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

<div>This document describes Geoscience Australia’s standard operating procedure for acquiring long-period magnetotelluric (MT) data using equipment supplied by LEMI LLC. It is current as at April 2024. Users should check periodically for updated versions.</div><div><br></div><div>The procedure is based on the use of the LEMI-424 magnetotelluric station, comprising:</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;LEMI-424 data logger</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;LEMI-039 3-component analog magnetometer and cable</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;LEMI-701 electrodes</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;GPS receiver</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;electric-line interface box</div><div><br></div><div>Geoscience Australia supplements this equipment with the addition of:</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a Pelican equipment box to hold and transport the equipment</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;an acrylic housing to protect the LEMI-039 magnetometer</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;four 50&nbsp;m electrode cables</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a brass earth stake and cable</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a 12&nbsp;V battery</div><div>·&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;a solar panel</div><div><br></div>

The AusLAMP-Victoria magnetotelluric survey was a collaborative project between the Geological Survey of Victoria and Geoscience Australia. Long period magnetotelluric data were acquired at 100 sites on a half degree grid spacing across Victoria in the south-east of Australia between December 2013 and September 2014. Some repeated sites were acquired in December 2017. Geoscience Australia managed the project and performed data acquisition, data processing, and data QA/QC. In this record, the field acquisition, data QA/QC, and data processing methodologies are discussed. A separate report will provide information on data analysis, data modelling/inversion, and data interpretation.

<div>The footprint of a mineral system is potentially detectable at a range of scales and lithospheric depths, reflecting the size and distribution of its components. Magnetotellurics is one of a few techniques that can provide multiscale datasets to understand mineral systems. The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is a collaborative national survey that acquires long-period magnetotelluric data on a half-degree grid spacing (about 55 km) across Australia. This project aims to map the electrical conductivity/resistivity structure in the crust and mantle beneath the Australian continent. We have used AusLAMP as a first-order reconnaissance survey to resolve large-scale lithospheric architecture for mapping areas of mineral potential in Australia. AusLAMP results show a remarkable connection between conductive anomalies and giant mineral deposits in known highly endowed mineral provinces. Similar conductive features are mapped in greenfield areas where mineralisation has not been previously recognised. In these areas we can then undertake higher-resolution infill magnetotelluric surveys to refine the geometry of major structures, and to investigate if deep conductive structures are connected to the near surface by crustal-scale fluid-flow pathways.</div><div> We summarise the results from a 3D resistivity model derived from AusLAMP data in Northern Australia. This model reveals a broad conductivity anomaly in the lower crust and upper mantle that extends beneath an undercover exploration frontier between the producing Tennant Creek region and the prospective Murphy Province. This anomaly potentially represents a fertile source region for mineral systems. A subsequent higher-resolution infill magnetotelluric survey revealed two prominent conductors within the crust whose combined responses produced the lithospheric-scale conductivity anomaly mapped in the AusLAMP model. Integration of the conductivity structure with deep seismic reflection data revealed a favourable crustal architecture linking the lower, fertile source regions with potential depositional sites in the upper crust. Integration with other geophysical and geochronological datasets suggests high prospectivity for major mineral deposits in the vicinity of major faults.</div><div> This study demonstrates that the integration of geophysical data from multiscale surveys is an effective approach to scale reduction during mineral exploration in covered terranes.</div> This Abstract was submitted to and presented at the 6th International Archean Symposium Target 2023, 28 July (https://6ias.org/target2023/)

<div>The Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP) is a collaborative national survey between federal government, state/territory governments and research organisations. The project aims to acquire long-period magnetotelluric (MT) data at half-degree spacing (~55&nbsp;km) across the Australian continent. AusLAMP started in 2013 and has completed about 1500 stations (~50% of total planned stations) to date. Over the last decade, regional-scale AusLAMP resistivity/conductivity models have been produced following data acquisition campaigns, but there is a strong demand for a single model. We produced a resistivity model from 1260 AusLAMP stations incorporating 85% of data acquired to date. The new AusLAMP resistivity model shows significant variations of resistivity at varying depths from a few kilometres to a couple of hundred kilometres in the crust and upper mantle. The model resolves the first-order resistivity structure of the Australian lithosphere across most parts of central and eastern Australia, including Tasmania. The resolved resistivity structures allow seamless interpretation across states and regions, broadly conform with identified major geological domains and crustal boundaries, and reveal significant variations within geological provinces, orogens and cratons. There are also strong spatial associations between crustal/mantle conductors and copper and gold deposits and carbonatites, which provide further evidence that major lithospheric conductors control the distributions of a range of mineral systems. This evidence aligns well with the conceptual model of mineral systems, that convecting mantle fluid and metal-rich magma can migrate into the crust through weak zones to form some ore deposits in the lithosphere. This new AusLAMP model demonstrates that long-period MT data are an important first-order reconnaissance dataset to resolve large-scale lithospheric architecture and provides a powerful tool with a bottom-up approach to highlight potential exploration areas, particularly in covered and under-explored regions. Presented at the 2024 Australian Society of Exploration Geophysicists (ASEG) Discover Symposium

