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 CO 2 and low 02 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 CO 2 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.
Two shallow gas-injection tests were carried out in a minor fault within the Siena Graben, Italy. Helium and carbon dioxide were injected 20 m below the ground surface through an inclined borehole intercepting the fault plane, into a sandy aquifer overlain by a clay formation. The gas-injection pressure was above hydrostatic plus capillary pressure, but below lithostatic pressure. Soil-gas, soil-exhalation flux and groundwater analyses were carried out to monitor gas breakthrough and behaviour. The experiment simulated gas rising naturally up a fault, where most of the geological boundary conditions were known. Despite its very low permeability to groundwater flow, the clay formation did not act as a complete barrier to gas rising via the fault. The gas velocity and migration pattern (namely gas channelling along the fault) were consistent with theoretical models of the migration of gas columns and gas slugs. Gas–water partitioning and the evolution of soil-gas and groundwater anomalies clearly reflected the conservative behaviour of He and the partial dissolution of CO 2 into the aquifer. A general model of gas migration and behaviour is suggested which may have implications in natural resource exploration, geological storage of toxic or nuclear waste, and tectonic studies.
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