Detectability and significance of the 12h barometric tide in the radon-222 signal, dripwater flow rate, carbon dioxide concentration and air temperature of an underground laboratory.

HR: 0800h
AN: H11B-0487 [Abstracts]
TI: Detectability and significance of the 12h barometric tide in the radon-222 signal, dripwater flow rate, carbon dioxide concentration and air temperature of an underground laboratory.
AU: * Richon, P
EM: richon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Equipe Géologie des Systèmes Volcaniques UMR 7154, 4 place Jussieu, Paris, 75252, France
AU: * Richon, P
EM: richon@ipgp.jussieu.fr
AF: Commissariat àl'Energie Atomique, Centre DAM-DIF,DASE/SRCE, BP 12, Bruyeres-le- Chatel, 91680, France
AU: Perrier, F
EM: perrier@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Equipe de Géomagnétisme, UMR 7154, 4 place Jussieu, Paris, 75252, France
AU: Pili, E
EM: eric.pili@cea.fr
AF: Commissariat àl'Energie Atomique, Centre DAM-DIF,DASE/SRCE, BP 12, Bruyeres-le- Chatel, 91680, France
AU: Boudon, G
EM: boudon@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Equipe Géologie des Systèmes Volcaniques UMR 7154, 4 place Jussieu, Paris, 75252, France
AU: Villemant, B
EM: villemant@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, Equipe Géologie des Systèmes Volcaniques UMR 7154, 4 place Jussieu, Paris, 75252, France
AU: Sabroux, J
EM: jean-christophe.sabroux@irsn.fr
AF: Institut de Radioprotection et de Sûreté Nucléaire, Centre de Saclay, BP68, Gif-sur- Yvette, 91192, France
AB: Radon concentration has been measured since 1995 in the Roselend dead-end tunnel, in the French Alps, together with other geophysical and geochemical parameters. Bursts of radon concentration, reaching 65,600 Bq m^-3 and up to several weeks duration, are observed over a background level of ca. 800 Bq m^-3. These bursts appear to be related to the bedrock deformation or to the hydrogeological processes associated with the yearly cycle of water level in the nearby artificial Roselend Lake. In order to work out a generation mechanism, for these bursts, we developed tools to characterize the transport properties in the host rocks. Here, we concentrate on the 12h (S₂) barometric tide. We first show, using real radon time series integrating synthetic signals, that a modified spectrogram method is more efficient than simple FFT to evidence weak periodic signals in such a context. Then, we apply this method to the radon concentration in the tunnel atmosphere measured by two different sensors: the AlphaGUARD^TM sensor based on volumetric detection in an ionizing chamber, and the BarasolTM sensor, based on surface detection. Using the time series recorded by the AlphaGUARD^TM, the S₂ line, difficult to see with a simple FFT method, emerges clearly with our spectrogram method. This S₂ line is not seen using the Barasol^TM time series, illustrating the superior sensitivity of the AlphaGUARD^TM for this particular purpose. Using a regular spectrogram analysis, we further show that the amplitude of the S₂ line in the AlphaGUARD^TM data depends on time, and appears particularly strong during the radon bursts. The presence of the S₂ line reveals a high sensitivity of radon exhalation flux from the tunnel wall to changes of atmospheric pressure, and thus supports the advective transport mechanism for the radon bursts. A small but clear S₂ component is also evidenced using our spectrogram method in a dripwater flow rate time series, representing a flow averaged over a 6 m² area of the tunnel ceiling, while it is not observed in the flow rate of a more localized dripping. This suggests that some water drippings can also be affected by atmospheric pressure variations. The temporal structures of the S₂ component in the flow rate and in the radon concentration, however, are not similar, indicating that water dripping from the ceiling cannot be the dominant source mechanism for the radon bursts. No S₂ component is observed in the time series of carbon dioxide, but an interesting pattern is revealed by the S₂ component of a temperature profile in the atmosphere. This study illustrates how a refined analysis to extract the S₂ component in various geophysical time series can provide interesting clues on the complex processes affecting transport of fluids in unsaturated fractured media under multiple influences.
DE: 0910 Data processing
DE: 0932 Radioactivity methods
DE: 3205 Fourier analysis (3255)
DE: 3389 Tides and planetary waves
DE: 5104 Fracture and flow
SC: Hydrology [H]
MN: 2007 Fall Meeting