To avoid silica scaling, three different pH conditions (5.0, 6.5 and 7.3) were examined by passing Sumikawa geothermal fluids (SiO₂ concentration ; 775 mg/L) through columns (50 mmφ, 55 cm in length) packed with alumina beads (1 or 2 mmφ). The flow rate decreased from 5 to 2∼3 L/min with time (within 72 hours) in all pH conditions. Silica was found to deposit mainly around the top of the column (within 20 cm from the top), where Al concentration in the silica deposits increased with time. The total amount of silica deposited during experiments increased drastically with increasing amount of fluid after some induction periods where no significant amount of silica was deposited. In the most acidic condition (pH 5.0), silica deposition was also observed instead of an iron material such as SS400 was corroded. These results indicate that around pH 6.5 is a better condition to avoid silica scaling instead of a small amount of silica is still deposited for fluids with high SiO₂ concentration (>700 mg/L).

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Kuju Volcano in central Kyushu, Japan has an active fumarolic field in the central part. Resistivity measurement was applied in order to detect the steam reservoir beneath the fumarolic field. As the result, an extremely resistive zone higher than 1000Ωm was detected at a depth of 10 to 50 m. A short directional hole (27 m long and 18m deep below the surface) was dug toward the resistive zone in May 1991. We obtained superheated steam from the margin of the resistive zone and then used it for material testing. The temperature of the steam was 233°C at the well head and was estimated to be 278°C at the bottom of the hole. In November 2001, we planned to get much hotter steam in order to conduct another experiment. Then a longer directional hole (47 m long and 30 m deep below the surface) was dug toward the center of the above-mentioned resistive zone. However, we obtained only saturated steam and the temperature at the well head was about 98°C. The saturated steam was discharged for about one month but stopped discharging because of deposition of sulfur inside the pipe. Repeat resistivity measurement was conducted in order to clarify the change in the thermal state in September 2002. The resistivity model showed that the resistivity of the above mentioned resistive zone decreased to several tens Ω m, that is, the resistive zone has disappeared. Such a drastic change would be explained if we assume that the superheated steam reservoir turned to be the liquid dominated reservoir, that is, the change in resistivity means cooling of the reservoir. The cold meteoric groundwater around the fumarolic field may be supplied to the steam reservoir. Phreatic eruptions occurred at about 300 m south of the active fumarolic field in October and December 1995. Geophysical monitoring after the eruption shows quick cooling of the central part of the volcano and also indicates a large amount of meteoric water supply to the central part of the volcano. The change in resistivity observed in this study may be one of the phenomena associated with the 1995 Phreatic eruptions.

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フローレス島マタロコ地域MT-1号井のカッティングスをX線解析した結果,変質帯は浅部からカオリナイト-α-クリストバライト帯（I帯）,ゼオライト帯（II帯）及びカオリナイト帯（III帯）に分帯され,II帯はヒューランダイト帯（IIa帯）,ローモンタイト帯（IIb帯）及びワイラカイト帯（IIc帯）に細分される。ゼオライトの相平衡とカオリナイト結晶度指数から推定される結晶化過程の地下温度は,静水圧沸騰曲線を超えて急上昇し,深度約180mで236℃に達する。この深度が水の2相分離曲線の最大エンタルピー点を表し,下部の蒸気卓越型貯留層と上部の蒸気凝縮帯とを分けていたらしい。蒸気凝縮帯の厚さは一般に360m程度であるが,本地域では岩石圧的被圧によりその半分であった。II帯の鉱物生成が不透水帯をつくり,蒸気卓越型貯留層に過剰な熱供給が続いた結果,岩石圧的被圧帽岩が出現した。現在,この岩石圧的被圧は解消されている。

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In order to evaluate the applicability of sodium fluorescein as geothermal tracer, in addition to the effect of pH on the fluorescence intensity of sodium fluorescein, the time dependence and the effects of dissolved ions on fluorescence intensity were examined. The fluorescence intensity decreased with decreasing pH when the pH of samples was lower than 9. On the other hand, the intensity was showed stable value when the pH was higher than 9. Therefore, for the samples without dissolved particular chemicals, the fluorescein concentration can be certainly measured when the pH of samples is adjusted to over 9 before measured using a fluorescence spectrophotometer. The fluorescein in the transparent and the brown glass bottles were easily decomposed under lights of . fluorescent lamps. On the other hand, the fluorescein in the bottle covered with aluminum foil was hardly decomposed during 1 month. As a result of examining the effects of dissolved ions on the fluorescence intensity, the effects of potassium, calcium, chloride, sulfuric acid or carbonate ion hardly could be observed. On aluminum ion, it was proven that the fluorescence intensity was not almost affected in the aluminum ion concentration of the degree that is also included for general geothermal brine (about 1 mg/l). Though the magnesium ion affects fluorescence intensity, adjusting pH over 12 could ease the effect. As the ferrous ion concentration is high, the accurate measurement becomes very much the difficulty since the fluorescence intensity seems to lower by the adsorption of the fluorescein on hydroxide as similar form of colloidal.Keywords : sodium fluorescein, fluorescent tracer, fluorescence intensity, peak wavelength, pH effect, salt effect

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