FRACTURE ZONE CHARACTERIZATION IN GEOTHERMAL FIELD USING SATELLITE IMAGE, FLUID FLOW ANALYSIS, AND RADON PROSPECTING: A CASE STUDY OF THE ASO CALDERA, SOUTHWEST JAPAN

摘要

Assemblage of main fracture and subordinate fractures form a fracture zone. Geometry, orientation, and permeability of fracture zone are significant factors for exploration and assessment of geothermal resources. Because these fracture attributes, which have an effect on the hydrothermal system represented by flow pattern and physical condition of hot fluids, are difficult to be clarified on a field scale, a combination of several methods is required for the fracture-zone characterization. This paper presents a case study of this characterization on the Aso caldera in southwestern Japan. The study consists of the three parts, lineament analysis, numerical simulation, and geophysical survey. Lineaments are derived from SPOT panchromatic imagery and used to calculate azimuths and dips of the major fractures associated with the origin of hot springs. A numerical simulation using finite element method is aimed at estimating the configuration and permeability of the fracture zone, formed by a gathering of the major fractures. By this calculation, the fracture zone was inferred to consist of two prisms that incline at angles of 70 deg -80 deg in the opposite direction of the mountain slope, and have permeability, 102 greater than that of the country, volcanic rocks. Moreover, the radon survey using scintillation counter method and soil gas near the fumaroles has been conducted to investigate the temporal changes of radon concentrations at the three sites, located on the two prisms. A difference in the temporal change patterns among the sites may result from the physical conditions of the hydrothermal fluids ascending through the prisms.