Seismic tomography of the excavation damaged zone of the gallery 04 in the Mont Terri Rock Laboratory

An endoscopic antenna is used to perform a seismic cross-hole tomography in the Excavation Damaged Zone (EDZ) of the new G04 gallery of the Mont Terri Underground Rock Laboratory (Switzerland) excavated in Opalinus clay. More than 800 seismic traces were recorded between two vertical boreholes by combining 22 source and 48 receiver locations. A vertical area of 1.2 x 3.4 m under the floor of the gallery is investigated with a high-resolution tomography. Data with a very good quality allow to determine the traveltimes and the amplitudes of a 40 kHz source wavelet propagating between the two boreholes. The analysis of the traveltimes shows that the wave velocity is homogeneous but anisotropic with a minimum value of 2490 +/- 45 m s(-1) in the direction normal to the bedding and a maximum of 3330 +/- 90 m s(-1) parallel to the bedding. The amplitude of the first arrivals strongly varies depending on the source-receiver locations, and suggesting an heterogeneous distribution of the attenuation coefficient of the seismic waves. A Bayesian inversion provides likely models of attenuation that are compared with geological observations. The areas where fractures or cracks exist in the Opalinus clay appear as highly absorbing the seismic waves.