AEGC 2019: From Data to Discovery – Perth, Australia 1

Geological insights of Northern Australia’s AusAEM airborne EM survey Yusen Ley-Cooper Ian Roach Ross C Brodie Geoscience Australia Geoscience Australia Geoscience Australia GPO Box 378, Canberra, ACT, 2601 GPO Box 378, Canberra, ACT, 2601 GPO Box 378, Canberra, ACT, 2601 Yusen.LeyCooper@ga.gov.au Ian.Roach@ga.gov..au Ross.Brodie@ga.gov.au

INTRODUCTION

As part of the Exploring for the Future (EFTF) program, Geoscience Australia, in partnership with state and territory geological surveys, commissioned the acquisition of 60,000 line kilometres of high-resolution pre-competitive airborne electromagnetic (AEM) geophysical data, now freely available from Geoscience Australia website (www.ga.gov.au). The AusAEM Survey (Ley-Cooper and Richardson, 2018) was collected with a fixed-wing TEMPEST system over the north-eastern Northern Territory and north-western Queensland (Figure 1). The survey provides new pre-competitive data over large areas of highly mineral-endowed geological provinces that have not yet been extensively explored and potentially opens a new search space for exploration companies. Detailed inversions of specific flight lines (Figure 2) show new under-cover geological features of interest that potentially could host new mineral deposits and groundwater resources. These inversion results highlight prospective areas that warrant further investigation by mineral exploration tenement-holders in the area.

Figure 1. oblique 3D view of the over 900 km long AusAEM conductivity depth-sections draped over the 1:1 million Surface Geology Map of Australia.

METHOD AND RESULTS To assist the interpretation of the AusAEM results, we have developed new functionality within Geoscience Australia’s sample-by-sample 1D layered-earth inversion algorithm GALEISBSTDEM (Brodie, 2015). Conventionally, for fixed-wing AEM data, we have jointly inverted the X- and Z-component data, and solved for the conductivity model plus three system geometry parameters: (i) transmitter-receiver horizontal inline and (ii) vertical separations and (iii) the pitch of the receiver coils. This is necessary to fit data within the estimated noise levels because of imprecise knowledge of the system geometry coupled with uncertainty in separating the measured electromagnetic response into primary and secondary parts. While jointly inverting two components and solving for geometry has been a successful approach for fixed-wing data, there are significant non-uniqueness trade-offs between the transmitter-receiver vertical separation and the receiver pitch parameters. Individually inverted soundings are then ‘stitched’ into section-models derived from this approach, leading to a degree of incoherence in the conductivity–depth sections along the flight lines, which impedes confident interpretation. The new functionality within our inversion algorithm allows us to invert the magnitude of the combined vector sum of the X- and Z-component data, instead of inverting both vector components jointly. For TEMPEST this means we invert 15 samples per sounding instead of 30 samples. In this new “combined XZ” inversion approach, we only solve for the transmitter-receiver horizontal and vertical separations. There is no need to invert for receiver-pitch because the input combined XZ data is not dependent (is insensitive to) receiver pitch. In eliminating the receiver-pitch parameter from the inversion, we have been able to improve the along-line coherency of our conductivity depth sections.

SUMMARY Airborne electromagnetic data generated by the AusAEM Survey are shown to map mineral deposit host rocks and regional geological features within the AusAEM Survey area. We have developed new functionality in Geoscience Australia’s sample-by-sample layered earth inversion algorithm, allowing inversion of the magnitude of the combined vector sum of the X- and Z-components of TEMPEST AEM data. This functionality improves the clarity of inverted interpretation products by reducing the degree of along-line incoherency inherent to stitched 1D inversions. The new inversion approach improves the interpretability of sub-horizontal conductors, allowing better mapping of geological features under cover. Examples of geological mapping by the AusAEM survey highlight the utility of wide line spacing, regional AEM surveying to improve geological, mineral systems and groundwater resource understanding in the regions flanking outcropping mineral deposit host rocks in northern Australia. Key words: AusAEM, TEMPEST, airborne, electromagnetic, inversion, AEM, geological mapping

Geological insights of Northern Australia’s AusAEM airborne EM survey Ley-Cooper et al.

