Some notes on the importance of airborne gamma-ray spectrometry in International Geochemical Mapping

Abstract

Abstract Airborne gamma-ray spectrometry (AGRS) is an important component of the International Geochemical Mapping (IGM) project. It permits geochemical mapping of radioactive elements (naturally occurring and man-made) by remote sensing. Data can be reported in terms of radioelement abundance and nuclide dose-rate. The technique offers continent-wide coherency because it may be applied across any land surface, with 90% of the response coming from the top 25 cm (termed the A25 horizon). Quantitative correlation of ground and airborne data can be assured within definable limits because airborne equipment is calibrated by relating airborne to upper regolith ground measurements, under controlled conditions, over test ranges several km in length. Evidence ranging from local to continental scales suggests that the reverse of the calibration process enables AGRS to provide an important supplementary function for global mapping. In compiling geochemical maps for large regions, based on conventional surficial geochemical sampling methods, significant differences are commonly observed in element base levels for individual survey blocks, especially where methods have not been standardized. Uncertainty then exists as to the extent to which apparent changes in base level are due to real geochemical differences, or to the methods employed. Continuous profiles of surface Th and K abundance, derived from airborne data, provide an independent reference level against which estimates of Th and K, derived from various direct sampling techniques, can be compared. This comparison can identify inconsistencies in Th and K data, which may in turn raise questions concerning the consistency of other data. This is important in the context of establishing reliable global geochemical baselines for all elements.