Imprint of Diurnal and Semidiurnal Cyclicity in Radon Time Series of MPGO, Ghuttu Garhwal Himalaya: Evidence Based on Singular Spectrum Analysis

Radon-222 is considered an important earthquake precursor because it is a noble gas and for its radioactive decay characteristics. However, radon anomalies are controlled not only by seismic activity, but also by environmental and other related natural phenomena such as atmospheric pressure, atmospheric temperature, and Earth tides. These effects must be assessed and eliminated from the radon signal to enhance its capability as a precursor. To identify these effects and their modes of variability in radon progeny time series, we have applied singular spectrum analysis (SSA). We analyzed hourly measurements of radon progeny for a 6-month period acquired from a 10-m-deep borehole at the Multi-Parametric Geophysical Observatory at Ghuttu, Garhwal Himalaya. The site also has a superconducting gravimeter for acquiring continuous gravity suitable to detect tidal waves with high accuracy. Our results show clear evidence of diurnal and semidiurnal daily periodicities, along with slight signatures of the M2–O1 waves in the radon time signal. Results show that even at 10 m depth these effects play a part in controlling radon release from the soil.