennant Creek region has a demonstrated iron oxide-hosted copper-gold-iron(-bismuth) mineral potential in the Paleoproterozoic and Mesoproterozoic basement and vast areas of regolith cover. Among the critical elements of this mineral system, the presence/absence of redox contrasts, iron enrichment, presence of sulfide minerals, and carbonaceous intervals can potentially be diagnosed by the elemental and isotopic composition of groundwater. The McArthur River region, in contrast, has demonstrated sediment-hosted stratiform lead-zinc-silver mineral potential in the Paleoproterozoic to Neoproterozoic basement and also vast areas of regolith cover. Here, critical mineral system elements that have the potential to be identified using groundwater geochemistry include the presence of felsic rocks (lead source), carbonate rocks (zinc source), basinal brines, dolomitic black shales (traps), and evaporite-rich sequences. Preliminary results will be presented and interpreted in the context of these mineral systems.

The EGGS database contains point depths to chronostratigraphic surfaces obtained from boreholes and four magnetic modelling methods. In regards to the depths obtained from inclined boreholes there are uncertainties on their measurement of the true vertical depths. These are currently being looked into and would be resolved in due course. Future development will include AEM, reflection seismic, MT and passive seismic. A Web Service will allow the public to download this data and will be accessed through the EFTF (Exploring for the Future) web portal.

The Geoscience Australia (GA) Inorganic Geochemistry database (GEOCHEM) contains chemical analyses and analytical metadata from rocks and regolith materials. The majority of analysed samples are from mapping and sampling programs in Australia by GA and its predecessor organisations (BMR, AGSO), along with a considerable collection from the Australian Antarctic Territory. A small number of analyses exist from Papua New Guinea and offshore sampling programs. The data set is currently used for internal GA consumption and is served off an application within the GA portal. As an enhancement, this data would be altered in terms of its structure adding more information out of such analyses. In addition, the data would me made compliant following GGIC standards. The data would be published within internal GA as well as to external third parties, through OGC web services viz. WMS and WFS.

3D seismic survey polygon area. The data within this layer only contains high level information regarding the individual surveys, not the actual survey. NOPIMS data is supplied by the petroleum industry. NOPIMS data is only offshore petroleum that belongs to the Commonwealth. A two dimensional (3D) seismic survey is a method of exploration used to capture seismic data beneath Earth's surface. 3D seismic provides continuous information of the subsurface within the extent of the survey. transect line of information in the survey. This method sends energy waves into the Earth to detect changes in the subsurface geology. The rock formations in the subsurface geology reflect the waves back to detector, where they are captured over the desired timeframe and converted into a seismic image. Depending on the age the survey was conducted will depend on the method used to capture the information. Older records more likely used explosives to capture seismic data whereas newer surveys use compressed air.

This OGC conformant web service delivers data from Geoscience Australia's Boreholes database (borehole header and directional survey data) and associated geological observations (lithostratigraphic data). The data includes records of boreholes drilled by Geoscience Australia and its predecessor organisations (BMR, AGSO), all boreholes drilled in Australian Commonwealth offshore marine jurisdictions, and a selection of onshore boreholes drilled by government and private entities. Where possible this service conforms to the GeoSciML v4.1 data transfer standard. Geoscience Australia is not a reporting or regulatory authority for borehole drilling. Borehole information in Geoscience Australia's Boreholes database is sourced from various regulatory authorities in the States, Northern Territory and Commonwealth governments for Geoscience Australia research purposes. Where Geoscience Australia is not the custodian of borehole data provided in this web service, the custodian agency provided with the data should be consulted as the authoritative source. The data dictionary for this web service is at <a href="https://d28rz98at9flks.cloudfront.net/144576/144576_00_3.pdf">https://d28rz98at9flks.cloudfront.net/144576/144576_00_3.pdf</a>. For information on borehole status definitions, refer to <a href="https://db-content.ga.gov.au/data_dictionary/Borehole_Status_vocabulary_2021.pdf">https://db-content.ga.gov.au/data_dictionary/Borehole_Status_vocabulary_2021.pdf</a>

