Fault detection using soil gas geochemistry

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 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.

Soil-Gas Geochemistry: Significance and Application in Geological Prospectings

“…Gas geochemistry has been proven to be a reliable and simple technique to apply, at different scales, to many geological scenarios (Annunziatellis et al, 2003;Lewicki et al, 2003;Baubron et al, 2002;De Gregorio et al, 2002;Ciotoli et al, 1998;Ciotoli et al, 1999;Lombardi et al, 1996;Hickman et al, 1995;Duddridge et al, 1991;Durrance and Gregory, 1988;Eremeev et al, 1973). The importance of fluid geochemistry is rooted in the fact that the Earth is an open system and that fluid-releasing crustal phenomena are the major means for the exchange of matter and energy at different depths.…”

“…Soil gas geochemistry involves the study of many gaseous species (radiogenic, trace and diagenetic gases); each of them can give specific information on the conditions that allow their formation, accumulation and/or migration. Field data can show the usefulness of the soil gas method for detecting, for instance, crustal discontinuities even when faults are buried or cut non-cohesive clastic rocks which makes surface recognition difficult using traditional field methods (Ciotoli et al, 1998;Lombardi et al, 1996;Duddridge et al, 1991;Durrance & Gregory, 1988). These characteristics as well as the rapidity and the low cost of the soil gas survey, make this method a powerful tool for geological investigation which can significantly contribute to hazard assessment and forecasting, especially when continuous monitoring is performed (Klusman, 1993;Reimer, 1990;King et al, 1996;Sugisaki, 1983).…”

Effects of gas-water partitioning, stripping and channelling processes on radon and helium gas distribution in fault areas.

“…Geochemical analyses of gases in ground and groundwater are increasingly recognised as a fundamental task for geological and environmen tal characterisation, including natural resource prospecting (McCarthy and Reimer, 1986;Klusman, 1993), seismic forecasting (Notsu et al, 1991;Bella et al, 1995;Igarashi et al, 1995), waste repository assessment (Gascoyne and Wuschke, 1990;Lombardi et al, 1996) and iden tification of buried faults (Duddridge et al, 1991;King et al, 1996). In these studies the determina tion of tracer gases in a phase of the system in vestigated is rarely accompanied by a robust analysis of partition phenomena or gas exchange between phases occurring in the system, especially when fluid approaches the surface.…”

Preferential flow paths in a complex coal seam system

“…The mechanisms of gas diffusion and groundwater flow transporting dissolved gases are likely to contribute a very slow gas transport, which cannot explain rapid methane and radon transport along faults zones (Duddridge et al, 1991). For instance, radon is an ideal gas tracer to investigate the geological process because of its short half-life (decay time) of 3.82 days.…”

“…As a non-invasive ground surface investigation technique, soil gas concentration mapping has been widely applied to detect buried faults and fracture zones (Fridman, 1990, Duddridge et al, 1991, Gascoyne et al, 1993, Fountain and Jacobi, 2000. High soil gas anomalies were shown to correspond to open faults and fractures which form permeable pathways for the gases (Fridman, 1990).…”