葛根田地熱地域の地熱貯留層が存在すると推定されている区域およびその周辺の3測点で,上下動地震計3台によるトリパタイト観測を実施し,微動の到来方向および見掛け位相速度の解析を行い,地熱微動の発生源および伝播経路について検討を行った。その結果,以下のようなことが明らかになった。1) トリパタイト観測による微動の到来方向は,観測時刻による変動は少なく,安定している。2) 蒸気生産施設近傍では,蒸気生産施設稼働中と生産井バルブ閉止後で到来方向に変化がみられ,生産施設稼働中には生産活動に関連する振動が伝播してきている。3) 3～7Hz程度の微動は貯留層の存在する方向から到来している。しかし,地表における地熱活動の活発な区域近傍では,波はその方向から到来する。4) 微動の見掛け位相速度は,周波数が高くなるにつれて遅くなる傾向,すなわち分散性を示し,当地域の地下速度構造モデルに基づくRayleigh波の理論分散とも対応している。

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Surface Sniff method, SS method, is conducted by settling the thermometer-attached N-packer (N is after the initial letter of inventor's name, Noda and that of cooperative corporation, NIC) on the ground and connecting it to the portable mercury analyzer PM-1A. Thus emitted mercury from the ground into the N-packer is measured, where the standard measurement is fulfilled by 2 times successive measurements for 5 min. at 0.41/min of suction rate to get stable intrinsic mercury concentration reflecting underground environmental condition. The change of air temperature in the N-packer is the undesired and uncontrolled factor giving a large difference in mercury emission, therefore it becomes necessary to correct data of mercury concentration against temperature change in order to compare them to each other. The mercury concentration changes according to air temperature in such a manner as bearing a linear relation between logarithmic mercury concentration and reciprocal air temperature in °K. The equation for the temperature-concentration correction is thus shown as follows, log D₀=log Dobs+a{(1/Tstd)-(1/Tobs)}, where 'D₀' is corrected essential mercury concentration, 'Dobs' observed mercury concentration, 'a' the temperature-concentration coefficient depending on field property, 'Tstd' standard temperature and 'Tobs' observed air temperature in the N-packer during measurement. The distribution map based on the corrected mercury concentration is found to harmonize well with underground geothermal condition in the area. Mechanism of mercury emission at geothermal area has been turned out as following through some experimental measurements in two instinctive geothermal fields, Hachimantai, Tohoku and Kirishima, Kyushu, Japan. There occures gaseous mercury atmosphere near ground surface whose concentration is in accordance basically with underground condition, especially geothermal activity and is recontrolled according to the surrounding air temperature. Such intrinsic concentration of mercury is immediately restored even though mercury above the ground diminishes by environmental effects such as wind-blow etc. The distribution patterns of mercury upward and laterally from a ground spot are diffusion-like, which suggests diffusion mechanism manages transportation of mercury from the ground.

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The mass spectral data of soil-gases obtained through a “PETREX Fingerprint” survey (Sakai, 1986, 1987) in the Okuaizu geothermal area of Fukushima Prefecture were subjected to principal components analysis in order to identify the detected gas components. The soil-gas composition at three of his sampling points was also analyzed by using a gas chromatograph-mass spectrometer to help the identification. On the basis of the results of these analyses, some of the detected gas components were interpreted as follows:(1) Components m/z43, 57, 71 and 85 are due to undecane and possibly other paraffins with carbon numbers greater than 6.(2) Component m/z106 corresponds to molecular ion of xylene and/or ethylbenzene.(3) Component m/z136 corresponds to molecular ion of monoterpene. Among the gas components, components m/z43, 57, 71 and 85 and component m/z106 directly reflect a geothermal structure of the Okuaizu geothermal area as follows:(1) An anomalous zone characterized by high emission of components m/z43, 57, 71 and 85 is found on up flow zones of the hot water system.(2) Anomalous zones characterized by high emission of component m/z106 also seem to be predominant on the same zones. The distribution of component m/z136 does not appear to reflect the geothermal structure. These distributions of gas components m/z 43, 57, 71 and 85 and component m/z106 are explained by the following hypothetical process:(1) Paraffin, xylene and ethylbenzene are generated by thermal cracking of organic materials in rocks of recharge zone. (2) Fluids containing the hydrocarbons are conducted into the geothermal reservoir by geothermal convection. (3) High temperature of the geothermal reservoir causes the hydrocarbons to vaporize. (4) The vaporized hydrocarbons form gas bubbles and ascend toward the surface, causing anomalous hydrocarbon concentration in the soil-gas

