滝ノ上(葛根田)地熱地域の地質は大きく4つに区分でき,それらは中新統雫石層群,葛根田層群,玉川溶結凝灰岩類および新期火山噴出物である。この地域の地熱流体は雫石層群の断層や褶曲に伴う断裂に胚胎している。褶曲形態や付随する断裂の解析を行うと,背斜部分はbendingとflexural flowの組合せによる機構によって特徴づけられ,向斜部分はflexural slipを伴ったbucklingによって形成されたことが判明した。これら相反する褶曲形成機構は先第三系基盤の傾動運動と東西圧縮応力の組合せによって説明できる。このような地下構造の場合,どのような場所に断裂ができやすいかを,地表の構造解析によって推定し,地熱坑井における掘さく時の逸泥の検討,産出指数の計算およびトレーサー試験等によって確認した。その結果,葛根田断層や北西―南東あるいは北東―南西方向の断層のような大きな断層,向斜構造のtroughおよび背斜を除いた部分での泥質岩と他の岩石との境界地層面で浸透率が高いことが判明した。

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Out of geothermal fields, mercury in a gaseous form uprises in proportion to the underground geothermal activity. Such volatile mercury is captured at a number of survey points scattered systematically on the ground surface and following quantitative mercury measurements provide its horizontal distribution map, which could be an effective method for geothermal prospecting. In order to capture volatile mercury selectively, a gold needle, 1 mm in diameter 10 cm in length, is hung in a hole 1 m deep in the ground and is left for several successive days, and then is collected for determination. Mercury adsorbed on the gold needle is reliesed by heating and determined using an atomic absorption spectrophotometer whose detection limit is satisfactorily low, 0.1 ng. The rate of mercury adsorption more than 1 ng/day indicates the existence of promising geothermal resources beneath. In the process of estimating the degree of underground geothermal acitvity, it becomes necessary to know not only the absolute amount of adsorbed mercury but its distribution patterns such as localization, extension and direction. Several survey attempts by this method have successfully been accomplished in some geothermal and non-geothermal fields in Japan. It is noteworthy that this prospecting method is applicable even in less active geothermal areas without any visual geothermal manifestations. According to some laboratory experiments, momentary slight mercury adsorption precedes regular adsorption process. Regular adsorption of volatile mercury to a gold needle occures in accordance with the first order reaction in which the adsorbed amount increases in proportion to the surrounding concentration and/or the lapse of standing time.

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The homogenization temperature of fluid inclusions in hydrothermal minerals from the Hatchobaru and the Kirishima geothermal fields in Kyushu has been measured. As a result it has revealed that the homo genization temperatures of the inclusions formed at reservoirs in the very active Hatchobaru geothermal field concentrate on a temperature near the boiling point curve of water, and agree well with the present reservoir temperature. Moreover, the geothermal fluid in the field seems to consist of the saturated or nearly saturated water, so that the pressure correction for the homogenization temperatue is unnecessary. On the other hand, in the less active Kirishima geothermal field the inclusions have a wide range of the homogenization temperature. the minimum values of which are quite close to the present underground temperature. This may indicate that the geothermal field has cooled down to some extent, forming the fluid inclusions at various stages of the cooling process. Some consideration has revealed that even the secondary inclusion has a homogenization temperature that is the same as that of the fluid at the time of necking down or refilling, if it traps only the saturated water. According, both primary and secondary inclusions are useful for knowing the thermal structure and the thermal history of geothermal fields.

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