Abstract
Toyoha geothermal field including Toyoha Mine of south-western Hokkaido is located in nearly the north end of Nasu volcanic zone. To make clear the subterranean thermal structure of this area, the writers tried following three steps of calculation. The first is the analysis of heat flow to seperate into the various heat sources, second comparison of observed temperature distribution and computed one by a thermal conduction model from large spherical heat source, and the third is calculation of stationary states of subterranean temperature distribution using cylindrical model of hydrothermal reservoir. For the first step, socalled “running mean method” is applied, especially taking the difference of two running mean, analysis of geothermal data (heat flow) in the southwestern part of Hokkaido can effectively be made. Namely, from the surface heat flow analysis, three subterranean heat sources are found with their sizes and ages. (ref. Fig. 3(b)). In the second step, surface temperature distribution at and adjacent Toyoha geothermal field is calculated by the thermal conductivity using the spherical heat sourc model, and this sesult is compared with the observed surface temperature distribution. Though generally speaking, the result of this comparison seems to be good, there appears a remarkable surface temperature anomaly at eastern part of Toyoha Mine. To explain this anomaly, as the third step, the calculation of thermal stationary states is made by using a cylindrical model, from which it becomes clear that the local surface temperature anomaly is due to the upward heat transportation by the liquid or steam. Consequently, the heat flow consists of thermal conduction and heat transportation at Toyoha geothermal field.
