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.

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.

Magnetotelluric (MT) data have been acquired in 2008 and 2009 at 40 broadband (0:01 s to 500 s) and 12 long-period (10 s to 10 000 s) sites along the east-west deep seismic reflection transect of northern Eyre Peninsula, South Australia. The MT survey is a joint project between the University of Adelaide and Geoscience Australia and is funded by the Australian Government as part of the Onshore Energy Security Program. Long-period sites are spaced 20 km apart and broadband sites infill this spacing to 10 km with also some 5 km spacing. This ensures sufficient coverage to map the upper crustal to upper mantle structures beneath northern Eyre Peninsula.

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.

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.

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.

PowerPoint presentations presented at the NORTH QUEENSLAND SEISMIC AND MT WORKSHOP in Townsville, June 2009.

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.

Magnetotelluric data were acquired for Geoscience Australia by contract along the north-south 08GA-C1-Curnamona seismic traverse to the east of Lake Frome from November 2008 to January 2009 as part of the Australian Government's energy security initiative. 25 sites were spaced an average of 10 km apart, and five-component broadband data were recorded with a frequency bandwidth of 0.001 Hz to 250 Hz and dipole lengths of 100 m. Apparent resistivity and phase plots are presented, along with dimensional analyses of the data based on rotational invariants, the representation of the data by the phase tensor, and Parkinson arrows. These analyses provide insight into the complexity of the Earth conductivity giving rise to the MT responses and are a useful precursor to modelling.

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.