<div>This dataset represents the second version of a compilation of borehole stratigraphic unit data on a national scale (Figure 1). It builds on the previous Australian Borehole Stratigraphic Units Compilation (ABSUC) Version 1.0 (Vizy &amp; Rollet, 2023a) with additional new or updated stratigraphic interpretation on key boreholes located in Figure 2. Its purpose is to consolidate and standardise publicly accessible information from boreholes, including those related to petroleum, stratigraphy, minerals, and water. This compilation encompasses data from states and territories, as well as less readily available borehole logs and interpretations of stratigraphy.</div><div>&nbsp;</div><div>This study was conducted as part of the National Groundwater Systems (NGS) Project within the Australian Government's Exploring for the Future (EFTF) program. 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. More information is available at http://www.ga.gov.au/eftf and https://www.eftf.ga.gov.au/national-groundwater-systems.</div><div>&nbsp;</div><div>As our understanding of Australian groundwater systems expands across states and territories, including legacy data from the 1970s and recent studies, it becomes evident that there is significant geological complexity and spatial variability in stratigraphic and hydrostratigraphic units nationwide. Recognising this complexity, there is a need to standardise diverse datasets, including borehole location and elevation, as well as variations in depth and nomenclature of stratigraphic picks. This standardisation aims to create a consistent, continent-wide stratigraphic framework for better understanding groundwater system for effective long-term water resource management and integrated resource assessments.</div><div>&nbsp;</div><div>This continental-scale compilation consolidates borehole data from 53 sources, refining 1,117,693 formation picks to 1,010,483 unique records from 171,396 boreholes across Australia. It provides a consistent framework for interpreting various datasets, enhancing 3D aquifer geometry and connectivity. Each data source's reliability is weighted, prioritising the most confident interpretations. Geological units conform to the Australian Stratigraphic Units Database (ASUD) for efficient updates. Regular updates are necessary to accommodate evolving information. Borehole surveys and dip measurements are excluded. As a result, stratigraphic picks are not adjusted for deviation, potentially impacting true vertical depth in deviated boreholes.</div><div>&nbsp;</div><div>This dataset provides:</div><div>ABSUC_v2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Australian stratigraphic unit compilation dataset (ABSUC)</div><div>ABSUC_v2_TOP&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A subset of preferred top picks from the ABSUC_v2 dataset</div><div>ABSUC_v2_BASE&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A subset of preferred base picks from the ABSUC_v2 dataset</div><div>ABSUC_BOREHOLE_v2&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ABSUC Borehole collar dataset</div><div>ASUD_2023&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;A subset of the Australia Stratigraphic Units Database (ASUD)</div><div>&nbsp;</div><div>Utilising this uniform compilation of stratigraphic units, enhancements have been made to the geological and hydrogeological surfaces of the Great Artesian Basin, Lake Eyre Basin and Centralian Superbasin. This compilation is instrumental in mapping various regional groundwater systems and other resources throughout the continent. Furthermore, it offers a standardised approach to mapping regional geology, providing a consistent foundation for comprehensive resource impact assessments.</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 package contains data generated in the field as part of stratigraphic drilling operations in the Delamerian region of the western New South Wales during 2023 funded through the Exploring for the Future program. A range of geological, geophysical and geochemical data are included, as well as associated borehole information such as core photographs. The data can be viewed and downloaded via the Geoscience Australia Portal - https://portal.ga.gov.au/. The data that is available is from several databases which are associated to this record. <i>These data are published with the permission of the CEO, Geoscience Australia. </i>

The Geoscience Australia Boreholes database (BOREHOLE) includes borehole header and directional survey data from: 1. boreholes drilled by Geoscience Australia and its predecessor organisations (BMR, AGSO), 2. all resource exploration boreholes drilled in Australian Commonwealth offshore marine jurisdictions, 3. a selection of Australian onshore mineral exploration, groundwater, geothermal and seismic boreholes, and 4. a small number of research-related boreholes outside of the Australian jurisdiction. Geoscience Australia is not a reporting or regulatory authority for borehole drilling. Borehole information in the Geoscience Australia Boreholes database is sourced from various regulatory authorities in the States, Northern Territory and Commonwealth governments for Geoscience Australia research purposes. Where Geoscience Australia is not the custodian of borehole data provided in this database, the custodian agency provided with the data should be consulted as the authoritative source.

This OGC conformant web service delivers data from Geoscience Australia's Boreholes database (borehole header and directional survey data) and associated geological observations (lithostratigraphic data). The data includes records of boreholes drilled by Geoscience Australia and its predecessor organisations (BMR, AGSO), all boreholes drilled in Australian Commonwealth offshore marine jurisdictions, and a selection of onshore boreholes drilled by government and private entities. Where possible this service conforms to the GeoSciML v4.1 data transfer standard. Geoscience Australia is not a reporting or regulatory authority for borehole drilling. Borehole information in Geoscience Australia's Boreholes database is sourced from various regulatory authorities in the States, Northern Territory and Commonwealth governments for Geoscience Australia research purposes. Where Geoscience Australia is not the custodian of borehole data provided in this web service, the custodian agency provided with the data should be consulted as the authoritative source. The data dictionary for this web service is at <a href="https://d28rz98at9flks.cloudfront.net/144578/144578_00_3.pdf">https://d28rz98at9flks.cloudfront.net/144578/144578_00_3.pdf</a>. For information on borehole status definitions, refer to <a href="https://db-content.ga.gov.au/data_dictionary/Borehole_Status_vocabulary_2021.pdf">https://db-content.ga.gov.au/data_dictionary/Borehole_Status_vocabulary_2021.pdf</a>