Los Alamos National Laboratory conceived the idea of the development of Hot Dry Rock geothermal energy in 1970 and they got hot water of about 150°C from a depth of around 2600 m in 1977 and 1978. This result demonstrates the technical feasibilities of HDR system so that they are constructing deeper and hotter heat-extraction loop called Phase II system to demonstrate the economic feasibilites. Japan and West Germany have participated in this program under the IEA agreement. This report outlines the activity and the problem to create Phase II system including hydraulic fracturing.

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A lineament analysis at the Hohi geothermal area, northern Kyushu, was made using the synthetic aperture radar (SAR) images. Several lineaments previously not defined were interpreted from the images. Of them, lineament which starts from Toyooka at the eastern coast of northern Kyushu to the southwestern portion of Miyanoharu has NE-SW trend and is the urgent geological concern in view of geothermal exploration. It obliquely crosses the geothermal field near Miyanoharu where intensive exploration is underway. We name the lineament “Toyooka-Miyanoharu Line” and discuss the geological implication on the basis of the verification survey around the Miyanoharu area. Due to poor outcrop at the geothermal field the cross survey was limitedly done by the soil gas survey at several survey lines along the lineament. Survey lines L-1 to L-7 were set traversing the lineament. Also survey lines of A-1 to A-4 crossing known active faults were chosen in order to correlate results of measurement at the lineament with these at active faults. Obvious increases of Radon and CO₂ concentration were observed on some points both the survey lines at the T-M Line and active faults. Result of the soil gas analysis using gaschromatograph suggests that the soil gas is not supplied from a deep underground to the surface through openings of fractures. However, anomalously high Radon and CO₂ concentration on the portion of traces of both the active faults and the T-M Line suggests the existence of mechanism which enables the concentration of radioactive material and biochemical activation. Taking into consideration of other geological data, we came to the conclusion that the T-M Line may consist of subtle fractures which reflect a significant fault system developed in the underlying pre-Tertiary rocks.

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