To investigate the standard electrical conductivity profile beneath a continent, we conducted a magnetotelluric (MT) observation with long dipole span near Alice Springs, central Australia. We utilized geomagnetic data acquired at the Alice Springs geomagnetic observatory operated by Geoscience Australia. Using the BIRRP processing code (Chave and Thomson, 2004), we estimated the MT and GDS (geomagnetic depth sounding) transfer functions for periods from 100 to 10 to 6 sec. The MT-compatible response functions converted from GDS response functions are resistive compared to the Canadian Shield (Chave et al., 1993) for periods around 10 to 5 sec. The calculated MT responses also have generally high apparent resistivity values over the entire period range. We inverted the average MT responses into a one-dimensional conductivity profile using Occam inversion (Constable et al., 1987). The resultant conductivity profile is extremely resistive (0.001 to 0.0001 S/m) down to the mantle transition zone. We compared this one-dimensional structure with electrical conductivity profiles predicted from compositional models of the earth's upper mantle by calculating phase diagrams in the CFMAS (CaO-FeO-MgO-Al2O3-SiO2) system. The on-craton and off-craton chemical composition models (Rudnick et al., 1998) were adopted for the tectosphere. The Perple_X (e.g. Connolly, 2005) programs were used to obtain mineral proportions and compositions with depth. The calculated conductivity profiles with on- and off-craton models show significantly larger magnitude than the observed. The result suggests the continental lithosphere (tectosphere) beneath Australia is extremely dry and its temperature profile is cooler than that used in the calculation.

Magnetotelluric (MT) techniques measure natural time variations of the Earth's magnetic and electric fields to infer subsurface electrical conductivity structure. Data are collected over a range of frequencies, providing insights into how this structure varies with depth. Depending on the Earth conductivity and frequencies used, information can be obtained from the near surface to depths of hundreds of kilometres. MT surveying has been used in a wide variety of geological scenarios, from investigations of continental-scale structures to mineral and geothermal exploration, and even in the search for ground-water, and many such surveys have now been undertaken in South Australia. Recently, surveys have been conducted by Geoscience Australia (GA) under the Australian Government's Onshore Energy Security Program (OESP) along deep crustal seismic reflection transects, in part in collaboration with the University of Adelaide (UA), the Geological Survey of South Australia, Primary Industry and Resources South Australia (GSSA, PIRSA) and the Australian National Seismic Imaging Resource (ANSIR) across the Gawler Craton and Curnamona Province. Given the wide range of applications for MT data, it is proposed to deliver these data online as industry-standard electrical data interchange (EDI) files, starting with the most modern datasets. This paper presents an overview of the MT data and reports presently available for South Australia. All MT data are available for download online from the South Australian Resources Information Geoserver (SARIG), and both seismic and MT data acquired by GA and collaborators under the OESP are available for download from the GA web site.

As part of the Australian Government's Onshore Energy Security Program and the Queensland Government's Smart Mining and Smart Exploration initiatives, deep seismic reflection surveys (~2300 line km) were conducted in North Queensland to establish the architecture and geodynamic framework of this area in 2006 (Mt Isa Survey; also involving OZ Minerals and pmd*CRC) and 2007 (Cloncurry-Georgetown-Charters Towers Survey; also involving AuScope). The purpose here is to use new geodynamic insights inferred from the seismic and other data to provide comments on the large-scale geodynamic controls on energy and other mineral potential in North Queensland.

This article presents the results of studies in North Queensland associated with the 2007 Mt Isa-Georgetown-Charters Towers seismic survey. Results include seismic interpretation, geophysical studies and 3D maps, tectonic and metallogenic syntheses and energy potential assessment.

The Record contains extended abstracts to accompany presentations at the GOMA (Gawler Craton-Officer Basin-Musgrave Province-Amadeus Basin) seismic and MT workshop, Adelaide 25 November 2010.

2009 Georgina-Arunta Seismic and MT Surveys - Acquisition and Processing

These presentations from Geosciece Australia staff form part of the 2011 AGES (Annual Geoscience Exploration Seminar) meeting. Extended abstracts associated with these presentations can be found in the Northern Terriorty Geological Survey Record 2011-003.

In 2007, three seismic lines were collected by Geoscience Australia and the Geological Survey of Queensland from Cloncurry to south of Charters Towers via Croydon and Georgetown, and a fourth line by AuScope to the northeast of Mt Surprise. Signals were recorded to ~20 seconds two-way travel time (TWT), which equates to about 60 kilometres in depth. The recent lines are among the latest in a series of deep seismic profiles conducted across Queensland since 1980.

The Australian Government's Onshore Energy Security Program (2006-2011) was completed recently by Geoscience Australia. The five year program provides pre-competitive geoscience data and value-added products for assessment of hydrocarbon, uranium, geothermal energy and mineral resources. As part of the program, broadband and long period magnetotelluric (MT) data have been acquired by Geoscience Australia in collaboration with relevant state and territory geoscience agencies and universities throughout Australia. The regional-scale MT profiles, which consist of more than 640 sites over 3700 km in distance, were obtained along 12 deep seismic reflection transects across potential mineral provinces and frontier sedimentary basins. New insights into the regional-scale electrical resistivity distributions and mechanisms gained from the MT data increase knowledge about lithospheric structures, tectonic processes, and regional geological features. For example, the MT results show resistivity contrasts at terrane boundaries and fault systems; Sedimentary basins, some shear zones, fluids, and melts exhibit significant low resistivity compared with the surrounding crust or upper mantle. The MT data complement deep seismic reflection, potential field and other geophysical and geological data for multi-disciplinary investigations of crustal architecture in the study regions. The integrated results demonstrate that there are significant spatial correlations between different geophysical data. The multi-disciplinary data reduce uncertainties and limitations of data considered separately and produce a more effective and reliable interpretation, especially for regions that have complex geological structures. They also improve the understanding of the mineral and energy potential in these regions.

Geoscience Australia, along with its partners, have used seismic reflection and magnetotelluric data, acquired under the Onshore Energy Security Program, and pre-existing geological and geophysical data, to provide new insights into the 3D architecture, geodynamics, mineral and the energy potential of the North Queensland region. The 3D architecture was constrained using all available data leading to an improved understanding of the North Queensland region. Innovative 3D geophysical techniques have been adopted to provide new understandings of the 3D alteration patterns associated with potential mineralisation and energy potential of the region. <p>Related material<a href="https://www.ga.gov.au/products/servlet/controller?event=GEOCAT_DETAILS&amp;catno=69862">3D Map and Supporting Geophysical Studies in the North Queensland Region - 3D data package</a></p>