KANEKO Katsuhiko Hokkaido Univ., Graduate School of Eng., Professor, 大学院・工学研究科, 教授 (20128268)
OHMI Michito Kumamoto Univ., Faculty of Eng., Professor, 工学部, 教授 (30040405)
|Budget Amount *help
¥6,500,000 (Direct Cost : ¥6,500,000)
Fiscal Year 1998 : ¥1,200,000 (Direct Cost : ¥1,200,000)
Fiscal Year 1997 : ¥5,300,000 (Direct Cost : ¥5,300,000)
In order to measure the radon concentration in soil gas continuously and stably, we have adopted the ionization chamber method and used the equipment, Alpha GUARD (Genitron Inc.), for the measurement By this equipment, the several kinds of data such as ^<222>Rn concentration (Bq/m^3), temperature, atmospheric pressure, humidity, and data quality can be obtained at time interval of 10 minutes. A hot spring area with the active fumaroles, situated in the western side of Mt. Aso and on the NNW-SSE trending fault, was chosen as the study site. The soil gas was inhaled at the rate of 0.5 l/mm from the 1-m depth observation hole. We paid special attention to rem'oving water drops from the inhaled gas and maintaining the temperature and the humidity around the Alpha GUARD almost constant.
By analyzing the 24,419 measurement data which have been taken from August 11, 1998, it was clarified that the correlation between the radon concentration and the weather factors such as the temperature, the atmospheric pressure, and the humidity were generally low and the radon concentration changed largely within the range of 0.4*10^5 Bq/m^3 in a day. But the average of radon concentrations in a day shows the temporal change that can be approximated by a smooth curve. This can be considered as trend component.
As the result of spectral analysis using the maximum entropy method, the changes of radon concentrations were revealed to contain predominant period of 24 hours and weak period of 12 hours. These periods correspond with those of two major elements in the earth tide. Furthermore, the radon concentrations were found to increase largely on the periods of high volcanic seismicity. It can be concluded that the disturbance of subsurface temperature and pressure resulted from the earthquakes affects the trend of radon concentrations and the earth tide induces the concentration changes with short periods superimposed on the trend.