This report presents the results of a study jointly conducted by the Geological Survey of South Australia and CSIRO Mineral Resources which aimed to enhance the expression of geological structure in geophysical images and to derive depth to...
This report presents the results of a study jointly conducted by the Geological Survey of South Australia and CSIRO Mineral Resources which aimed to enhance the expression of geological structure in geophysical images and to derive depth to magnetic source estimates over Region 6 of the Gawler Craton Airborne Survey; viz. the southern half of the GAIRDNER 1:250k map sheet area and the northern half of the YARDEA 1:250k map sheet area. The study was based on magnetic field data acquired in sorties made during the period 31 July 2018 to 23 January 2019 by this airborne magnetic and radiometric survey commissioned by the Geological Survey of South Australia, that have been combined with ground gravity data from the South Australian state gravity database. The 2017-19 PACE Copper Gawler Craton Airborne Survey (GCAS) provides both higher resolution and more consistent mapping of the magnetic field than are available from previous coverage by multiple geophysical surveys of lesser extent. Advantages of the new survey data are quite evident upon inspection of the primary total magnetic intensity (TMI) data, but it is through the enhancement of that TMI data to assist in recovery of geological information that the advantages are most clearly expressed. Many of the enhancements presented in this report are necessarily of limited application to the TMI data previously obtained across the Gawler Craton area, mainly because of known numerous insufficiencies and imperfections in those data, and they would be hampered by the unavoidable effects of abrupt signal strength contrast that appear on passing between survey datasets acquired on different line spacings, flying heights or flight-line orientations. The GCAS data acquisition consistency and close line spacing therefore support higher resolution and more confident source depth mapping from the magnetic field data. Local magnetic field variations arise exclusively from ferromagnetic minerals which may only constitute of the order of 2% or less of the rock (even for what are considered strongly magnetised rocks), while lateral variations in geology which have no associated variation in magnetisation have no direct expression in the magnetic field imagery. In contrast, gravity data respond to variations in density, to which all components of the rock contribute. Gravity field variations therefore provide a complementary mapping of geology. Suitable combinations and contrasts of gravity and magnetic fields provide more diagnostic information about the subsurface than does the sum of the two fields processed and imaged independently. The output of this study is a collection of images and digital data products, downloadable herein, which have been generated to facilitate geological interpretation. The products are not themselves interpretive, but provide more direct access to interpretation than do directly measured datasets treated alone. These products, and in particular the magnetic source depth estimates, are designed to provide the genesis of a ‘live’ resource which can be progressively upgraded rather than simply being replaced when further studies are undertaken in the area, the depth solution database is added to, or when new drillhole information is reported.
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