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The ROSELEND-Gas Project: Research On Sollicitation Effects in a Large-scale Experiment under Natural Dynamics with Gas.



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Titre du congrès :2008 American Geophysical Union Fall Meeting Ville du congrès :San Francisco Date du congrès :15/12/2008


A 128-m long dead-end tunnel has been used since the 90's as an Underground Research
Laboratory at 55 m below ground surface in the unsaturated zone of a fractured granitic
body in Roselend (French Alps). As part of a multidisciplinary research project (Provost et al.,
2004-EOS) aiming at understanding the impact of mechanical, meteorological,
hydrogeological, and geochemical stimuli on transport in unsaturated fractured media,
experiments have been run from and to the tunnel, based on long-term, high-resolution
monitoring of a variety of physical and chemical parameters. In particular, transient
phenomena have been studied within this highly dynamic system. The tunnel is located close
to and above the artificial Roselend Lake where large variations in water level induce
reproducible deformation and hydrogeological disturbances. This mountainous area is also
characterized by contrasted water infiltration regimes. Solute transport presents a large
variability in space and time due to variations in water content, flow pathways, and chemical
reactions. Tracing experiments with natural and artificial tracers have been performed
between ground surface and dripping water collecting points in the tunnel. Dripwater fluxes
and water chemistry have been monitored for several years along sections of the tunnel
showing different geological structures and flow rates in order to get water representative of
different contributions of matrix porosity and fractures. Up to now, major findings include: -
mechanisms of earthquake precursors (Trique et al., 1999-Nature; Pili et al., 2004-EPSL), -
ventilation and the spatial and time variations of radon-222 concentrations in the tunnel
(Richon et al., 2005-J.Env.Rad., Perrier et al., 2005-STOTEN), - stereological analysis of
fractures and permeability determination (Patriarche et al., 2007-WRR), - reactive transport
and residence time determination (Pili et al., 2008-Developments in Earth & Environmental
Sciences). The new research project in the Roselend URL focuses on gases. It aims at
understanding the origin and dynamics of natural gas sources, as well as the transfer
mechanisms of gases from the tunnel toward the atmosphere through the unsaturated
fractured rocks. This research program is of interest for the understanding natural emissions
of greenhouse gases, the design of surface monitoring technologies for baseline
determination and leakage detection in the framework of CO2 sequestration, as well as for
improving tools for detection of concealed nuclear tests in the framework of the CTBT. At
the Roselend site, CO2 is naturally produced in soils by plant and microbiological activity and
in rocks by carbonate dissolution promoted by protons gained from pyrite oxidation upon
water infiltration. Radon is emitted from rocks and transported through fractures. Naturally
occurring CO2 and radon will be used as tracers in addition with an artificial mixture of noble
gases. Tracers will be injected from an isolated section of the tunnel. High-frequency
sampling will be followed by classical laboratory gas analyses (elemental and isotopic), and
will be completed with high-resolution field monitoring, including the determination of the
carbon isotope ratio in CO2 by diode-laser spectrometry.