地熱地帯の地球物理学的構造

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Natural steam was first utilized practically in 1913 in Tuscany, Italy, as a source of thermal energy for electric power, and today steam from numerous bores is said to be producing more than 350, 000 kilowatts of power. Recently, since 1949, scientists and engineers in New Zealand have also studied very carefully the factors involved in the utilization of geothermal steam for power and they succeeded in developing power from geothermal steam in 1955. At Wairakei in New Zealand, drilling of production boreholes has now reached a stage where sufficient steam is available to generate approximately 150, 000 kilowatts (Smith, 1958). Iceland, the U. S. S. R., Mexico and the United States are also planning the exploitation of steam energy, and some of them have already succeeded in preliminary experiments. In Japan, the volcanic zones extend both along and across the islands. This country has many active and dormant volcanoes, fumaroles and hot springs, consequently people have utilized the heat of fumaroles and springs for medical and therapeutic purposes. In 1947, a geothermal advisory committee was set up in the M. I. T. I. (Ministry of International Trade and Industry), and it made a plan to obtain basic scientific data relating to the evolution of geothermal energy. In response to this proposal, scientific investigation was begun by the staff of the Geological Survey of Japan, and they succeeded in producing some power in Kyushu. It was only an examination and various experiments were carried out by this pilot plant. Since that time, test surveys have been continued covering many geothermal areas including Kyushu, Tohoku districts and Hokkaido in Japan (Studt 1957 ; Grange 1955 ; Mon. Rep. Geol. Surv., Japan 1955). Recently, we have been carrying on geophysical prospectings on two areas in Onikobe and Matsukawa, both situated in Tthoku distinct, and we are approaching to the success. Besides this, the Geothermal Society of Japan has been set up and many scientists have been able to discuss geothermal problems easily. Therefore, the writer believes this is a good time to reconsider the method of study and to present his ideas on this problem. The first part of this paper is devoted to the consideration on the geothermal source (Fig. 1) and the geophysical studies of some active volconoes (Fig. 2). The middle part deals mostly with the transportation of heat associated with steam and, secondarily, with the conduction and convection of it through the reservoir and fissured formations. This is followed by discussions on the method of exploration, applying geophysical prospecting. However unsolved the problem of the origin of heat may be, we could imagine the existence of magma or magma as the geothermal source of heat. After once the magma forms, the heat of this melt would be gradually infiltrating and penetrating the surrounding country rocks ; consequently, this melt would begin cooling during solidification. According to the writers calculation, the temperature decrease of magma takes 104 or 105 years by assuming a spherical model as the heat source (Fig. 3). Needless to say in heat conduction, , the time elapsed is proportional to the squares of distances. Therefore, if we assume the radius of this sphere as three times of the case in Fig. 3, the time scale becomes ten times the initial states. (for example, 10, 000 years→100, 000 years.) Therefore, the result does not contradict that of average life of a volcano.
社団法人東京地学協会の論文

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早川正巳
地質調査所

地熱地帯の地球物理学的構造

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