Temperature gradients were obtained from temperature logging of drillholes for geothermal explorations. These data were compiled with the published temperature gradients for terrestrial heat flow measurements to make a temperature gradient map of the Japanese Islands. Since a temperature-depth profile obtained from a drillhole near a volcanic zone is often distorted by fluid convections, it is difficult to estimate the conductive heat transfer. This complicated nature of heat transfer requires detailed heat flow studies and sufficient coverage over a volcanic zone. The proposed map is based on direct evidences of many drillhole measurements in and around the volcanic zones ranging from several hundreds meters to 3000 meters in depth. Consequently, the map can represent shallow reliable geothermal structures of volcanic zones. A significant feature of the map is a clear difference between backarc (2∼10°C/100 m) and forearc sides (1∼3°C/100 m). Temperature gradient of the Japan Sea side is predicted to be 3∼4°C/100 m from a number of data sets. The spatial variation of the Curie depths are in remarkable agreement with the temperature gradient map.
Heat transfer between fluid flowing in a slit and particles packed in it and that between the fluid and the slit wall are important for the extraction of heat from the hot dry rock. Mass transfer coefficients, instead of the heat transfer coefficients, on the packed particles in the slit and on the slit wall were measured by making use of the electrochemical method. The result indicates that the mass transfer coefficient on the particles increases with decreasing void fraction and approaches to that in the usual packed bed. The mass transfer coefficient at the wall increases also with decreasing void fraction. New empirical equations for the mass transfer coefficients on the packed particles in the slit and on the slit wall are presented.
A neo-granitic pluton which is petrographically composed of tonalite and quartz diorite has been found at 2.2 km-2.8 km depth at the Kakkonda geothermal field, Iwate Prefecture, Japan. Pre-Tertiary, Miocene strata and Pliocene dykes of tonalite and dacite were thermally metamorphosed by the granitic pluton. The youngest metamorphic protolith is 4.9 ± 1.0 Ma in age. The width of metamorphosed area is over 2.0 x 2.5 km.The metamorphic minerals are green or brown biotite, muscovite, cordierite, anthophyllite, andalusite, hastingsite, cummingtonite, tremolite, pyrrhotite and magnetite. The biotite and cordierite isograds envelope the granitic pluton above ca.1.0 km and 0.7 km respectively, which sink toward the southwest in 15-30 degrees. The roof of the granitic pluton is inferred to be parallel with the surface of the mineral isograds. This estimation was proven by two deep geothermal wells.The productive fractures exist at the margin of the granitic pluton and its neighboring pre-Tertiary formation. The deep and high temperature (300-350°C≤) geothermal fluid, which is slightly acid water (pH 3.5-4.5), exists in these fractures. The depth of the roof of the granitic pluton can be estimated from the isograds of biotite and cordierite.
A new system has been developed for accurately measuring the longitudinal wave velocity in rock specimens under simulated hydrothermal conditions. The pulse transmission method was adopted for the velocity measurement. Error in the velocity measurement is less than 1.5%. The pressurized hot water condition was realized by using an autoclave. High temperature piezoelectric transducers were used for the measurements. Travel time of the longitudinal wave was carefully calibrated in the range of 5-250°C and 0.1-20 MPa. Measurements were made in the temperature range between 10 and 240°C, under confining pressure of 15 MPa. The relationship between velocity and temperature, which is essential for interpreting seismic surveys in the geothermal area, was investigated in six kinds of rocks. In general, velocities in all specimens gradually decrease with increasing temperature. Hysteresis in velocity between heating and cooling processes was recognized in four kinds of rock specimens; especially, those in marble and sandstone specimens were very remarkable. On the other hand, no hysteresis was observed for tuff and andesite specimens. The mechanism of velocity hysteresis in marble and granite specimens was investigated quantitatively. We conclude that the hysteresis in crystalline rocks has resulted from irreversible process of generation of microcracks due to the thermal expansion of constituent minerals.
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