AEGC 2019: From Data to Discovery – Perth, Australia 2

Figure 3 shows conductivity sections comparing results from the two approaches along an 80 km long portion of flight line 1020002, located in the western margin of the AusAEM survey in the Northern Territory (Figure 2). Geology in this line consists of gently dipping, folded and faulted Wiso Basin rocks overlying probable Warramunga Group rocks of the Tennant Creek region to the right-hand side, and faulted against Davenport Province rocks to the left-hand side of the diagram. The interpretability of the conductivity section resulting from this new combined XZ inversion approach (Figure 3b) is superior to the conventional approach (Figure 3a). We make this assessment based on the fact that vertical striping is reduced; sub-horizontal conductors are now resolved more discretely; and, spurious model-related conductivity anomalies in the base of the section are removed. It could be expected that by combining the X- and Z-components, we would potentially have lost information content implicit in the vector nature of the individual component data. This might be true; however, there is no apparent loss of information between the combined XZ inversion (Figure 3b) compared to the conventional inversion (Figure 3a). We assess that any loss of information has been adequately compensated for by a reduction in the non-uniqueness trade-offs and associated instabilities. Figure 4Figure 3 shows a combined XZ vector sum inversion conductivity-depth section for part of flight line 2018002, in the Lawn Hill area of northwestern Queensland (Figure 2). The section highlights the utility of the AusAEM survey data for mapping Proterozoic cover rocks in the survey area, in this case, folded rocks of the McArthur Basin in the west (left) of the line. In the far east of the line, conductive sedimentary rocks of the Carpentaria Basin, part of the Great Artesian Basin, overlie McArthur Basin rocks and effectively trap the AEM signal. Figure 5Figure 2 illustrates an oblique 3D view of inversion sections in the Mount Isa-George Fisher area of Queensland. Inversion results positively identify electrically conductive Mount Isa Group rocks, which consist of folded and faulted dolomitic, carbonaceous and pyritic siltstones and shales (Forrestal, 1990). These rocks outcrop around Mount Isa and extend, partially, under shallow cover. The illustration highlights the ability of the AusAEM dataset to map potential mineral deposit host rocks in the Mount Isa area. Similar conductivity features identify electrically conductive host rocks under cover over 100 m thick in some areas to the east of Mount Isa.

CONCLUSIONS

The ability to better interpret conductivity sections resulting from Geoscience Australia’s new combined XZ inversion approach is superior to our original method of jointly inverting the X and Z EM components of data, and solving for the conductivity and system geometry parameters. Vertical striping is diminished, sub-horizontal conductors are more discretely resolved, and spurious inversion-related conductivity anomalies at the base of the conductivity sections are removed.

The AusAEM Survey has proven to be useful in areas with a significant thickness of surface cover. The geological insights it has unveiled have made the current AusAEM a successful program. It has attracted global interest and has acquired data over extents never formerly attempted. These extensive coverage broad-spaced surveys come from GA’s long-lasting practice of supplying precompetitive data as a way of promoting exploration in new frontiers, particularly in areas with a paucity of data. These regional and continental surveys support several applications, ranging from geological mapping, mineral and energy exploration, and land management/ environmental studies.

ACKNOWLEDGEMENTS The AusAEM Survey was co-funded by Geoscience Australia’s Exploring for the Future Programme and the Queensland and Northern Territory governments. This abstract is published with the permission of the CEO, Geoscience Australia.

REFERENCES

Brodie, R. C., 2015, User Manual for Geoscience Australia’s Airborne Electromagnetic, Inversion Software. Online: https://github.com/GeoscienceAustralia/ga-aem.git. CGG Aviation, 2018. AusAEM (NT-QLD) Year 1 Survey, Australia, 2017-2018, TEMPEST Airborne Electromagnetic Survey: Logistics and Processing Report, (available from http://pid.geoscience.gov.au/dataset/ga/124092). Ley-Cooper, A. Y., and Richardson, M., 2018, AusAEM; acquisition of AEM at an unprecedented scale: ASEG Extended Abstracts, v. 2018, no. 1, p. 1-3. Forrestal, P. J. 1990. Mount Isa and Hilton Silver-Lead-Zinc Deposits. In: Hughes, F. E. ed. Geology of the Mineral Deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, 927-934.

Geological insights of Northern Australia’s AusAEM airborne EM survey Ley-Cooper et al.

AEGC 2019: From Data to Discovery – Perth, Australia 3

Figure 2. Flight line map of the AusAEM Survey (cyan) with selected lines used here as examples (black).

Figure 3. Conductivity sections for part of AusAEM Line 1020002, from the western margin of the survey in the NT, flown North-South. Panel a) shows the section derived from the conventional inversion approach, panel b) shows a section from the new combined XZ inversion, highlighting improvements in the coherency of models at depth, a better resolution of sub-horizontal conductors, and removal of jaggedness between individual soundings at the base of the conductivity section.

Geological insights of Northern Australia’s AusAEM airborne EM survey Ley-Cooper et al.

AEGC 2019: From Data to Discovery – Perth, Australia 4

Figure 4. Segment from AusAEM Line 2018002, from the north-western margin of the survey near the Queensland NT border, flown east-west. This conductivity section highlights the utility of AusAEM data to map gently folded rocks of the McArthur Basin in the Lawn Hill area of NW Queensland.

Figure 5. Oblique 3D view of combined XZ inverted conductivity sections from four adjacent AusAEM flight lines over Mt Isa and George Fisher, projected over the local 1:250,000 scale geological maps. Conductivity anomalies are associated with Mount Isa Group rocks, which host the mineralisation in this area.