利用土壤逸氣調查掩覆斷層及破裂帶之可能分佈：以潮州斷層為例

Abstract

空氣成份與源自於地殼深處的氣體成份截然不同，地表附近深入地殼的斷層或破裂帶，可成為地底深處氣體向上遷移的通道，而使該處地表土壤氣體成份異常。台灣南部潮州斷層為一被地表沖積層覆蓋的活動斷層，本研究沿著幾條明顯穿過構造線的剖面採集土壤氣樣本，進行氦氣、二氧化碳、甲烷、氧氣、氬氣及氮氣等氣體成份的分析，配合鄰近地區已有的地質、地球物理探勘及地形資料，探討潮州斷層之地表分佈。 分析結果顯示土壤氣中氦氣與二氧化碳濃度，在每條剖面的異常值出現處，呈南北向分佈，與已有文獻所報導的潮洲斷層分佈位置吻合。因此在本研究區域內，氦氣與二氧化碳成為指示斷層位置非常有效的氣體。本研究區域的土壤氣體，除了地表之空氣成份以外，還可以辨識出兩個端成份：一為來自深斷裂的氣體，其氦氣異常程度隨著二氧化碳含量增高而增加；另一則源於淺破裂的氣體，其所含之氦氣濃度並未隨著二氧化碳含量升高而有明顯變化，顯示其二氧化碳來自淺處。 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.