T. Higuchi et al., TIME-SERIES ANALYSIS OF GROUNDWATER RADON USING STOCHASTIC DIFFERENTIAL-EQUATIONS, Journal of Physics of the Earth, 43(2), 1995, pp. 117-130
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.