Uranium-series disequilibrium in Australian soils and its effect on aerial gamma-ray surveys

Measurements of U and Ra-226 in 445 soil samples collected from areas throughout Australia show that these soils have a mean U content of 2.0 mg kg(-1) and activity ratio (U/Ra-226) of 1.09. There is a wide spread in individual values but, overall, more soils are U-rich than Ra-226-rich. Some indications of regional disequilibrium were found, with Ra-226-rich soils being noted in the Southern Cross-Kambalda area (WA), in south-eastern Eyre Peninsula (SA) and in an area north of Cobar (NSW). Uranium-rich soils were found in two Carboniferous volcanic terrains in north-eastern Queensland. The spread in activity ratio values indicates that disequilibrium will contribute to the noise in U channel data obtained by aerial gamma-ray surveying. The combination of statistical noise from low count rates (reflecting the low U concentrations in soils) and U-Ra-226 disequilibrium combine to give variances of 40-60% in typical U channel data. Data appearance can be improved to some extent by appropriate filtering.