The radiometric map of Australia

Abstract

Geoscience Australia and State and Territory Geological Surveys have systematically surveyed most of the Australian continent over the past 40 years using airborne gamma-ray spectrometry to map potassium, uranium and thorium elemental concentrations at the Earth?s surface. The quality of the radiometric data acquired over this period varies markedly. Early surveys (prior to about 1990) were flown with a line spacing of about 1500 m and a 16 litre detector at a flying height of 150 m agl. Later surveys have been flown with a detector volume of 32 litres and a line spacing of 500 m, or closer, and flying heights of 100 m, or better. The use of high-performance survey aircraft have now enabled flying heights to be lowered to heights of 60-80m for regional surveys and even lower for detailed surveys. For early surveys, the results were usually reported in units of counts per second. Thus the magnitudes of these data values depend on both the instrumentation used in the survey (such as crystal volume) and the survey parameters (such as nominal flying height). This means that the results from early surveys that used different instrumentation and survey parameters are not directly comparable. Also, even where survey acquisition systems were calibrated to report results as equivalent concentrations of the radioelements, limitations in the calibration of these instruments and temporal variations in radiation output from the earth often result in mis-matches between surveys along their common boundaries. These problems limit the usefulness of the gamma-ray spectrometric data, as surveys are not easily combined into regional compilations, and quantitative comparisons between radiometric signatures from different surveys are difficult. The solution is to adjust all of Australia?s public-domain gamma-ray spectrometric data to a common datum. This will enable surveys to be easily merged into larger regional compilations, and thus facilitate the recognition and interpretation of broad-scale regional features in the data. This paper describes the adjustment of Australia?s National Radioelement Database to a common datum. We have used an Australia-wide Airborne Geophysical Survey (AWAGS) to adjust all the public-domain radiometric surveys in Australia to the International Atomic Energy Agency?s (IAEA) Global Radioelement Datum. The levelled database has been used to produce the first ?Radiometric Map of Australia? - levelled and merged composite potassium (% K), uranium (ppm eU) and thorium (ppm eTh) grids over Australia at 100 m resolution. Interpreters can use these grids 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.