<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.</div><div><br></div><div>During February and March in 2023, Geoscience Australia undertook the Curnamona Cube Extension Magnetotelluric (MT) Survey in western New South Wales and eastern South Australia. The survey complements the University of Adelaide/AuScope Curnamona Cube MT survey by extending the coverage from the Curnamona Province into the Delamerian Orogen. Geoscience Australia contracted Quantec Geoscience Ltd. and its subcontractor Australian Geophysical Services to conduct the data acquisition and processing.&nbsp;Audio and broadband MT data was acquired at 99 sites on an approximately 12.5-25&nbsp;km grid with denser sites across known geological structures and along seismic lines acquired by Geoscience Australia in 2022 (L213 Darling-Curnamona-Delamerian (DCD) 2D Seismic Survey, eCAT # 147423). Instruments were set up to record five channels (three magnetic and two electric fields) for a minimum of 24 hours with a target bandwidth of 0.0001 – 1000 s. Processed data show good quality at a majority of the survey sites, except a few sites affected by environmental and cultural noise. The acquired data will be used to derive resistivity models, and to enhance the understanding of the geodynamics and mineral potential in the Curnamona Province and Delamerian Orogen.&nbsp;</div><div><br></div><div>This data release contains a field logistic report; processed data in EDI format containing spectra and site locations in shape file and .txt format. Time series data in ASCII format is available on request from clientservices@ga.gov.au - Quote eCAT#147904.</div><div><br></div><div>Geoscience Australia acknowledges the traditional landowners, private landholders and national park authorities within the survey region, without whose cooperation these data could not have been collected.</div><div><br></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

The magnetotelluric (MT) method is increasingly being applied to map tectonic architecture and mineral systems. Under the Exploring for the Future (EFTF) program, Geoscience Australia has invested significantly in the collection of new MT data. The science outputs from these data are underpinned by an open-source data analysis and visualisation software package called MTPy. MTPy started at the University of Adelaide as a means to share academic code among the MT community. Under EFTF, we have applied software engineering best practices to the code base, including adding automated documentation and unit testing, code refactoring, workshop tutorial materials and detailed installation instructions. New functionality has been developed, targeted to support EFTF-related products, and includes data analysis and visualisation. Significant development has focused on modules to work with 3D MT inversions, including capability to export to commonly used software such as Gocad and ArcGIS. This export capability has been particularly important in supporting integration of resistivity models with other EFTF datasets. The increased functionality, and improvements to code quality and usability, have directly supported the EFTF program and assisted with uptake of MTPy among the international MT community. <b>Citation:</b> Kirkby, A.L., Zhang, F., Peacock, J., Hassan, R. and Duan, J., 2020. Development of the open-source MTPy package for magnetotelluric data analysis and visualisation. 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>Geoscience Australia’s Exploring for the Future program (EFTF) 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.</div><div><br></div><div>One main component of the EFTF program is the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP), which is a collaborative national survey by federal government, state and territory governments, and research organizations since late 2013. The project acquires long-period magnetotelluric data on a half-degree grid spacing across Australia and provides first order electrical conductivity/resistivity structure of the Australian continental lithosphere. This reconnaissance dataset improves the understanding of lithospheric structures and tectonic evolution of Australian plate. It provides a framework and a bottom-up approach to identify newly resource potential regions for infill surveys and further study. The dataset also uses for assessment and prediction of geomagnetic storm’s nature hazards. </div><div><br></div><div>This data release contains a 3D resistivity model and site locations. The 3D model was derived from publicly available AusLAMP data in Australia (excluding western Australia). The model was projected to GDA94 MGA Zone 54 and was converted into SGrid/ASCII format and geo-referenced TIFF format.</div><div><br></div><div>We acknowledge the traditional custodians of the country where the data were collected. We also acknowledge the support provided by individuals and communities for land access and data acquisition, without whose cooperation these data could not have been collected. The 3D model was produced on the National Computational Infrastructure, which is supported by the Australian government.</div><div><br></div>

This OGC compliant service provides access to magnetotelluric data and associated products, which have been produced by Geoscience Australia’s Magnetotelluric Program. This program includes regional magnetotelluric projects and the Australian Lithospheric Architecture Magnetotelluric Project (AusLAMP), a collaborative project between Geoscience Australia, the State and Northern Territory geological surveys, universities, and other research organisations. The data provided in this service comprise resistivity model depth sections and the locations of sites used in these studies.