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.

Geoscience Australia has been acquiring deep crustal reflection seismic transects throughout Australia since the 1960s. The results of these surveys have motivated major interpretations of important geological regions, contributed to the development of continental-scale geodynamic models, and improved understanding about large-scale controls on mineral systems. Over the past five years, Geoscience Australia has acquired over 6000 km of deep crustal seismic reflection data under the auspices of the Predictive Mineral Discovery Cooperative Research Centre (pmd*CRC), Onshore Energy Security Program (OESP), AuScope Earth Imaging (part of the National Collaborative Research Infrastructure Strategy), and all mainland State and Territory governments. These seismic datasets continue to underpin fundamental research into the geodynamics of the Australian continent and provide the third dimension for pre-competitive geoscience information related to mineral and energy resources in selected provinces and basins. Regional seismic reflection surveys currently utilise three Hemi 50 or 60 vibrators at 80 m VP with 40 m group interval, resulting in 75 fold data to 20 s TWT. In-house processing is aimed at providing a whole of crust image, without sacrificing shallow detail. Gravity readings are also collected along the lines at 400 m intervals to assist integrated regional interpretations based on the seismic traverses. Magnetotelluric (MT) soundings, including both broad-band and long period, have been acquired along most traverses. MT provides an image of the conductivity of the crust which is complementary to the structural information obtained from reflection seismic. Geoscience Australia is currently developing an in-house MT processing and modelling capability.

Description of the Youanmi MT acquisition and processing along the 10GA-YU1, 10GA-YU2, 10GA-YU3 seismic lines. A collaborative project with the Geological Survey of WA.

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>

The Natural Fields EM Forum was held in Brisbane, Queensland, Australia, on February 26, 2012, in conjunction with the ASEG 22nd International Geophysical Conference & Exhibition 2012. The forum was organised to review the current state of development of natural field EM methods (NFEM), being those methods that utilise the ambient electromagnetic field rather than deploying an additional active source as an element of a survey. NFEM methods are used to acquire data from which various parameters can be obtained to help interpret the electrical characteristics of the subsurface.

Geoscience Australia has been acquiring deep crustal reflection seismic transects throughout Australia since the 1960s. The results of these surveys have motivated major interpretations of important geological regions, contributed to the development of continental-scale geodynamic models and improved understanding about large-scale controls on mineral systems. Under the Onshore Energy Security Program, Geoscience Australia has acquired, processed and interpreted over 5000 km of new seismic reflection data. These transects are targeted over geological terrains in all mainland states which have potential for hydrocarbons, uranium and geothermal energy systems. The first project was undertaken in the Mt Isa and Georgetown regions of North Queensland. Interpretations of these results have identified several features of interest to mineral and energy explorers: a previously unknown basin with possible hydrocarbon and geothermal potential; a favourable setting for iron oxide uranium-copper-gold deposits; and, a favourable structural setting for orogenic gold deposits under basin cover. Other geophysical data were used to map these features in 3D, particularly into areas under cover. Seismic imaging of the full thickness of the crust provides essential, fundamental data to economic geologists about why major deposits occur where they do and reduces risk for companies considering expensive exploration programs under cover.

World class mineral systems, such as those found in the Archaean Yilgarn Craton, are the product of enormous energy and mass flux systems that were driven by lithospheric scale processes. These processes can create big footprints or signatures on the lithosphere that can be observed at a range of scales and via a range of methods: including geophysics, isotopes, tectono-stratigraphy and geochemistry. This paper uses these datasets to describe both the architecture (structure) of the world-class gold systems of the Yilgarn Craton and the signatures of their formation. By applying the understanding of the most critical elements of the process and its signature, new areas (especially undercover) may be targeted more predictably than before. Knowledge of the major architectural elements of the Yilgarn Craton has increased greatly over the last decade through the collection of a wide range of geophysical and geochemical/isotopic data sets. These data sets range from 1) lithospheric-scale studies that provide information on the entire craton down to depths in excess of 350 km, through; 2) regional-scale studies that provide information at the province scale and down to depths of 30-40 km, to; 3) mine- and camp-scale studies providing information on the local-scale down to the top few kilometres of the crust. Deep pathways in the upper mantle and lower crust can be inferred from broad-band tomography and analysis of long wavelength gravity data. Geophysical data (e.g., magnetotellurics and seismic) also provide evidence for the signatures of the flow of fluids through this architecture (the pathways) and they illustrate the scale of the systems is many orders of magnitude larger than the immediate deposit itself. Of particular importance is the role of deep-crust penetrating shear zones or faults that link the mantle with domes in the upper crust.

As part of the Australian Government's Energy Security Program (2006-2011), Geoscience Australia (GA) has acquired magnetotelluric (MT) data (more than 3000 km in distance) in conjunction with deep crustal seismic reflection along 12 transects in Queensland, South Australia, Northern Territory and Western Australia. These data, along with total magnetic intensity, gravity and geological data form the basis for multi-disciplinary investigations of energy and mineral potential and crustal architecture, providing pre-competitive information to industry and researchers. These MT projects have been undertaken by GA in collaboration with relevant state and territory geological surveys and The University of Adelaide. MT data were collected using AusScope MT instrumentation through ANSIR (National Research Facility for Earth Sounding) agreement and by a contractor with different equipment. Different survey design and acquisition parameters were used for different projects. While processing, analysis and modelling of data are ongoing, preliminary results of two dimensional models confirm the complementary value of MT to seismic interpretations. However, electrical resistivity of Earth materials is a complex property and MT responses represent a more complex Earth than one or two dimensions.

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.

Magnetotelluric (MT) data were acquired in September 2009 in a collaborative project by Primary Industry and Resources, South Australia (PIRSA), Geoscience Australia and the University of Adelaide (UA) along the east-west southern Flinders ranges seismic traverse in South Australia. The seismic and MT data acquisition are part of the Australian Government's energy security program, with main funding being provided by PIRSA under the Plan for Accelerating Exploration (PACE) initiative. The MT data form a valuable complimentary addition to the seismic data for the investigation of energy potential and crustal architecture of this region. National facility Auscope MT instruments based at UA were used (through ANSIR agreement) to record both broadband data with a frequency range 200 Hz to 0.008 Hz and long period data with a frequency range of 10 Hz to 0.0001 Hz. This enables sensing of Earth electrical conductivity from near-surface in the crust to depths well below the Moho. Two orthogonal components of the magnetic field were measured with induction coils for the broadband acquisition, and three components of the magnetic field were recorded with fluxgate sensors for the long-period data. Two horizontal components of the electric field were measured at each site with orthogonal NS and EW dipoles ~50 m long. Data were recorded at fifteen sites with a nominal spacing of 10 km covering a profile ~150 km in length. Data are processed to industry standard EDI files prior to the generation of apparent resistivity and phase plots. A suite of plots are created to investigate dimensionality including, skew angle, phase tensor ellipses and Parkinson arrows. Parkinson arrows point to regions of high conductance and away from more resistive blocks. Preliminary analysis of the long period data has revealed that the Parkinson arrows generally point to the east at higher frequencies. At lower frequencies these arrows swing southerly pointing to the south east.