Soil-gas method for tracing neotectonic faults in clay basins: the Pisticci field (Southern Italy)

Abstract

Two soil gas surveys were performed in 1991 and 1993 over an area of 95 km 2 at Pisticci, Southern Italy. The research was aimed at assessing the relationship between soil gas distribution and fracture systems in a Plio-Pleistocene clay basin located in the Bradanic trough. More than 400 samples were collected to measure the He, CO 2 and 222Rn concentration in soil atmosphere. Very large helium anomalies (ΔHe), up to 21 000 ppb above atmospheric reference, were detected in 1991 in correspondence with the youngest fractures reported in the literature. In order to verify the persistence of such soil gas anomalies, a survey was conducted again in 1993 in subareas where major helium anomalies were previously detected. The values of CO 2 and 222Rn have shown about the same average as that in the 1991 survey, while ΔHe values have revealed a sharp decrease with maximum values, not exceeding 3000 ppb. This considerable difference is tentatively related to the seismic activity that occurred just before the 1991 survey, whereas no appreciable seismic activity was reported in 1993. The results of the soil gas surveys show that the shape and spatial distribution of the gas anomalies are related to the structural pattern of the area, whereas the magnitude of gas leakage is controlled by the occurrence of deep gas-bearing traps (hydrocarbon reservoirs) and possibly triggered by seismic activity.