Long-term and multi-scale monitoring with passive geophysical methods: principles and measurements at the Mont Terri underground research laboratory

Document available in extended abstract form only. Full text of publication follows: Geophysical methods, used to investigate the extension and physical properties of an Excavation Damaged Zone (EDZ) in sedimentary or crystalline rock masses, enable non-invasive testing around the excavation of galleries, tunnels, niches or boreholes. Seismic, acoustic emission and electric methods allow to characterize the rock mass in term of anisotropy and damage for instance, but also to monitor the EDZ evolution during an excavation process. Hydraulic and pneumatic methods (e.g. hydraulic conductivity, gas injection or extraction tests in boreholes) allow to delineate the EDZ and to estimate its transport property changes with time. But most of those conventional techniques require controlled sources which might become unsustainable when dealing with long-term monitoring of geological repositories. In order to approach the scientific challenge of long-term and sustainable monitoring of underground repositories, geophysical methods based on the monitoring of natural signals are proposed: (1) barometric signal and radon-222 activity. The comparison of pressure signals measured in obturated boreholes and in the tunnel pressures shows time-lag and attenuation of the barometric signal propagated through the EDZ, both depend on the pneumatic diffusivity. Radon-222 (3.82 days of half-life and coming from the decay chain of the uranium-238 present in argillite) is used as a geophysical tracer of transport property changes of the EDZ at different scales. Barometric signal and radon-222 activity concentration have been measured since December 2011 in three different niches characterized by particular EDZ and different argillite facies. Radon activity concentration is continuously measured in niches and boreholes. Data analyses is associated with numerical modeling of the permeability of the EDZ and laboratory characterization of radon emanation process. (2) ambient seismic noise and (3) magneto-telluric signals. The first objective consists in cataloging and characterizing the natural sources available on site, by the use of continuous noise recording. The second one is to determine the potential of the innovating methods, as well as their resolutions in space and time. Hence, as a first step, in situ experiments are being developed at the Mont Terri URL. Continuous recordings of ambient seismic noise have been measured since February 2012, along a line of ten geo-phones which crosses a main fault. Characteristic acoustic signatures can be identified, associated to the main tunnel, the boreholes drillings, the compressor, etc. From the seismic noise correlation, it should be possible to pinpoint which areas and which elastic properties are concerned by the changes observed in the obtained Green functions. Continuous recordings of electromagnetic signals coming from the ionosphere and magnetosphere should inform about the macroscopic properties of the rock mass above the Mont Terri URL. A preliminary test has been performed in the EZ-G04 on February 10, 2012: the recording of electrical field allows the detection of anthropic noise that have to be filtered. A second station will be installed on the mountain of Mont Terri to record continuously the magnetic field variations. Some numerical modeling will be associated to the analysis of signals recorded in the gallery. The proposed methods are being tested at different scales of applications, from the damaged zone around a borehole to a damaged tunnel, or even to a large scale of a whole repository. (authors)