Open AccessDetectability and significance of 12 hr barometric tide in radon‐222 signal, dripwater flow rate, air temperature and carbon dioxide concentration in an underground tunnel
Author(s) -
Richon Patrick,
Perrier Frédéric,
Pili Eric,
Sabroux JeanChristophe
Publication year - 2009
Publication title -
geophysical journal international
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.302
H-Index - 168
eISSN - 1365-246X
pISSN - 0956-540X
DOI - 10.1111/j.1365-246x.2008.04000.x
Subject(s) - radon , carbon dioxide , amplitude , flow (mathematics) , atmospheric pressure , volumetric flow rate , geology , advection , atmosphere (unit) , atmospheric sciences , soil science , environmental science , mineralogy , mechanics , chemistry , meteorology , physics , thermodynamics , optics , organic chemistry , quantum mechanics , oceanography
SUMMARY 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.