レーダ画像による豊肥地域のリニアメント解析と土壌ガス法による現地検証

抄録

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.