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Recognition of buried fault and/or fracture by soil gas method: an example of the Chaochou Fault
soil gas,active fault,Chaochou Fault,helium,carbon dioxide,Taiwan,
|Publication Year :||2006|
氦同位素 (0.52 ~ 1.05 Ra) 顯示，本研究大部分樣本主要成份為空氣，部分可能有地殼氣體成份混合，但無明顯的地函來源。二氧化碳之碳同位素值介於–11.8 ~ –23.4 ‰之間，顯示有機物質與石灰岩混合的結果。氦同位素和碳同位素表示本研究區域的確有多種的氣體來源。由連續的觀測結果亦發現，斷層帶土壤氣體成份的變化，可能與當地的地殼應力變化有關；因此非常適合日後進行斷層活動的監測。
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. In this work, soil-gas compositions are measured and synthesized in conjunction with the geological, geophysical and geomorphological information along the Chaochou Fault, which is considered as an active fault in southern Taiwan.
Soil-gas samples were collected along several traverses crossing the observed structures and analyzed for He, CO2, CH4, O2 + Ar and N2. 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 CO2 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 concentration and low He concentration. Moreover, helium isotopic compositions of representative samples vary from 0.52 to 1.05 Ra (the 3He/4He 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 (δ13C) of carbon dioxide in the soil samples vary from –11.8 to –23.4 ‰, which could be the result of mixing between organic and limestone components. Both helium and carbon isotopic results support the multiple gas sources in studied area. Meanwhile, continuous monitoring 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 activity.
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