Gravity anomalies at the Hohi geothermal field, central Kyushu, reflect the density contrast between the pre-Tertiary basement rocks and the Cenozoic sedimentary and volcanic rocks. Recent volcanic rocks tend to distribute around the areas of local gravity high anomalies. Hatchobaru, well known geothermal field, is located in one of such areas. The basement structures analyzed from gravity anomalies can be calssified into two types. One is active and the other is dormant. The active structures are inferred from common characteristics with active faults, linear features of drainage pattern, or water divide, which are formed through recent tectonic movements. The dormant structures have little in common with above mentioned tectonic features suggesting ceased geologic movements. The tectonic movements which have formed basement structure originated in the Tertiary as the displacement of basement analyzed from gravity anomaly is too large to be accounted for by the rate of the Quaternary tectonic movements. Three gravity basins at Kokonoe, Kujuu, and Shonai have mutually common characteristics such as linear boundary in parallel with each other, steep gradient of gravity on boundary, comparable depth, and similar areal extent, suggesting common tectonic origin, although the only Kokonoe basin has been supposed to have close relation with volcanic activities.

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The concept of geothermal energy development using ultralarge-scale heat pipes is briefly described. A simple way to estimate the heat transfer rate to heat pipes placed in upwelling geothermal fluids within geothermal reservoir is presented. First, effective thermodynamic and hydrodynamic parameters for a reservoir are evaluated. Then, similar transformations are locally implemented in order to compute the rate of heat transfer to the heat pipes in the uniform porous medium. Two slightly different transformations were developed locally. Both of them were extensively tested against the complete two-dimensional numerical results: the validity was confirmed. According to the present analyses, the heat transfer rate to an evaporator with 0.1 m in radius and 1 km in length can be as large as 1600 kW, if a 200 K temperature difference between the evaporator and geothermal fluids is maintained over the entire length and the flow velocity is 1 mm/sec.

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The heat extraction through a geothermal reservoir on an oblique fault plane in a hot dry rock is consid ered. The fault is modeled mechanically as a frictional contact interface of two elastic bodies. During the heat extraction, the surface of the reservoir is cooled by the fluid and the thermal contraction of rock occurs. For the purpose of clarifying the behavior of the reservoir and the leakage of fluid through the fault, the analysis is attempted on the basis of the two dimensional theory of quasi-static thermoelasticity. It is concluded from numerical results that a geothermal reservoir can be created on an oblique fault plane and the opened region of the plane increases gradually with time during the heat extreaction. It is also found that the large thermal stresses on the fault plane outside the opened region tend to stop the leakage of fluid through the fault plane.

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Alteration ages have been determined for 30 samples collected mainly from white colored alteration halos and a few green colored ones. The mineral used for thermoluminescence dating is secondary quartz separated from silicified or argillized rocks, and phenocrystic quartz of altered rhyo-dacitic rocks. Obtained ages range from 0.021 to 2.22 Ma with the accuracy of±30%. Hydrothermal activity periodically occurs in separate places. Recognized activities have the ending times of 2.2, 1.5, 1.2, 1.0, 0.5 and 0.3 Ma, respectively. At the Kakkonda area, zonal arrangements of alteration ages are observed from less than 0.1 Ma in the central part of the presently active zone to 0.3 Ma of the marginal zone. This indicates that the hydrothermal activity of the Kakkonda area started about 0.3 Ma and has continued to present time. Overlap of hydrothermal activity is clearly recognized in the Kakkonda and Omatsukurazawa areas. Recent activities occurred at about 0.3 Ma in both areas. Old activity in the Kakkonda area ended about 0.7 Ma whereas that in the Omatsukurazawa area ended about 0.9 Ma. The difference of hydrothermal activity (widely developed conduction dominated type in the Kakkonda area, and locally formed volcano genic fluid supply type in the Omatsukurawa area) may have cause such a gap of ending times.

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A seismic noise survey was carried out from 1982 to 1983 in the Takinoue (Kakkonda) geothermal area, Iwate prefecture. The variation of spectra and amplitudes of seismic noise before and after the closure operation of wellhead valve (“build-up test”) were investigated.
The instruments used for in-situ measurements consisted of three component seismometers, amplifier and tape recorder. The system has flat response to ground velocity in the frequency range of 1 to 40Hz. The data were analyzed in the laboratory with a microcomputer. The original analog data were degitized at 100/sec. Fourier spectra of 2048 selected samples were computed using the fast Fourier transform.
The results are summarised as follows:
1) When the production wells are closed, the amplitudes of seismic noise in the survey points near the geothermal reservoir decrease significantly and the spectral characteristics are not constant.
2) When the production wells are closed, a large decrease of spectral amplitudes in the frequency range of 4 to 12Hz included commonly in the survey points of Base C1 and C2 near the geothermal reservoir is recognized, but that for frequencies higher than 8Hz in the Base 3 and 6, far from the reservoir, is not so large.
3) The depth of origin of seismic noise is estimated to be in the range of 200 to 500m during the operation of production wells.
4) The dominant frequency 4-12Hz did not change after the closure of the production wells. This means that the geothermal seismic noise in the Takinoue area is nature in origin.

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