Abstract
Abstract. Two hourly-sampled time-series of soil-gas radon concentrations of durations of the order of a year have been investigated for periodic and anomalous phenomena. These time-series have been recorded in locations having little or no routine human behaviour and thus are effectively free of significant anthropogenic influences. One measurement site, Sur-Frêtes, is located in the French Alps, with saturated soil conditions; the second site, Syabru-Bensi, is located in Nepal, in a river terrace with unsaturated soil conditions. In such conditions, periodic components with periods ranging from 8 h to 7 days are often weak and intermittent and therefore, even in the presence of stationary forcing, difficult to identify. Two spectral decomposition techniques, Empirical Mode Decomposition (EMD) and Singular Spectrum Analysis (SSA), have been applied to these time series and yield similar results. For Sur-Frêtes, weak diurnal and semi-diurnal components are observed with EMD, while SSA reveals only a diurnal component. In Syabru-Bensi, both EMD and SSA reveal a strong diurnal component and a weaker semi-diurnal component. Tidal components M1 and M2 are also suggested by EMD in Sur-Frêtes, while these frequencies are not observed in Syabru-Bensi. The development of such analytical techniques can help in characterising the multiple physical processes contributing to the surface and subsurface dynamics of soil gases.
Highlights
Analysing time-series of radon emissions and concentrations for the presence of anomalies and/or cycles has the potential to reveal important information regarding crustal and surficial structures and processes, e.g. location and behaviour of faults, response to tidal forces and changes in stresses associated with earthquakes
IMF-1 comprises high-frequency “sampling” noise; IMF-2 contains frequency spectra centred at 4 cycles per day, having little consistent distinct structure; IMF-3 contains distinct frequency components centred at 2 cycles per day; IMF-4 contains distinct frequency components centred at 1 cycle per day and IMFs 5–12 comprise intermittent low-frequency quasi-periodic components
Empirical Mode Decomposition (EMD) has revealed “noisy” sets of diurnal and semi-diurnal frequency components in both time-series. Both time-series contain S1 (24-h) and S2 (12-h) solar tidal harmonics but these are much more clearly defined in the Syabru-Bensi data than in the Sur-Fretes data
Summary
Analysing time-series of radon emissions and concentrations for the presence of anomalies and/or cycles has the potential to reveal important information regarding crustal and surficial structures and processes, e.g. location and behaviour of faults, response to tidal forces and changes in stresses associated with earthquakes. While some external forces such as tidal influences are fundamentally stationary, the radon response from the soil or bedrock can be highly non-linear (Richon et al, 2009), and result in non-stationary variations Harmonic components in such geophysical time series, when present, are expected to be affected by significant temporal modulations. These spectral decomposition methods have been used successfully to indicate the presence of diurnal and sub-diurnal cycles in radon concentration which we provisionally attribute to solid-earth and barometric tidal influences These methods have been used to enhance the identification of short-duration anomalies in radon timeseries, attributable to a variety of causes including, for example, earthquakes and rapid large-magnitude changes in weather conditions (Crockett and Gillmore, 2010)
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