Title : In situ seismic measurements in claystone at
Tournemire (France)
Abstract of article in text form :
Compressional and shear wave seismic measurements were
performed in an old railway tunnel and in galleries excavated in
a 250-m-thick Toarcian claystone formation in the Tournemire
experimental station (France). Three component (3C) geophones
and three orthogonal orientations of the vibroseismic force
source were used. Additionally, vertical seismic profiling (VSP)
measurements were recorded with a 3C borehole geophone, a
hydrophone and a microphone in a 159 m deep borehole (ID180) in
the tunnel. The seismic data show that Toarcian claystone has
strong transverse isotropy (TI) with a vertical symmetry axis.
The qP, SH and qSV wave propagation velocities in horizontal
directions—the plane of isotropy of the TI medium—are measured
as 3550, 1850 and 1290 m/s, respectively. The zero-offset VSP
reveals that only one shear wave propagates in the vertical
(depth) direction and the P- and S-wave velocities are 3100 and
1375 m/s, respectively. Four elastic moduli of the TI medium are
determined from the seismic velocities and from the bulk density
of 2.53 g/cm^3 : c11 = 31.9 GPa, c33 = 24.3 GPa, c44 = 4.5 GPa
and c66 = 8.7 GPa. A walkaway VSP with the borehole geophone at
50 m depth in borehole ID180 and shot points in the galleries
leads to oblique seismic ray paths which allow us to determine
the fifth elastic modulus of the TI medium to c13 = 16 GPa. The
tube wave recorded by a hydrophone in the water filled lower
part of the borehole propagates with 1350 m/s, which confirms
the estimate of the elastic constant c66. The analysis of body
wave and surface wave data from a seismic experiment in Galerie
Est shows reflections from several fracture zones in the gallery
floor. The thickness of the excavation damaged zone (EDZ) in the
floor of Galerie Est is estimated to 0.7 m.
Ref : Zillmer et al. (2014),
Geophysical Journal International 199, 1798-1807.
