To increase the safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel has become a valuable tool. One recent successful forward looking approach is based on the excitation and registration of tunnel surface-waves. For further development and for finding optimal acquisition geometries it is important to study the propagation characteristics of tunnel surface-waves. 3D seismic finite difference modelling and analytic solutions of the wave equation in cylindrical coordinates reveal that at higher frequencies, i.e., if the tunnel-diameter is significantly larger than the wavelength of surface-waves, these surface-waves can be regarded as Rayleigh-waves confined to the tunnel wall and following helical paths along the tunnel axis. For lower frequencies, i.e., when the tunnel surface-wavelength approaches the tunnel-diameter, the propagation characteristics of these surface-waves are similar to S-waves. We define the surface-wave wavelength-to-tunnel diameter ratio w to be a gauge for separating Rayleigh- from S-wave excitation. For w > 1.2 tunnel surface-waves behave like S-waves, i.e. their velocity approaches the S-wave velocity and the particle motion is linear and perpendicular to the ray direction. For w < 0.6 they behave like Rayleigh-waves, i.e., their velocity approaches the Rayleigh-wave velocity and they exhibit elliptical particle motion. For 0.6 < w < 1.2 a mixture of both types is observed. Field data from the Gotthard Base Tunnel (Switzerland) show both types of tunnel surface-waves and S-waves propagating along the tunnel.
[1]
Thomas Bohlen,et al.
Paralel 3-D viscoelastic finite difference seismic modelling
,
2002
.
[2]
Kay Krüger,et al.
Seismic investigations of the Piora Basin using S-wave conversions at the tunnel face of the Piora adit (Gotthard Base Tunnel)
,
2008
.
[3]
Thomas Bohlen,et al.
Rayleigh-to-shear wave conversion at the tunnel face — From 3D-FD modeling to ahead-of-drill exploration
,
2007
.
[4]
Thomas Forbriger,et al.
Inversion of shallow-seismic wavefields: I. Wavefield transformation
,
2003
.
[5]
David Saiang,et al.
The excavation disturbed zone (EDZ) at Kiirunavaara mine, Sweden—by seismic measurements
,
2007
.
[6]
Jianmin Qu,et al.
Relationship between Rayleigh wave polarization and state of stress.
,
2006,
Ultrasonics.
[7]
A. Zehnder,et al.
Superconducting Sn/Sn-oxide/Sn tunneling junctions as high-resolution x-ray detectors
,
1987
.
[8]
H. Alheid,et al.
Seismic investigation of the Excavation damaged zone in Opalinus Clay
,
2001
.
[9]
C. Klose,et al.
In situ seismic investigations of fault zones in the Leventina Gneiss Complex of the Swiss Central Alps
,
2005,
Geological Society, London, Special Publications.