Map of galleries and positions of seismometers. The upper left inset show schematically the orientation of the galleries below the mountain together with the sensor locations and the vertical borehole that joins facility to the surface topography.

Contact : Stéphane Gaffet, gaffet@geoazur.unice.fr

La Grande Montagne shields instruments from many noise sources and the LSBB is located within the regional natural park of the Luberon, with few anthropogenic perturbations, centrally positioned at known distances away from large seismogenic faults of Provence (Baroux, 2003). This quiet area allows in situ access to the carbonate reservoir (Jurgawczynski, 2007) and in situ survey of hydraulic flows across the unsaturated porous and fractured zone, which is below the near-surface weathered zone and above the saturated zone of the Fontaine-de-Vaucluse aquifer (Garry et al., 2008).

The black box symbols in the figure above show the locations of the six broadband STS2 3-components seismometers recorded with Agecodagis digitizers (http://www.agecodagis.com/) with 125 samples per second. The 3D array contains one near-surface station together with the five deep stations. One sensor station labeled as VES is located near the top of the mountain at an elevation of 1062 m (and 5 m below the surface). The other five sensor stations are located at approximately the same elevation, between 535 m and 549 m (the reference. geoïd). The depth of these five stations at the LSBB tunnel level with overburden coverage below the surface topography varies from 20 m for Station GGB (near the LSBB entrance) to 518 m for Station RAS, the deepest sensor, at the center of LSBB. The deepest Station RAS also has a CMG5 accelerometer.

The prime motivations for installing this array are

(i) to monitor the seismic noise variations at the LSBB in order to discriminate between various noise sources that may perturb all other experiments running in LSBB (e.g., change in water saturation, hence wave velocities);

(ii) to determine the azimuth and apparent velocities of high-frequency body waves, given the very low noise level and the horizontal extension of about 1.5 km;

(iii) to allow for a comparison at different scales between solid block rotations of the surrounding mountain derived from the 3D array with direct tilt measurements conducted inside the mountain.

The establishment of an LSBB seismic array facilitates many other observations, including the studies of magnetometry and water chemistry, as well as the study of poroelastic coupled-processes dynamics induced by seismic waves that propagate within the medium (Cappa et al., 2006).

Example of data recorded for the Mw8.1 Samoa event occurred in september 2009, showing the great coherency for all components and waveform effect on free surface sensor 

The data recorded by the 3D broadband seismic array, available at http://websismo.unice.fr/, are fed in real time via SeedLink to the European Union seismic network (ORFEUS, http://www.orfeus-eu.org/) and are now also available via IRIS, http://www.iris.washington.edu/.

Sensors EGS, MGS, RAS, GAS, GGB and VES depicted in the figure above correspond to the streams RUSF_03, 04, 01, 05, 06, and 07 respectively. Data are also available by autoDRM (Kradolfer, 1993, 1996) using autodrm@geoazur.unice.fr, and via the Fosfore portal http://www.fosfore.ipgp.fr/ by NetDC using netdc@ipgp.jussieu.fr or netdc@geoazur.unice.fr.

Gaffet S, J. Wang, M. Yedlin, G. Nolet C. Maron, D. Brunel, A. Cavaillou, D. Boyer, C. Sudre, M.Auguste, (2009) A 3D Broadband Seismic Array at LSBB, IRIS DMS Newsletter, vol11 no3

also here

http://www.iris.edu/news/newsletter/vol11no3/3d_seismic_array.htm