Seasonal emanation of radon at Ghuttu, northwest Himalaya: Differentiation of atmospheric temperature and pressure influences.

Continuous monitoring of radon along with meteorological parameters has been carried out in a seismically active area of Garhwal region, northwest Himalaya, within the frame work of earthquake precursory research. Radon measurements are carried out by using a gamma ray detector installed in the air column at a depth of 10m in a 68m deep borehole. The analysis of long time series for 2006-2012 shows strong seasonal variability masked by diurnal and multi-day variations. Isolation of a seasonal cycle by minimising short-time by 31 day running average shows a strong seasonal variation with unambiguous dependence on atmospheric temperature and pressure. The seasonal characteristics of radon concentrations are positively correlated to atmospheric temperature (R=0.95) and negatively correlated to atmospheric pressure (R=-0.82). The temperature and pressure variation in their annual progressions are negatively correlated. The calculations of partial correlation coefficient permit us to conclude that atmospheric temperature plays a dominant role in controlling the variability of radon in borehole, 71% of the variability in radon arises from the variation in atmospheric temperature and about 6% of the variability is contributed by atmospheric pressure. The influence of pressure variations in an annual cycle appears to be a pseudo-effect, resulting from the negative correlation between temperature and pressure variations. Incorporation of these results explains the varying and even contradictory claims regarding the influence of the pressure variability on radon changes in the published literature. Temperature dependence, facilitated by the temperature gradient in the borehole, controls the transportation of radon from the deep interior to the surface.

Radon variability in borehole from Multi-Parametric Geophysical Observatory of NW Himalaya in relation to meteorological parameters.

Radon has been measured continuously at an interval of 15 min at 10 m depth in a 68 m deep borehole. Five years of high resolution data of radon sampled at 15 min shows a complex trend with strong seasonal and diurnal trends. This temporal variation of radon has high emanation during summer and low concentration in winter. Diurnal radon variability was observed mainly during March-June and September-November each year. The existence of these diurnal periodicities in radon is related to the borehole temperature gradient i.e. difference between external atmospheric and internal borehole temperature at 10 m depth in the air column. The measured radon values are characterized by a diurnal cycle with a maximum in the afternoon and a minimum in the morning in all the seasons except in winter and during rainy periods. Minimum radon variations are recorded in the winter months primarily in December and January. Sudden unsystematic jumps in radon counts are observed each year in the rainy period (July and August). The atmospheric temperature was found to positively correlate with radon emanation. The data set of the borehole indicates a good correlation between atmospheric temperature and radon concentration that is observed throughout the year except in the rainy season. The spectra of radon and atmospheric temperature time series of 5 years data clearly show prominent and clear peaks at 1 and 2 cycles per day.

Seasonal and daily variation of radon at 10 m depth in borehole, Garhwal Lesser Himalaya, India.

Mostly accepted and widely reported radon (Rn(222)) measurements, a tool for earthquake precursor research, is a part of multi-parametric geophysical observation in the Garhwal Lesser Himalaya for earthquake related studies. Radon is being recorded continuously at an interval of 15 min at 10 m depth in a 68 m deep borehole. Three years high resolution 15 min data at 10 m depth shows a complex trend and has a strong seasonal effect along with some diurnal, semi-diurnal and multi-day recurring trends. A well-defined seasonal pattern is prominent with a high emanation in summer and low values in winter accounting for about a 30% decrease in count values in winter when the atmospheric temperature is very low at this station located 1.90 km above mean sea level. Diurnal, semi-diurnal and multi-day trends in this time-series are mainly observed during April-May and October-November. This is the period of spring and autumn when there is a high contrast in day-night atmospheric temperature. Hence the high fluctuation in Rn concentration is mainly caused by the temperature contrast between the air-column inside the borehole and the atmosphere above the earth's surface.