Construction of Evaluation System for Hydrothermal Resources Using Soil Gas Method

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The system to estimate the condition of hydrothermal fluids in the shallow subsurface has been proposed based on the temporal and spatial variations of radon concentration contained in soil gas. The results obtained are summarized as follows.(1)The observation holes of 1 m depth for the radon measurement were equipped at the three hot spring areas with fumaroles in the western side of the Aso volcano, central Kyushu, southwest Japan. These areas are considered to be located on a north-south striking fault. Using the scintillation counter method, the number of alpha particles attributable to the decay of ^<220> Rn and ^<222> Rn for 20 minutes were counted. These measurement data were processed by the calculation theory for the number of radon at the start of measurement, and the number of ^<222> Rn which has longer half-life than ^<220> Rn and the age of radon gas were obtained. It was found that the temporal variations of those quantities correspond with the activities of volcanic micr … More o-earthquakes in the Aso area.(2)The lineaments around the hot spring areas were automatically extracted using the Segment Tracing Algorithm and the subscene of the SPOT Panchromatic imagery with high spatial resolution. By the combination of these lineaments and the digital elevation model data, the fault which has a role of conduit for the hydrothermal fluids was estimated to be a reverse-fault type and dip 80ﾟ east.(3)A numerical simulation method for the three-dimensional flow analysis of hydrothermal fluids has been constructed using a finite element method. Three rock types, the basement rocks, the Quaternary volcanic roocks, and the fault fracture zone, were assigned to the study area for the simplification of analysis, and the geologic structure was modeled by the distribution of these rock types. Using this model and the simulation method, the changes of temperature and pressure in the shallow subsurface, which may be caused by the change of hydrothermal fluids condition in a deep part, were calculated. This calculation clarified that these physical quantities change sensitively at the site with large convection of hydrothermal fluids, and consequently, the radon concentration varies remarkably. The site at which the temporal changes of radon concentration are related closely to the volcanic earthquakes, therefore, is considered to have high geothermal potential.Based on the above results, an evaluation of hydrothermal resources reguires the three steps : estimation of three-dimensional distribution of continuous fractures through the combination of satellite images and digital elevation model ; detection of a site having distinctive feature in temporal variation of radon concentration ; and execution of numerical simulation for hydrothermal flow, taking the change of radon concentration and the fault distribution into account. Less