Spectral-decomposition techniques for the identification of periodic and anomalous phenomena in radon time-series

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.

Temporal signatures of advective versus diffusive radon transport at a geothermal zone in Central Nepal

“…14c, where the corresponding powers of the peaks, obtained by standard wavelet extraction, are shown as a function of time. These results confirm the observations made earlier with this time-series using other spectral techniques (Crockett et al, 2010).…”

“…In RefN, the radon probe was also associated with one small soil temperature data logger TinyTagÔ. A preliminary time-series was also recorded at point RefN in 2005 (Perrier et al, 2009a;Crockett , 2010). For this test, the sensor was lying in the soil without any pipe, in a small trench at a depth of 30 cm.…”

Relationship between seismicity and water level in the Enguri high dam area (Georgia) using the singular spectrum analysis

“…Such analysis is complicated by the stochastic nature of geophysical records and the range of natural influences these are susceptible to (Crockett et al, 2010).…”

Intrinsic vs. spurious long-range memory in high-frequency records of environmental radioactivity

“…There exist a multiplicity of techniques for decomposing time series into components corresponding to oscillatory variability patterns with more or less distinct natural frequencies. In the context of radon time series, wavelet methods [9,69,70], singular spectrum analysis (SSA) [42,43] as well as empirical mode decomposition (EMD) [41] have been applied in recent studies. Among these techniques, discrete wavelet analysis [9] has the potential disadvantage of providing a decomposition into frequency bands rather than signals with specific periodicities.…”

“…We emphasize that a more detailed assessment of the relevant statistical and dynamical properties of such multi-scale dynamics is a challenging task by itself and may relate to phenomena such as multifractality, but also the necessity of proper time-scale decomposition of the data. Regarding the variability of radon concentrations, the two latter aspects have been addressed recently by various authors [39][40][41][42][43] for some specific geological settings.…”