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 applie d to a clay basin in Southern Italy (Ofanto valley) to delineate tectonic f eatures. the resulting distribution of previous soil-gas surveys (helium an d radon) and the location and orientation of field-observed brittle deforma tions (faults and fractures) were compared with air-photo interpreted morph otectonic features. The results show that the highest helium and radon valu es occur preferentially along elongated features shown by mesostructural an d geomorphological analyses, i,e. anti-Apennine, Apennine and, secondarily, N-S orientations. Furthermore, the application of geostatistical technique s in a testing area has enhanced the semi-quantitative evaluation of this a nisotropic soil-gas distribution (linked to the gas-bearing properties of t he local brittle deformations). The correspondence between soil-gas distrib ution and mesostructural/geomorphological features, as well as the results from the geostatistical analysis, suggest that gas leakage towards the surf ace is controlled by the same structural pattern which created some morphol ogical features. Geostatistical analysis of the geochemical data combined w ith the other geological techniques has been shown to improve the interpret ation of soil-gas results for neotectonic studies in clay basins ic here te ctonic discontinuities have no surface expression.