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 applie d to a basin filled with clayey sediments in southern Italy (Ofanto Valley) to delineate tectonic features. More than 100 soil-gas samples were collec ted and analysed for CO2, Rn and He, and the resulting distribution was com pared with the location and orientation of field-observed brittle deformati ons (faults and fractures), and air-photo interpreted morphotectonic featur es. The results show that the highest helium, radon and CO2 values occur pr eferentially along elongated zones similar to the most representative trend s obtained by geomorphological and mesostructural analyses, i.e. anti-Apenn ine, Apennine and, secondarily, N-S orientations. Furthermore, the developm ent of geostatistical techniques has allowed the semi-quantitative evaluati on of the anisotropic soil-gas distribution. The gas-distribution pattern i s considered to result from the combination of the anisotropic distribution of fracture traces and the randomly distributed background field. The corr espondence between soil-gas distribution and geomorphological/mesostructura l features, as well as the results from the geostatistical analysis, sugges t 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 d efine the leakage of deep-seated gases along tectonic discontinuities which have no surface expression. (C) 1999 Elsevier Science B.V. All rights rese rved.