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

摘要

Assemblage of main fracture and subordinate fracturesform a fracture zone. Geometry, orientation, andpermeability of fracture zone are significant factors for exploration and assessment of geothermal resources.Because these fracture attributes, which have an effecton the hydrothermal system represented by flowpattern and physical condition of hot fluids, aredifficult 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 threeparts, lineament analysis, numerical simulation, andgeophysical survey. Lineaments are derived fromSPOT panchromatic imagery and used to calculateazimuths and dips of the major fractures associatedwith the origin of hot springs. A numerical simulationusing finite element method is aimed at estimating theconfiguration and permeability of the fracture zone,formed by a gathering of the major fractures. By thiscalculation, the fracture zone was inferred to consist of two prisms that incline at angles of 70°-80° in the opposite direction of the mountain slope, and havepermeability, 102 greater than that of the country,volcanic rocks. Moreover, the radon survey usingscintillation counter method and soil gas near thefumaroles has been conducted to investigate thetemporal changes of radon concentrations at the threesites, located on the two prisms. A difference in thetemporal change patterns among the sites may resultfrom the physical conditions of the hydrothermalfluids ascending through the prisms.