Predictive maps of the subsurface can be generated when geophysical datasets are modelled in 2D and 3D using available geological knowledge. Inversion is a process that identifies candidate models which explain an observed dataset. Gravity, magnetic, and electromagnetic datasets can now be inverted routinely to derive plausible density, magnetic susceptibility, or conductivity models of the subsurface. The biggest challenge for such modelling is that any geophysical dataset may result from an infinite number of mathematically-plausible models, however, only a very small number of those models are also geologically plausible. It is critical to include all available geological knowledge in the inversion process to ensure only geologically plausible physical property models are recovered. Once a set of reasonable physical property models are obtained, knowledge of the physical properties of the expected rocks and minerals can be used to classify the recovered physical models into predictive lithological and mineralogical models. These predicted 2D and 3D maps can be generated at any scale, for Government-funded precompetitive mapping or drilling targets delineation for explorers.

The Broken Hill Managed Aquifer Recharge (BHMAR) project is part of a larger strategic effort aimed at securing Broken Hill's water supply and identifying significant water-saving measures for the Darling River system. Hydrogeological investigations to rapidly identify and assess potential MAR targets and groundwater resources over a large area (>7,500 km2), included acquisition of an airborne electromagnetics (AEM) survey, a 7.5 km drilling program (100 sonic and rotary mud holes), and complementary field and laboratory hydrogeochemical investigations. The study identified an excellent aquifer (the Calivil Formation), with high storage capacity, very high transmissivities (up to 50 l/s), and significant volumes of fresh groundwater. The aquifer is sandwiched between variably thick clay aquitards, and can be characterised as varying from a confined to a 'leaky confined' system. The hydraulic properties make the Calivil Formation aquifer potentially suitable for groundwater extraction and/or MAR injection, with excellent recovery efficiencies predicted. Mapping identified a number of potential suitable locations for MAR options, for which entry-level risk assessments were carried out. Targets were prioritised, and a pre-commissioning semi-quantitative residual risk assessment carried out for a priority site. Assessment of 12 hazard types included hydrogeological modelling, laboratory column clogging studies and geochemical assessment to assess source water treatment requirements. The study found that all of the scientific/technical risks for MAR at the priority target are low. The integrated analysis has identified a range of possible MAR options including injection, passive or enhanced recharge, and/or conjuctive use involving a combination of surface, groundwater extraction and/or MAR options.

The National Geochemical Survey of Australia project represents an essential component of the Australian Government's Onshore Energy Security Initiative. The national geochemical survey involves the use of field-tested methods for collection and analysis of transported regolith samples representative of catchments covering most of Australia. The project is a collaboration between Geoscience Australia and State and Northern Territory geoscience agencies, which will provide an internally consistent geochemical dataset useful for calibration and ground-truthing of airborne radiometrics surveys. The survey also will help to fill gaps in current airborne radiometrics and geochemical coverages of Australia, provide multi-element characterisation and ranking of radiometric anomalies and aid in first-order investigation of the nature of geothermal hot-spots. As a result it will support and add value to numerous other Onshore Energy Security Initiative projects and have wider applications in mineral exploration and in environmental assessment and management. This report details the methodology underpinning the determination of the theoretical sampling points using terrain and hydrological analysis; and the protocols for sample collection. It will be used for knowledge transfer during training sessions for the State and Northern Territory field parties who also will receive field equipment and consumables which will ensure there is consistent sampling throughout the project. A digital data entry template has been designed to enable efficient and consistent in-field data capture, which also will streamline data entry into Geoscience Australia's corporate databases.

This publication is an outcome of a meeting entitled "Transient and Induced Variations in Aeromagnetics" that was held in Canberra on 18 September 1996 to discuss the effects of rapid fluctuations of the geomagnetic field on high-resolution aeromagnetic surveys and airborne detection systems. The meeting brought together people from the exploration and mining industry, Defence, Government Science, and Universities with common interests in the nature and applications of external magnetic fields and of the electromagnetic properties of the Earth's crust and oceans. Inevitably, much of the focus was on the use of base stations and tie lines for correcting for the influence of geomagnetic fluctuations in survey data. However, the discussion ranged widely from magnetospheric physics to the magnetic effects of ocean swells at aeromagnetic elevations.

