Potential of embedded Fiber Bragg Grating networks for seismic characterization of an Excavated Damaged Zone

Seismic methods offer a non-destructive and repeatable solution adapted to the long-term monitoring of the zone affected by the excavation of galleries in deep geological repositories. This study provides preliminary observations and feedback from an in-situ experiment conducted in the Bure Underground Research Laboratory (Meuse, France). A network of Fiber Bragg Grating (FBG) sensors was embedded in boreholes drilled perpendicularly to the wall of a gallery through the first meters affected by the excavation, to assess our ability to measure P-wave propagation velocities and their temporal variations. A newly developed interrogation system was tested in real conditions. Reference accelerometers were embedded for comparison. Two high-frequency active seismic campaigns (1-5 kHz) were conducted for this purpose. Preliminary velocity models are presented. The benefits of this approach and the challenges to overcome in terms of signal-to-noise ratio are summarized in this contribution.