Geophysical monitoring of the EDZ during a gallery excavation in the Opalinus clay of the Mont Terri URL: design and principles of specific in situ experimental setups

Document available in extended abstract form only. At the Mont Terri Underground Rock Laboratory, the gallery Ga08 was excavated in August 2008 to join the end-face of the pre-existing gallery Ga04. The Excavation Damaged Zone (EDZ) induced during the gallery construction was monitored from the instrumented face of Ga04. The EZ-G08 experiments required original acquisition setups, specifically designed to be installed in small boreholes and to ensure long term measurements. Different geophysical methods have been experimented for the EZ-G08 project and specific design of the experiments were required in order to coordinate multiple and simultaneous acquisitions, including acoustic emission (AE) acquisition setup (16 acoustic receivers), manufacturing of a combined electrical (256 electrodes) and acoustic tomography setup (64 receivers) to be introduced in horizontal boreholes with small diameters (56 mm), design of an acoustic source, design and manufacturing of a combined electrical tomography (64 lead electrodes) and self-potential measurements (64 un-polarizable electrodes). Other arrays of electrodes have also been placed at the end-face of Ga04 (715 inox electrodes and an array of 16 un-polarizable electrodes). The self-potential (SP) monitoring took place in the borehole BEZ-G5 equipped in mid- February 2008 with a specific device on which custom-made electrodes were fixed. To avoid polarisation effect and guaranty a satisfactorily signal-to-noise ratio, SP was measured with 64 un-polarizable electrodes, with an offset of 15 cm made of a metallic wire in chemical equilibrium with a surrounding solution containing a salt of the same metal. The electrical contact between the solution and the natural medium is ensured by a porous material. The electrodes were mounted upward on a half-cylinder fixed onto an inflatable pipe and introduced in BEZ-G5. The internal pipe was inflated to ensure a good electrical contact between the electrodes and the borehole wall. The combined electrical and seismic tomography setup consists in four arrays inserted into each small-diameter boreholes BEZ-G12:15. Specifically designed holding poles were made of two half cylinders with an inflatable membrane located between them: one half cylinder to hold the electrodes, while the other half was designed to hold the acoustic receivers. No interactions were observed between these two types of equipment during the experiment. The purpose of the inflatable membrane, pressurized with air at 1.4 bar, was to push the sensors against the borehole wall and to ensure a good coupling between each set of sensors and the borehole wall. On each holding pole, of 3.2 m in length, 16 piezo-ceramics with an offset of 20 cm were fixed on one side and on the other side, 64 lead electrodes were fixed with an offset of 5 cm. In order to measure acoustic surveys inside the rock mass, an active source was designed to be introduced in BEZ-G5: the omnidirectional source allowed to sound the medium with different travel paths to study the anisotropic structure of the rock. More than one month of measurements took place in boreholes BEZ-G12:15, both for the electrical and the acoustic acquisition, which produced a very large data base. (authors)