Abstract

Soil gas surveys have been carried out at research sites in Great Britain and Italy to test soil gas geochemistry as a site investigation technique for the detection of faults and discontinuities. At a site on Oxford Clay in Gloucestershire, soil gas anomalies of high He, Rn and CO2 and low O2 were shown to correspond to the outcrop of a fault, identified by drilling and geophysics. Other apparently random anomalies remained unexplained and lateral migration of gas through superficial horizons complicated interpretation. Using three parallel sample lines at the same distance apart as the sample spacing it has proved not only possible statistically to remove spurious anomalies, but also to enhance and concentrate clusters of high values resulting from gas migration.

This method was used to investigate a fault in Neogene clay at Narni in Italy where a gas pathway permeable to Rn and CO2 was identified, corresponding to a geoelectrical anomaly indicating displaced strata. Further work was carried out over a fault revealed by trenching in the Caithness Flags in Scotland. For each three-line traverse an integrated gas anomaly map was produced to aid interpretation by allowing both sharp and diffuse anomalies to be more easily identified.

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