Seismic Modelling

Motivation

Seismic modelling proves very helpful to support the interpretation of tunnel seismic data and to validate reflection imaging. Modelling is particularly important in tunnel seismology because poor spatial resolution may often leave room for ambiguities and tunnelling noise often compromises data quality. Wave scattering at or close to the excavation damaged zone around the tunnel is an important target for seismic modelling, in particular tunnel surface waves that circulate around the tunnel. A technique that is suitable for tunnel seismic modelling must be able to treat correctly arbitrarily complex geologies. In particular seismic interactions at heterogeneities from empty microcracks to water-filled breccia zones as well as the free surface of the tunnel must be modelled correctly for arbitrary measurement configurations.

Method

What method should we choose? Ray-tracing techniques are valid for high frequencies only and may not be able to deal with the required complexity of models, at least not with an affordable computational effort. Finite element methods represent a possible choice, in particular if the modelling space can be divided into areas of distinctively different scales of heterogeneity. We prefer finite-difference (FD) methods because they are easier to use and their computation times are nearly independent of the model heterogeneity. FD schemes discretise the wave equations on a grid. Finite differences over neighboring gridpoints replace spatial derivatives. Among the different published algorithms we prefer the diagonal differencing algorithm because it is stable even at locations of extreme elastic contrasts as they are of particular importance in tunnel seismology. In a recent publication (Kneib, G., Leykam, A., 2004, Finite Difference Modelling for Tunnel Seismology, Near Surface Geophysics, 2, 71-93.), we present an extension of the diagonal differencing algorithm in three ways: (1) We apply FD operators which are high-order accurate in space and time, (2) we automatically switch to 2nd order spatial operators at strong heterogeneities, and (3) we implement effective absorbing boundaries. This allows 3D simulations of realistic models on affordable PC-hardware.

Example

The example below illustrates wave scattering ahead of a tunnel boring machine. The 3-D elastic model of 6m X 6m X 2m includes the high-velocity steel body of the tunnelling machine and continues with the scratching tools which reach into the loose sandy ground ahead. Around and ahead of the machine we observe an excavation-damaged zone where velocities and density gradually decrease towards the tunnel. The undisturbed ground has a moderate vertical gradient.
The source that is integrated in the cutting wheel of a tunnel boring machine applies a directional force in the forward (Z) direction, emitting P-Waves towards the viewer and mostly S-waves to the sides. The ten snapshots below are taken every msec in a plane immediately ahead of the cutting wheel. They show orthogonal components X, Y, Z, the S-wave (curl), and the P-wave (divergence).
Apart from the direct P- and S-waves we observe strong scattering and wave conversions. Shear-wave scattering is dominant because the source emits S-waves into the plane shown and because dominant S-wavelengths are about 20 cm that is the approximate dimension of the tools. Near-source S-wave scattering results in several wavefronts immediately following each other.

P-Wave velocity modelP-Wave velocities for the 3-D elastic modelling. Notice the high-velocity steel body, the scratching tools and the excavation damaged zone around the tunnel. S-waves are more than three times slower in our loose-ground model.
Wave scattering ahead of a tunnelling machineWave scattering ahead of a tunnelling machine. A source applies a directional force towards the viewer. Notice the strong S-wave scattering at the tools of the cutting wheel.

Last updated November 1, 2005. © 2005, Tunnelseis. All rights reserved.