Extended abstract regarding the Frome AEM data set and Murray Basin geology and landscape evolution

The holistic inversion approach for frequency domain AEM data (Brodie and Sambridge, 2006) has previously been employed to simultaneously calibrate, process and invert raw frequency-domain data where prior information was available. An alternative formulation has been developed, which is suitable in the case where explicit prior information is not available. It incorporates: a multi-layer vertically-smooth conductivity model; a simplified bias parameterization; horizontal smoothing with respect to elevation; and cluster computer parallelisation. Without using any prior data, an inversion of 8.0 million data for 3.4 million parameters yields results that are consistent with independently derived calibration parameters, downhole logs and groundwater elevation data. We conclude that the success of the holistic inversion method is not dependent on a sophisticated conceptual model or the direct inclusion of surrey-area specific prior information. In addition, acquisition costs could potentially be reduced by employing the holistic approach which may eliminate the need for high altitude zero-level measurements.

Short article describing detection of interpreted unconformity between Coolbro Sandstone and Rudall Complex rocks near the Kintyre uranium deposit, Western Australia

Data from a VTEM airborne electromagnetic survey over resistive terrain is examined. Forward modelling and analysis of high-altitude lines shows that the amplitudes of random noise, bucking error, processing corrections and geological signals can be large compared to the geological signal in the resistive terrain. The negative impacts of the low geological signal to noise ratio on conductivity estimates generated by layered-earth inversion and conductivity transformations are demonstrated. The reader is alerted to the degree of uncertainty and non-uniqueness that is inherent in conductivity estimates generated from similar datasets.

The Frome airborne electromagnetic (AEM) survey is the last and largest (in line kilometres and area) of the three AEM surveys flown under the Onshore Energy Security Program at Geoscience Australia. The survey covered about 95 000 km2 (about six 1:250 000 map sheets) and covered the Frome Embayment, northern Murray Basin and the flanks of the Eromanga Basin in South Australia. Survey data are providing new insights into the stratigraphy of the Frome Embayment and Murray Basin, and will add to our knowledge of the sources and sinks of sandstone related uranium mineral systems, as well as base metals, gold and copper, in this highly prospective area, as well as ground water resources. This seminar will present highlights of the survey and demonstrate AEM data integrated with other data sets to develop 3D models to explorable depths, lowering exploration risk.

Airborne Electromagnetic data are being acquired by Geoscience Australia (GA) under the Australian Government's Onshore Energy Security Program (OESP) in areas considered to have potential for uranium or thorium mineralisation. The surveys have been managed and interpreted by GA's Airborne Electromagnetic Acquisition and Interpretation project. In contrast to deposit scale investigations, conducted by industry, these surveys are designed to reveal new geological information at a regional scale. The Pine Creek airborne electromagnetic survey show in Figure 1 is comprised of three survey areas; Woolner Granite, Rum Jungle and Kombolgie. TEMPESTTM data were acquired for the Woolner Granite and Rum Jungle surveys and VTEMTM data were acquired for the Kombolgie survey. The Kombolgie survey, in the Pine Creek Orogen of the Northern Territory, covered sections of the Alligator River, Cobourg Peninsula, Junction Bay, Katherine, Milingimbi and Mount Evelyn, 1:250 000 map sheets (Costelloe et al., 2009). A total of 8 800 line km of VTEMTM data were acquired in 2008, covering an area of 32 000 km2. In 2009 the processed response data and EM FlowTM commercial version 3.30 (Macnae et al., 1998, Stolz and Macnae 1998) conductivity estimates to 600 m depth, produced by the survey contractor Geotech Airborne, were made available to the public in the GA Phase-1 data release. In this article we discuss an enhanced set of conductivity estimates, which are now available from the GA website free of charge. These new conductivity estimates, reveal new geological information to depths approaching 2 km in the more resistive portion of the survey area. They were generated by GA using the most recent version (5.23-13) of EM FlowTM.

