ABSTRACT

Subsurface gases have been surveyed in the fault zones around the Median Tectonic Line in western Shikoku Island. Thirty-two bubble gas samples have been collected for this purpose, and their chemical compositions have been determined gas chromatographically. The He/Ar, Ne/Ar, N₂/Ar and CH₄/Ar ratios of the bubble gas samples suggest that substantial separation of He from Ne has been revealed to interfere seriously with the He/Ar determination using carrier gas of oxygen when the He/Ne ratio of sample gas is as low as in the atmospheric air. Some experimental conditions to reduce the interference have been proposed. The He/Ar, Ne/Ar, N₂/Ar and CH₄/Ar ratios of the bubble gas samples suggest that substantial amounts of He and CH₄ produced in subsurface rocks are added to groundwaters and that almost all Ne, Ar and N₂ of subsurface gases are of the atmospheric origin. Concentrations of major and minor gas components of the bubble gases are considerably variable reflecting the types of subsurface rocks. CO₂-rich bubble gases with relatively high CH₄/Ar ratios are commonly observed in the Sambagawa schist zone, and their He/Ar and CH₄/Ar ratios are highly fluctuating according to unsteady discharges of CO₂-rich gases containing He and CH₄ from underground. N₂-rich bubble gases with relatively low CH₄/Ar ratios are predominant in the Ryoke granite zone. Some of their He/Ar ratios are fifty to ninety times as high as the atmospheric ratio. The intimate relationship between the bubble gas compositions and the types of subsurface rocks may encourage seismo-geochemical studies of subsurface gases and may suggest that CH₄/Ar as well as He/Ar is a possible seismo-geochemical parameter in this region.