TIME-SERIES ANALYSIS OF GROUNDWATER RADON USING STOCHASTIC DIFFERENTIAL-EQUATIONS

This paper provides a new approach to detect changes in the groundwate r radon concentration related to an earthquake. We express changes in radon concentration in a radon-detection chamber by using stochastic l inear differential equations. These equations are represented by the s tate space notation, and then its solution is replaced by an estimatio n of the state vector at discrete points in time with an assumption th at the coefficients describing the stochastic differential equations a re constant for a sufficiently small time interval. Since the solubili ty of radon in water depends strongly on temperature, the separation o f radon from liquid water, which is necessary for radon detection, cau ses fluctuations in the observed radon concentrations due to water tem perature changes in the chamber. We applied our procedure to some actu al data sets on groundwater radon concentration with those on simultan eously observed water temperature, and found that the temperature effe cts on the fluctuations in the observed radon concentration can be sat isfactorily described by our procedure. Furthermore, we were able to e stimate the original radon concentration in groundwater before it was introduced into the radon-detection chamber, which was not affected by water temperature changes. The obtained original radon concentrations are very stable during normal periods, and anomalous changes associat ed with earthquakes were easily detected. Our new method will be very useful to examine time-variation patterns of changes in groundwater ra don and will provide important information about the mechanism of rado n changes related to earthquakes.