Geochemical survey of geothermal resources, 2

Exploration of geology and geothermal resources by use of radon in a soil gas was performed in fields. A purpose of this paper is to check the applicability of two methods of estimating radon concentration in the soil gas; one is a measurement of γ-ray exposure rate in the soil, and the other is track-etch method. Both the methods have been used frequently instead of radon meter. Studied areas include two sites without geothermal features where exist faults in the basement and three sites of geothermal areas. Three methods were employed, measurement of γ-ray exposure rate, track-etch method, and direct measurement of radon (Rsub(n), Tsub(n)) by radon meter. Application of the three methods indicates; 1) γ-ray exposure rate is not well correlated with the concentration of (Rsub(n) and Tsub(n)), and 2) α-track density attained by the track-etch method is much more controlled by (Tsub(n)) concentration rather than (Rsub(n)) concentration in the soil gas. Therefore, the radon meter should be used for properly estimating the (Rsub(n)) concentration in the soil gas. The α-track density along some lines crossing a fault reaches to its maximum on the fault, and the distribution pattern has occasionally different density between the two sides of the fault, whereas some patterns has no correlation with the fault. The (Rsub(n)) concentration in the soil gas of the two geothermal areas is lower in the soil above zones producing geothermal fluids than in the surrounding soils. This indicates that the upward migration of the fluid may be disturbed by extraction of the geothermal fluid. (author)