Thermal Condition of the Geothermal Field around Mt. Waita in Central Kyushu, Japan (1) Distributions of Subsurface Temperature and Heat Discharge

Abstract

A wide geothermal field, consisting of six basins, around Mt. Waita in central Kyushu contains a large number of steaming grounds and hotsprings. Two geothermal power plants are also located in this area. Since FY 1978, a national project was started to survey deep geothermal resources in this field. As a part of the project, 81 bore holes (80m-depth) were drilled. The mean heat conductivity of the mostly andesitic cores from these bore holes is 3.90×10^-3 cal sec^-1 cm^-1 deg^-1. The distributions of subsurface temperature and heat flow observed in these bore holes indicate five anomalous areas with high heat flow. Formulae providing the standard temperature at 20m-, 50m- and 80m-depth with respect to the height above the mean sea level were developed using the temperature data obtained from 39 bore holes located in the normal-heat-flow areas. Geothermal gradient in lowland areas is larger than in the highlands around Mt. Waita. In order to discuss the geothermal condition of the field, the heat discharges from the six basins are estimated. Their total is about 143, 500 kcal sec^-1, of which 5, 030 kcal sec^-1 by conduction, 9, 690 kcal sec^-1 by hotsprings, 27, 550 kcal sec^-1 by fumaroles and the remaining 101, 200 kcal sec^-1 by steaming wells including the two power plants. A half of the total water discharge (1, 980 thr^-1) is attributed to hotsprings. Comparing the convective heat discharge from the field, except Kuju-Iwoyama and the two power plants, with the two nearby geothermal fields, Beppu and Yufuin, the field has 22% of the total discharge of Beppu and is about 1.7 times larger than Yufuin.