Probable younger date of the southward overthrust of the Palaeozoic Chichibu rocks upon the Mesozoic Hidaka than the Eocene Nakaoku Bed is an interpretation of the writer for various geological accounts given by I. Shiida in his work “Stratigraphical and Geotectonical Studies of the Paleozoic Chichibu and Mesozoic Hidaka Terrains in the Central Part of Kii Mountainland Southern Kinki, Japan”, 1961, with special reference to the Omine lobe of the overthrust-sheet.
Exploration of geothermal field should be regarded experimental because the condition of geothermal fields varies so greatly from one field to another. Practically geothermal condition in Italy differs greatly from New Zealand's geothermal fields. Nevertheless, the resemblance of geothermal structure between geothermal fields may be pointed out. Such a resemblance is basement fault which acts as the path of geothermal energy, geothermal reservoir and sometimes cap rock of reservoir. Nature of geothermal fluids depends upon the characteristics of original fluids and geothermal structures. To clarify these geothermal conditions, the following should be explored : nature of surface heat output, temperature distribution near the surface, hydrothermal alterations of rocks, geological structure, physical properties of rocks, underground water, magmatic mass, . heat output from drillhole, pressure and temperature distribution, chemistry of the discharge. These exploration should be carried out systematically in accordance with regional survey, . preliminary reconnaissance, comprehensive survey and proving. Methods and techniques used in these surveys are geological, geophysical and geochemical works, land surveying and drilling. If we could develop our knowledge regarding geothermal features and techniques of exploration in geothermal fields, it would be possible to obtain more information concerning underground geothermal condition and consequently to develop the valuable resources of geothermal energy.
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 writer's 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.
Geothermal areas in Japan are distributed along Quaternary volcanic zones. Fumaroles and hotsprings in these areas issue from younger volcanic rocks and origin of them seems to be related to Quaternary volcanic activities, though a few jet from Tertiary granitic rocks Thermal activities in these areas are divided into two types : One is caused by emanation of shallow magmas of active volcanoes like as Atosanupuri and Showa-shinzan in Hokkaido and Iwojima in Kyushu which are characterized by high temperature and strongly acidic gases, the other is originated by consolidation of deeper magmas. One of characteristic appearances of geothermal areas in Japan is that surface thermal indications are poor. But, according to investigations made in some areas, it has become clear that geothermal products are reserved in porous beds and permeable hard rocks of younger volcanics and their basements. For example, hot water occurs from Tertiary formation covered by younger andesite lava flow in spite of lacking remarkable surface indications, in Matsukawa area, Iwate Prefecture. Occurrences in other areas may be analogous to that of Matsukawa. To know more clearly subsurface structures in geothermal areas, investigations by drilling are necessary.