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The Nullarbor Plain is famous for having one of the longest straight roads in the world, but it is beneath the ground that new frontiers are beginning to open up. TIM CONNELL writes

Aircraft undertaking magnetic survey over the Nullarbor ©  Geological Survey of South Australia (GSSA)

Across the border and east of the plain’s famous 90 Mile Stretch, in South Australia, yawns the Gawler Craton.

At about 440,000 square kilometres, it is the oldest and largest geological province in the state.

It is also the site of one of the largest airborne geophysical surveys ever carried out.

Australia's largest geological survey

The Gawler Craton Airborne Survey, concluded in 2019, was led by the South Australian government and Geoscience Australia.

But the record undertaking, which surveyed more than 1.66million line kilometres, was supported by expertise from CSIRO Mineral Resources.

The resulting products and reports will light the way into the craton for mineral explorers.

The significance of this geophysical visualisation is critical to anyone with an interest in finding mineral deposits in Australia, where the adage 'needle in a haystack' is rarely adequate.

Deposits are not just small targets. They can be locked underground with little or no surface expression to provide a clue.

The search for minerals, says Clive Foss, Senior Principal Research Scientist with CSIRO Mineral Resources, often relies on remote sensing.

Remote sensors map Nullarbor magnetisation

The most widely used method is aeromagnetics, or mapping a magnetic field resulting from magnetisation beneath the surface using aircraft.

In Australia, miners and other explorers with little surface exposure of mineralised subterranean basement use the resulting images of magnetic field variations as proxies for geological maps.

State and territory governments routinely fly surveys, typically at line spacings of one hundred to four hundred metres, and elevations of 50 to 80 metres.

Private companies can then fly more detailed surveys of their tenements at tighter line spacings and lower elevations to improve resolution.

Aircraft contracted for the Geological Survey of South Australia to undertake aerial surveys above the Nullarbor Plains are specially fitted with magnetometers situated in ‘stingers' (long arm off back of aircraft) to remove the sensor from the metal and electrical currents of the aircraft. ©  GSSA

"What we're doing is physics; geophysics is physics in its application to the earth," Dr Foss says.

"Mapping the magnetic field variations we measure in the air reveals the position of magnetisation in the rocks beneath the ground but we still have a lot of work to estimate how deep they are."

"This 'inverse' problem of estimating the source from the measured field is a major challenge with no assurance that the answer we get is correct."

"There are many 'quick and easy' methods to get an answer but CSIRO Mineral Resources has developed a robust method to give at least the highest chance of being correct."

"We term this the 'sweet-spot' method."

Finding the sweet spot

The sweet-spot method is prone to non-uniqueness just like any other, however, seeks better solutions by selecting only the best data, which are our 'sweet spots'. Individually, these are only small packets of data and we can run intensive computations on them quite quickly.

"We liken this method to a climbing wall where you're only allowed to use the specific hand and footholds, with no assistance from the large areas of smooth wall to which those holds are attached," Dr Foss says.

In the Gawler Crater Airborne Survey, CSIRO Mineral Resources generated more than 4,600 hand-crafted sweet-spot depth estimates, piecing together the most comprehensive study of its kind ever undertaken.

The fruits of that work, from raw data to models, have all been made available for download.

It is a monumental resource that lets users investigate and test depth estimates and incorporate them into more interpretive geological models.

The exercise has greatly enriched the pool of primary magnetic field data, Dr Foss says, "for only an incremental cost".

Digital resources for Gawler Craton now available

In March 2021 the Gawler Craton Airborne Survey released troves of such digital riches; anyone interested in radiometric and elevation grids can download packages compiled by the South Australian government and CSIRO laden with magnetic data and models of depth to prospective rocks.

These depths are essential for planning drill programs.

Mineral companies generally look for shallower depths that can be economically tested by drilling.

Petroleum companies whose prospective ground is mostly above the strongly magnetized basement rocks prefer greater depths.

Magnetic source depth estimates can also be used in groundwater and engineering studies.

The methods honed by CSIRO, while designed for resource identification, are already equipping Australia and the world for clean energy.

There is a global urgency to discover battery metals such as lithium, nickel, manganese and cobalt, and accurate mapping will be critical.

The Gawler Craton also has some of the world's largest copper mines and will aid in equipping Australia’s future energy needs.

You can download the data from the Gawler Craton Airborne Survey home page from Energy and Mining South Australia.

The data can also be downloaded via the national Geophysical Archive Data Delivery System (GADDS) which is hosted by Geoscience Australia.

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