Reconnaissance of soil gas composition over the buried fault and fracture zone in southern Taiwan

Abstract

The soil-gas method is based on the principle that faults and/or fractures are highly permeable pathways in rock formation where gases can migrate upward from the deep crust and/or mantle and retain their deep-source signatures in the soil cover. This method is adopted because it can give results in short time and at low costs. In this work, soil-gas compositions are measured and synthesized in conjunction with the geological, geophysical and geomorphological infor- mation along the Chaochou Fault, which is considered as an active fault in southern Taiwan. More than 500 soil-gas samples were collected along 18 traverses crossing the observed structures and analyzed for He, CO 2 , CH 4 , O 2 + Ar and N 2 . The results show that both helium and carbon dioxide concentrations in the soil gas have anomalous values at the specific positions in each of the traverses. The trace of these positions coincides with the N-S trending faults and/or fractures, that is, the postulated trend and pattern of the faults in southern Taiwan. Hence, helium and carbon dioxide are useful index gases in this area. Based on the helium and carbon dioxide concentrations of the soil gases, at least three components are required to explain the observed variations. In addition to the atmospheric air component, two gas sources can be recognized. One is the deep crust component, exhibiting high He and CO 2 concentrations, and considered as best indicator for the surface location of fault/fracture zones in the region. The other component could be a shallower gas source with high CO2 concen- tration, and low He concentration. Moreover, helium isotopic compositions of representative samples vary from 0.52 to 1.05 Ra (the 3 He/ 4 He ratio of air), illustrating that most samples have soil air component and may be mixed with some crustal component but no significant input of mantle component. Carbon isotopic composition (δ 13 C) of carbon dioxide in the soil samples vary from -11.8 to -23.4‰, which could be the result of mixing of organic and limestone components. Both helium and carbon isotopic results support the multiple gas sources in studied area. Meanwhile, continuous monitor- ing indicates that soil gas variations at fault zone may be closely related to the local crustal stress and hence, is suitable for further monitoring on fault activities.