Geostatistical analysis for the assessment of rare-gas soil distribution in detecting concealed faults: The Ofanto clay basin

An integrated geochemical, morphological and structural analysis was applied to a clay basin in Southern Italy (Ofanto valley) to delineate tectonic features. the resulting distribution of previous soil-gas surveys (helium and radon) and the location and orientation of field-observed brittle deformations (faults and fractures) were compared with air-photo interpreted morphotectonic features. The results show that the highest helium and radon values occur preferentially along elongated features shown by mesostructural and geomorphological analyses, i,e. anti-Apennine, Apennine and, secondarily, N-S orientations. Furthermore, the application of geostatistical techniques in a testing area has enhanced the semi-quantitative evaluation of this anisotropic soil-gas distribution (linked to the gas-bearing properties of the local brittle deformations). The correspondence between soil-gas distribution and mesostructural/geomorphological features, as well as the results from the geostatistical analysis, suggest that gas leakage towards the surface is controlled by the same structural pattern which created some morphological features. Geostatistical analysis of the geochemical data combined with the other geological techniques has been shown to improve the interpretation of soil-gas results for neotectonic studies in clay basins ic here tectonic discontinuities have no surface expression.