February 2020 marked the completion of primary data delivery from the Gawler Craton Airborne Survey (GCAS) data with provision of the Region 5, Streaky Bay data package. Beginning in 2017, the 1.6 million line-kilometre survey has been systematically delivering high-quality magnetic, radiometric and elevation data from its 16 survey regions over two years (Fig. 1). Complementing the primary data are value-added data packages and magnetic source models, with the last in the series released on 3 March 2020 to coincide with the opening of ExploreSA: The Gawler Challenge.

Figure 1 A preliminary total magnetic intensity (TMI) grid merge from the 16 GCAS regions, all now available online, via SARIG and the GCAS web page.

The new data surpasses the previous patchwork of historical surveys and the 200 m line spacing ensures gridded data products offer a higher resolution and more consistent mapping of the magnetic field than previously available. The result is an internally consistent set of geophysical datasets, creating image products that merge seamlessly to provide high-quality geological information over the Gawler Craton, South Australia’s premier mineral producing region.

A major initiative to extend the GCAS body of work has been through a collaboration with CSIRO. A series of enhanced geophysical imagery, magnetic source depth models and reports have been generated to facilitate geological interpretation using the TMI data, complemented by regional gravity data. These products can be progressively upgraded rather than simply replaced as further studies are undertaken in the area, the depth solution database is added to, or new drillholes are reported.

Rohan Cobcroft, Geological Survey of South Australia Director, says ‘The completion of this world-leading work speaks to delivery of geoscience excellence. Our Survey is developing the next evolution of data science over this region with the fusion of new geology, geophysics, geochemistry, landscape evolution studies, detailed structural interpretation and exciting innovation.’

Accessing data

The Geological Survey and Geoscience Australia have worked closely with four survey acquisition contractors engaged during the life of the acquisition and processing to ensure that rigorous data standards established nationally through Geoscience Australia and Australian state geological surveys were achieved. A number of survey specifications updated for GCAS have become the new standard, in line with technological improvements in acquisition equipment and platforms.

The primary GCAS data release consists of:

• final located data: magnetic, radiometric and elevation
• gridded magnetic images: TMI, RTP TMI, RTP TMI 1VD in geodetic and projected coordinate systems
• gridded radiometric images: dose, uranium, thorium, potassium, ternary (RGB) with NASVD and no NASVD, in projected and geodetic coordinate systems
• gridded radar and laser altimeter derived elevation: in geodetic and projected coordinate systems.

The value-added package data releases consist of:

• a range of additional analytic magnetic and gravity products
• magnetic source depth models and imagery.

This data can be viewed and downloaded via the South Australian Resources Information Gateway (SARIG). SARIG’s advanced geophysical search capability can be used to download portions of the GCAS located data or grids, plus its new airborne surveys time-slice tool provides links to direct download. Links to the data packages are also available via the GCAS community information webpage utilising Amazon Web Services (S3).

A final project report that encapsulates all of the activities, data and learnings from the survey will be released in the fourth quarter of 2020.

Next steps

Individual GCAS grids will be merged into sets of continuous surfaces covering the entire project area. This will involve re-gridding of the individual survey regions using a single, consistent set of gridding parameters with careful attention paid to the maintaining grid co-nodularity across the region. This eliminates resampling during processing to ensure an end product with minimal effects introduced by processing, which may affect the final product. While it is a straightforward process for TMI and elevation, radiometric grid merging is more complicated as it requires both shifting and rescaling to achieve a robust product. To improve the radiometric merge process, data from the Whyalla test lines – flown as part of the GCAS to enable a direct comparison of acquired data over the same set of seven flight lines – will be used to generate scaling coefficients using the average radioelement concentrations from each aircraft to scale the data to a set of reference survey concentrations (one of the Whyalla concentration sets). This novel procedure will remove the requirement to scale all surveys to the national reference dataset – only a shift will be required to perform the final grid merge.

– Laz Katona, April 2020

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