Numerical Model of Takenoyu-Sugawara Geothermal Area and an Interpretation of a Low Resistivity Zone with KYU88-1, the Simulator for Thermal Water Flow

Abstract

A three-dimensional steady state liquid phase simulator for hydrothermal systems was developed. It is called KYU88-1. In this simulator, thermal water flow through a porous medium is assumed to obey Darcy's law. The governing equations are approximated by the finite difference equations and calculated by the successive overrelaxation method. Using this simulator, a base model of a real field (the Takenoyu-Sugawara geothermal area in the middle Kyushu, Japan) is constructed. From this model, it is inferred that the hydrothermal system of this area extends to more than -3000 m above sea level and the Takenoyu fault, along which Takenoyu and Hagenoyu hot springs appear, has high permeability at the shallower part but not so high permeability at the deeper part. Moreover, a low resistivity zone near the well DW-7 detected by ELF-MT surveys is explained by this model, and the low resistivity zone is regarded as not a well fractured high permeable zone but a low permeable alteration zone which acts as a barrier to the hot water flow from eastern side.