In many floodplain landscapes in Australia, surface-groundwater interactions are poorly understood. There is limited mapping of recharge and discharge zones along the major river systems, and only generalised quantification of hydrological fluxes based on widely spaced surface gauging stations. This is compounded by a lack of temporal data, with poor understanding of how surface-groundwater interactions change under different rainfall, river flow and flood regimes. In this study, high resolution LiDAR, in-river sonar, and airborne electromagnetic (AEM) datasets (validated by drilling) have been integrated to produce a detailed 3-Dimensional map of surface geomorphology and hydrogeology. These maps enable potential recharge zones in the river and adjacent landscape to be identified and assessed under different flow regimes. These potential recharge zones and groundwater flow pathways were then compared against the spatial continuity of (and presence of) 'holes' in near-surface and deeper aquitard layers derived from the AEM. These 3D mapping constructs provide a framework for considering groundwater processes. Hydrochemistry data, allied with hydraulic data from a bore monitoring network, demonstrate the importance of recharge during significant flood events. In many places, the AEM data also affirm the spatial association between fresher groundwater resources and sites of river/floodplain leakage. At a more localised scale, hydrogeochemical data allows discrimination of lateral and vertical fluxes. Overall, this integrated approach provides an important conceptual framework to constrain hydrogeological modelling, and assessments of sustainable yield. The constructs are also invaluable in an assessment of managed recharge options and locations.

Provides an overview of the Project in the context of the Onshore Energy Security Program in advance of the presentations dealing with the acquisition, data, procedures and interpretation results of the survey.

Geoscience Australia (GA) has recently released regional AEM data in two survey areas of the Pine Creek region. The Woolner Granite-Rum Jungle survey in the western part of the region was flown using TEMPESTTM and the Kombolgie survey in the eastern part was flown using VTEMTM. These data assist mapping shallow subsurface geological features for mineral explorations including uranium. Conductivity estimates derived for the Woolner-Rum Jungle surveys using GA's Layered Earth Inversion (GA-LEI) algorithm reveal several unconformities and conductors in sedimentary sequences interpreted mostly within the top 300-400 m of the crust, including: 1. The Depot Creek unconformity over the Paleoproteroic Pine Creek Orogen and Archean. A 3D model derived from these data shows the geometry and lateral extent of part of the Depot Creek unconformity with a clarity previously not available; 2. Strong conductors of carbonaceous and pyritic materials in the South Alligator River Group and Mt Partridge Group, 3. Moderate to week conductors revealed in the sedimentary sequences of the Birrindudu, Daly and Bonaparte Basins, and 4. Faults and folds. EM FlowTM (Version 5.23-13) sections from the Kombolgie survey not only map the Kombolgie unconformity over the Pine Creek Orogen and Archean and other conductors in the Katherine River Group, but also map conductors in pre-Kombolgie basement to a depth up to 2 km. Penetration to this depth represents a major breakthrough in AEM survey capability, achieved in part due to the extremely resistive rocks in the area. Geological cross-section interpretations based on conductivity images also incorporate available drill hole data to help improve section reliability. The implications of these AEM data for uranium and gold mineral systems in the Pine Creek region are discussed in the companion paper by Jaireth et al. (this volume).

The continuing world demand for potash (potassium salts) is driving a new exploration boom in the Australian minerals industry for this valuable resource, listed by Geoscience Australia (GA) as a strategic commodity (Mernagh 2013). The Food and Agriculture Organization of the United Nations (FAO) predicts a rising demand for fertilizers, with potash demand increasing at 3.7% per annum (FAO 2012), and Rabobank predicts that demand will exceed supply by up to 100% by 2020 (Rabobank 2012). This demand is driving the application of airborne electromagnetics (AEM) to map salinity as a proxy for potential potash resources in salt lakes. This short paper describes a few of the applications and is written in response to an industry request to GA for information on how AEM might be used to explore for potash.