AWAGS Radiometrics and Magnetics Project

 

This project concluded on 30 June 2008.

Overview

A common problem with past national airborne geophysical coverages is that the surveys were flown in patchwork fashion over many years and are not all registered to the same datum. In the case of airborne gamma-ray spectrometric data, acquisition equipment, system calibration and data processing procedures have changed significantly over time. Older surveys in Australia were reported in units of count per second, while modern surveys are reported in units of radioelement concentration. Also, environmental effects such as soil moisture and radon emanation can affect the base level of gamma-ray surveys. This means that gamma-ray spectrometric surveys seldom match exactly along their common borders, making it difficult to merge surveys into regional or continental-scale compilations. This limits the usefulness of these data because regional compilations facilitate the interpretation of large-scale features in the data and the comparison of features large distances apart.

A similar problem occurs with magnetic surveys, with inadequate reference field removal introducing base-level shifts. Also, the cross-over tie levelling procedure commonly applied to airborne magnetic data introduces a range of spurious wavelengths into the levelled data. Both gamma-ray spectrometric and magnetic surveys can be levelled and merged into continental-scale compilations by using the differences in areas where the surveys overlap to estimate correction factors. However, without independent control, this merging procedure can introduce long-wavelength errors into the merged data.

The Australian Government's solution to this problem was to acquire gamma-ray spectrometric and magnetic data over the entire Australian continent at 75km flight line spacing. These data (the Australia Wide Airborne Geophysical Survey - AWAGS) have been used to level and merge all public-domain gamma-ray spectrometric and magnetic data in Australia to common datums. The gridded merged data have been called the Radiometric and Magnetic Maps of Australia, respectively.

Project outputs

AWAGS survey

The AWAGS survey was completed in June 2008, and the proejct is continued as part of the Continental Geophysics Project. The data are available via the Australian Government's Geoscience Portal Geophysical Archive Data Delivery System (GADDS) .

Radiometric Map of Australia

A new radioelement map of Australia, the Radiometric Map of Australia, has been compiled showing the distribution of airborne measured potassium (percent K), uranium (ppm equivalent U) and thorium (ppm equivalent Th) over 80 percent of the Australian continent at 100 metre resolution (Minty et. al., 2009). The map has been calibrated with the AWAGS to adjust all the public-domain radiometric surveys in Australia to the International Atomic Energy Agency's (IAEA) Global Radioelement Datum . The new datum provides a baseline for all current and future airborne gamma-ray spectrometric surveys in Australia.

Interpreters can use these grids of potassium, equivalent thorium and equivalent uranium to reliably compare the radiometric signatures observed over different parts of Australia. This enables the assessment of key mineralogical and geochemical properties of bedrock and regolith materials from different geological provinces and regions across the continent. These data support a range of different applications, including geological mapping, mineral and petroleum exploration, geomorphological studies and environmental mapping.

Play the movie to learn more about how the map was developed, how to interpret it and its range of applications.

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Revealing Australia's Secrets
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© Geoscience Australia

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The Radiometric Map of Australia dataset is made up of 12 separate grids (see table below).

Grid Data Types
Grid Type Description Units
Unfiltered K K element concentrations % K
Unfiltere U U element concentrations ppm eU
Unfiltered Th Th element concentrations ppm eTh
Unfiltered Dose Terrestrial dose rate derived as a linear combination of the unfiltered K, U and Th grids described above (see Minty et al.,2009) nG/h
Filtered K Low-pass filtered K element concentrations % K
Filtered U Low-pass filtered U element concentrations ppm eU
Filtered Th Low-pass filtered Th element concentrations ppm eTh
Ratio Th/K Ratio of Th over K derived from the filtered Th and K grids dimensionless
Ratio U/K Ratio of U over K derived from the filtered U and K grids dimensionless
Ratio U/Th Ratio of U over Th derived from the filtered U and Th grids dimensionless
Filtered Dose Terrestrial dose rate derived as a linear combination of the filtered K, U and Th grids described above (see Minty et al.,2009) nG/h
Total Dose Total dose rate due to terrestrial and cosmic radiation, derived by adding the estimated cosmic dose contribution to the filtered dose due to terrestrial sources described above nG/h

The gridded digital data which underpins the Radiometric Map of Australia is available free of charge for download using the Geophysical Archive Data Delivery System (GADDS) . Because the grids are large (approximately 5.5GB each), users planning to access entire grids at full resolution should contact minerals@ga.gov.au to arrange alternative means of acquiring the digital data.

Magnetic Anomaly Map of Australia

A 5th edition full-colour Magnetic Anomaly Map of Australia at 1:5 million scale, and accompanying composite dataset with cell resolution of 3 seconds of arc, have been compiled. It is estimated that 27 million line-kilometres of survey data was acquired to produce this new edition, which is eight million line-kilometres more than for the previous edition released in 2004. New independent airborne total-field magnetic data acquired in 2007 during the AWAGS have been used to increase the accuracy of intermediate wavelengths of the continental-scale merge of the grids.

Information in the new magnetic anomaly map and associated grid database provides insights into the distribution of magnetically susceptible minerals within the Earth's crust. Such insights are of great value to energy and mineral exploration companies and for research into the solid Earth and the environment. Magnetic minerals in small amounts are widespread in the crust and become concentrated in zones which highlight the structure of the crust. This is particularly important for areas that have a significant thickness of surface cover (regolith and sedimentary basins) which can mask the underlying crystalline basement rocks. The magnetic signatures of the basement are measured through the cover and provide important information to help determine the nature and depth of the basement.

Grid data types: The Magnetic Map of Australia composite grid dataset is available in ERMapper IEEE4ByteReal binary format.

The gridded digital data which underpins the Magnetic Anomaly Map of Australia is available free of charge for download using GADDS . Because the grid is large (approximately 8.3GB), users planning to access the entire grid at full resolution should contact minerals@ga.gov.au to arrange alternative means of acquiring the digital data.

References

Minty, B, Franklin, R, Milligan, P, Richardson, LM and Wilford, J 2009, 'The Radiometric Map of Australia', Exploration Geophysics, vol.40, no.4, pp. 325-333.

Milligan, PR, Minty, BRS, Richardson, M and Franklin, R 2009, 'The Australia-wide Airborne Geophysical Survey - accurate continental magnetic coverage', Preview, vol.138, p. 70.

Minty, BRS, Milligan, PR, Luyendyk, T and Mackey, T 2003, 'Merging airborne magnetic surveys into continental-scale compilations', Geophysics, vol.63, pp. 1986-1996.

Briggs, IC 1974, 'Machine contouring using minimum curvature', Geophysics, vol.39, pp. 39-48.

Savitzky, A and Golay, MJE 1964, 'Smoothing and differentiation of data by simplified least squares procedures', Analytical Chemistry, vol.36, pp. 1627-1639.