Use of airborne gamma radiometric data for soil mapping

Variations of naturally emitted gamma radiation have been used in geological prospecting for over 20 years to detect anomalies associated with exploitable ore deposits. We examined their ability to detect spatial variation of soil material by comparing simultaneous ground and airborne measurements of gamma emissions with ground observations over a catchment in south-western Australia. Measurements were taken in the spectral windows for K-40, U-238 and Th-232. Variations of gamma radiation corresponded with the distribution of soil-forming materials over the landscape, and were used to distinguish between highly weathered residuum and fresh material from granitic outcrops. Gamma radiometric data also discriminated clearly between doleritic, lateritic and granitic soil parent materials. Airborne data indicated the distribution of these materials through the catchment, with the exception of dolerite dikes, which were too narrow to be detected using pixels greater than 20 m wide. It is concluded that gamma radiometric data can provide valuable insights into the spatial distribution of soil-forming materials but, given their limitations to provide direct information of pedological alteration, such data are likely to prove most valuable to soil survey when considered jointly with other information such as terrain models or aerial photography.