The detection of concealed faults in the Ofanto Basin using the correlation between soil-gas fracture surveys

An integrated geochemical, morphological and structural analysis was applied to a basin filled with clayey sediments in southern Italy (Ofanto Valley) to delineate tectonic features. More than 100 soil-gas samples were collected and analysed for CO2, Rn and He, and the resulting distribution was compared with the location and orientation of field-observed brittle deformations (faults and fractures), and air-photo interpreted morphotectonic features. The results show that the highest helium, radon and CO2 values occur preferentially along elongated zones similar to the most representative trends obtained by geomorphological and mesostructural analyses, i.e. anti-Apennine, Apennine and, secondarily, N-S orientations. Furthermore, the development of geostatistical techniques has allowed the semi-quantitative evaluation of the anisotropic soil-gas distribution. The gas-distribution pattern is considered to result from the combination of the anisotropic distribution of fracture traces and the randomly distributed background field. The correspondence between soil-gas distribution and geomorphological/mesostructural 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 also created some morphological features. This technique has been shown to be a useful tool for neotectonic studies; this is especially true in basins filled with clayey sediments, as soil gas is even able to define the leakage of deep-seated gases along tectonic discontinuities which have no surface expression. (C) 1999 Elsevier Science B.V. All rights reserved.