The inversion analyses presented in our paper and now extended in this Reply were ultimately only one part of the AEM system selection process for the BHMAR project. Both Derivative and Inversion analyses are in their nature theoretical, and it is impossible, in a theoretical analysis, to capture all of the aspects relevant for real surveys with little margin for error in political time frames. In reality, neither the Derivative nor Inversion analysis provided the degree of certainty required (by the project manager and client) to ascertain whether any of the candidate AEM systems were able to map the key managed aquifer recharge targets recognized in the study area. Consequently, a decision was made to acquire data over a test line with the 2 systems (SkyTEM and TEMPEST) that performed best in the Derivative and Inversion analysis studies. This approach was vindicated with quite distinctive and very different performance observed between these two systems, especially when compared with borehole and ground geophysical and hydrogeological data over known targets. Data were inverted both with contractors' software and with reference software common to all systems and the results were compared. Ultimately, it was the test line, particularly in the near-surface (top 20metres), thatmade the SkyTEM system stand out as the best system for the particular targets in the project area. SkyTEM mapped the key multi-layered hydrostratigraphy and water quality variability in the key aquifer that defined the key MAR targets, although the TEMPEST system had a superior performance at depths exceeding 100metres. Importantly, the SkyTEM system also mapped numerous, subtle fault-offsets in the shallow near-surface. These structures were critical to mapping recharge and inter-aquifer leakage pathways. Further analysis has demonstrated that selection of the most appropriate AEM system and inversion can result in order of magnitude differences in estimates of potential groundwater resources. The acquisition of SkyTEM data was an outstanding success, demonstrating the capability of AEM systems to provide high-resolution data for the rapid mapping and assessment of groundwater and strategic aquifer storages in Australia's complex and highly salinized floodplain environments. The SkyTEM data were used successfully to identify 14 major new groundwater targets and multiple MAR targets, and these have been validated by an extensive drilling program (Lawrie et al., 2012a-e). Increasingly, the demand from clients for higher certainty in project decision making, and quantifying errors, will see development of new system comparative analysis approaches such as the Inversion analysis approach documented in our initial paper. Ultimately, system fly-offs are likely in high-profile projects where budgets permit.

Short article describing a new method of defining depth of investigation for airborne electromagnetic surveys

More recently the O'Farrell government has called for expressions of interest to explore for uranium across NSW. Fugro Airborne Services Pty Ltd also called for expressions of interest in flying a large TEMPEST AEM survey in NSW covering the NSW Curnamona Province and portions of the Murray-Darling Basin and Lake Eyre Basin, abutting the SA border, to complement the Frome AEM Survey. The following is a brief summary of some of the main points discussed and presented during 3 presentations at the NSWGS on 19 September 2012, and in follow-up discussions on 20 September 2012. Approximately 40 people attended the three presentations. A discussion after the talks centred around using AEM in NSW for regional mapping including for uranium, porphyry copper-gold systems and massive sulphide systems. PowerPoint presentations were left with NSWGS. Three abstracts describing these presentations are included at the end of this document.

Conceptual MAR targets in the Broken Hill region were identified in previous investigations (Lewis et al., 2008; Lawrie et al., 2009a). In the BHMAR Phase 2 study, the project team is required to make recommendations on the presence and suitability of potential MAR sites with an 80% confidence level. While this will be attempted through a combination of AEM, borehole analysis and seismic reflection data acquisition, AEM is the prime dataset required to map the aquifer targets in 3D.

Airborne Electromagnetic data are being acquired by Geoscience Australia in areas considered to have potential for uranium or thorium mineralisation under the Australian Government's Onshore Energy Security Program (OESP). The surveys have been managed and interpreted by Geoscience Australia's Airborne Electromagnetic Acquisition and Interpretation project. In contrast to industry style deposit scale investigations, these surveys are designed to reveal new geological information at regional scale. The Paterson airborne electromagnetic data were acquired at line spacings of between one and six kilometres, a total of 28 200 line km and covers an area of 47 600 km². The outcomes of the Paterson AEM survey include mapping of subsurface geological features that are associated with unconformity-related, sandstone-hosted and palaeovalley-hosted uranium mineralisation. The data are also capable of interpretation for other commodities including metals and potable water as well as for landscape evolution studies. The improved understanding of the regional geology resulting from the Paterson survey results will be of considerable benefit to mining and mineral exploration companies. This Data Package is for Archive to the internal area of the CDS and contains all data, grids, images, mxd, shape files, documentation, licenses, agreements, interpretations and scripts used to create the Paterson deliverables. At the projects completion (2012) all directories are required to be moved off the NAS. The reason to keep all the files is that more work is to be done on this data in the 2012-2015 period and these files may be needed in this future work.

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.