Evidence for radon transport by carrier gas through faulted clays in Italy

Abstract

Extensive soil-gas surveys in sedimentary basins in Italy were performed to study the potential of some naturally occurring gases as indicators for concealed fracture zones, hydrocarbon and geothermal fluids. One conclusive result is a positive correlation between anomalously high values of radon and carbon dioxide in the soil-air over faults. The correlation coefficient for 1173 gas samples is 0.41. Statistically derived contourlines of Rn and CO₂ anomalies show similar locations, shapes and directions. Fairly good Rn−CO₂ coupling evidence appears even on a point-to-point analysis. Furthermore, it was recognized that the highest Rn values are in contrast to the low Ra content of the underlying clayey rocks and that conventional Rn transportation mechanisms seem to be inadequate for the clay sequences. All these facts strongly suggest that Rn is transported from the subsoil, through fault-linked pathways, by carrier gases of which CO₂ could be one of the major components. The theory of geogas microbubbles is a possible explanation of the observed results. The carrier effect of ascending microbubbles can explain both the origin of soil-gas Rn anomaly and the Rn−CO₂ coupling phenomenon.