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Recently geothermal fluid flows have become to be regarded as mainly controled by fracture permeability rather than formation permeability, especially on a reservoir scale. In order to clarify the role of the fracture system for geothermal fluid flow and reservoir behaviour, the properties of fractures and their associated hydrothermal veins were analyzed on surface outcrops and drill core samples in the Yuzawa-Ogachi geothermal field, Akita, Japan. This field can be divided structurally into the Okumaemori-Oyasudake upheaval belt and the Kijiyama subsidence belt by the Doroyu and associated faults. The former is characterized by high-angle-fractures in NW-SE trend, mid-angle-fractures in N-S trend, and hydrothermal veins in W-E trend, and the latter is characterized by high-angle-fractures in WSW-ENE and WNW-ESE trends, low-angle-fractures, and hydrothermal veins which are oblique to dominant fracture trends. Bore hole temperature logging data have revealed the detailed subsurface thermal structure in this field, and the isothermal contours suggested the coupling of two convection cells around the Doroyu and the Oyasu, and two geothermal heat source regions around the Takamatsu-dake and the Oyasu-dake. Based on these fracture and subsurface isothermal contour data, the deep seated geothermal fluids around the Doroyu can be inferred to ascend obliquely north-northwestward from the geothermal heat source region to geothermal reservoir zones along N-S and NW-SE trending, high-dipping fractures at deeper part, and preserved and/or deposited as hydrothermal veins in W-E trend fractures at shallow part. It is assumed that the essential geothermal fluids obliquely ascend by transferring several fractures rather than passing through a single fracture beneath the reservoir. Whereas around the Oyasu, the Sanzugawa formation consists of impermeable siltstone, which can be inferred to play an important role on fluid flow control as a cap rock to the geothermal reservoirand a lateral geothermal fluid flow. Based on the stress analysis of conjugate faults, the low angle fractures around the Oyasu can be regarded to be formed as the reverse faults under NE-SW trending compressive stress, and other high angle fractures around the Doroyu and the Oyasu can be regarded to be formed as the normal or lateral faults.

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The first experiment for the Downhole Coaxial Heat Exchanger (DCHE) was carried out successfully at the HGP-A well on the Island of Hawaii using an interval from the surface down to a depth of 876.5m as a cooperative project between the Engineering Advancement Association of Japan and the Pacific International Center for High Technology Research (Morita et al., 1992). Analysis of the experimental results was carried out using a simulator developed by Morita and others (Morita et al., 1984; Morita and Matsubayashi, 1986), to investigate the insulation performance of the inner pipe used in the DCHE and the heat transfer characteristics in the formation. Another purpose of this analysis was to confirm the accuracy of the simulator which has been used in this series of studies. The equivalent thermal conductivity of the pipe was estimated to be 0.06W / m · k. This indicates that the performance of the insulated pipes used as the inner pipe is sufficiently high for DCHE application, hence the highly efficient DCHE is feasible. Analysis also indicated that the heat transfer mechanism during the experiment in the main heat extraction interval between 300m in depth and the bottom of the DCHE was almost pure conduction and that the thermal conductivity of the formation in this interval was 1.6W/m·K. The observed heat transfer mechanism is concordant with the physical conditions of the well. The estimated thermal conductivity of the formation is concordant with that of water saturated Hawaiian basalt (Horai, 1991). Also, excellent agreement between measured values and computed values was obtained. This indicates that the simulator used in this series of studies is sufficiently accurate for DCHE application. Hence the results which have been obtained using the simulator in these studies were confirmed to be correct.

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