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Detectability and significance of 12 hr barometric tide in radon-222 signal, dripwater flow rate, air temperature and carbon dioxide concentration in an underground tunnel

Abstract : Searching for small periodic signals, such as the 12 hr (S 2) barometric tide, and monitoring their amplitude as a function of time, can provide important clues on the complex processes affecting fluid transport in unsaturated fractured media under multiple influences. Here, first, we show that a modified spectrogram analysis (MSA) is more efficient than simple Fourier transform to reveal weak periodic signals. Secondly, we show how transient periodic signals can be monitored as a function of time using spectrograms. These methods are applied to time-series of radon and carbon dioxide concentration, dripwater flow rates and air temperature measured during several years in the Roselend dead-end tunnel, located in the French Alps near an artificial lake. A weak S 2 line is evidenced in radon concentration, with enhanced amplitude during transient radon bursts. Similarly, the S 2 line is observed using MSA in dripwater flow rates which sample mainly fracture flow, as suggested by a hydrochemical analysis, while it is not seen in dripwater flow rates sampling matrix flow. In the absence of a strong 24 hr line, the presence of a S 2 line suggests sensitivity to barometric pressure , and thus a significant advective contribution in radon and some dripwater transport. No S 2 line is observed in the carbon dioxide time-series. The temporal structure of the S 2 component, however, is not similar in the radon concentration and the dripwater flow rates, suggesting, in particular, that dripwater does not play a significant role in the generation of radon bursts. Temperature time-series exhibit a significant S 2 contribution, induced by atmospheric pressure, spatially organised in the tunnel, decreasing vertically upwards. A remarkable transient temperature inversion during radon bursts suggests that the additional advective air contributions responsible for the radon bursts occur from the non-saturated rocks below the tunnel.
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Patrick Richon, Frédéric Perrier, Eric Pili, Jean-Christophe Sabroux. Detectability and significance of 12 hr barometric tide in radon-222 signal, dripwater flow rate, air temperature and carbon dioxide concentration in an underground tunnel. Geophysical Journal International, Oxford University Press (OUP), 2007, 176 (3), pp.683-694. ⟨10.1111/j.1365-246X.2008.04000.x⟩. ⟨insu-01298739⟩

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