地熱地域におけるγ 線スペクトル探査

抄録

We carried out a gamma-ray spectra survey to discuss the relation between geothermal manifestations and gamma-ray intensities for three radioactive elements, ¹⁰K, ²¹⁴Bi and ²⁰⁸Tl, in the Komatsu geothermal area, near Hatchoubaru geothermal power station in Oita prefecture, where a typical hydrothermal fractured type reservoir was developed. A data analysis method, based on Covell (1959) method, including the error estimation of observed data was used in this study Using detection limits of gamma-ray from each element, based on Currie (1966) method, was also proposed in this data processing. The results of field survey showed high gamma-ray intensity areas for Bi well correspond to high lm-depth temperature areas. This means that ²²²Ra, the parent element of Bi, comes up to the surface together with circulating thermal water. High intensity areas for Tl correspond to the locations of fumaroles distributed around high lm-depth temperature areas. This fact is interpreted that Tl is accumulated in places where the thermal water comes up to the surface rapidly, because ²²²Rn, the parent element of Tl, has a very short half-life. We can conclude the high gamma-ray intensities of Bi and Tl to be potential indicators for the existence of upflows of thermal water.