Mapping the natural radioactivity of Elba Island by means of geostatistical interpolation of airborne gamma-ray data.

We present the maps of K, eU, and eTh radioelement abundances of Elba Island (Italy) realized by means of a geostatistical interpolation of airborne "-ray data, performed with a module of four NaI(Tl) crystals of 16 L mounted on an autogyro. We applied the Collocated Cokriging (CCoK) multivariate estimator in a non-conventional way for interpolating the under-sampled airborne gamma-ray data using the geologic map as constraining ancillary variable. An arbitrary number has been assigned to each geological formation and used in the algorithm for estimating the radioelement abundances: the independence from the random assignment process has been tested for three distinct models. The spatial variability shows well-defined structures for the linear coregionalization models. The abundance maps indicate a distinct correlation between the geological formation and radioactivity content. High K, eU and eTh abundances were estimated in the intrusive granitic complex of Mt. Capanne and low abundances in the geological formations in the N-E sector of Elba Island. However, a clear anomaly of high K content in the Mt. Calamita promontory confirms the presence of felsic dykes and hydrothermal veins not reported in geological map of Tuscany Region at scale 1:10000. This result confirms that the internal variability of the radiometric data is not biased by the multivariate interpolation.