ABSTRACT

Active seismic methods employ controlled sources that release energy at specified times and locations. They haven proven that they can predict the 3-D spatial distribution of rock properties ahead of the tunnel face. Examples are propagation velocities of compressional or shear waves or their reflectivities. Ground porosity or seismic attenuation can be estimated with lower spatial resolution. A 3-D image of P-wave reflection strength ahead can be obtained automatically and continuously along a tunnel when a vibration source and receivers are placed on the cutting wheel of a tunnel boring machine. A similar 3-D model is computed using seismic drilling noise exclusively recorded on the tunnel boring machine and shows the same obstacle. Such passive tunnel seismology promises predictions at less operational and hardware costs but it requires a different signal processing compared to active methods.

1 TUNNELSEISMOLOGY

Tunnel construction engineers need information about the geotechnical properties of the rock ahead of the tunnel face to allow them to adapt advance parameters and to be warned against obstacles. Seismic methods have proven to be successful in predictions of rock properties ahead of the tunnel or in mining due to their robustness, flexibility, low cost and comparably high spatial resolution of the order of meters and good energy penetration. Most commercial seismic systems for tunnelling in soft ground or hard rock aim at imaging of structures that reflect elastic waves ahead of the tunnel or in its neighborhood. They all use controlled sources to initiate the wave propagation. Kneib et al. (2000) describe a soft ground tunnel seismic system (named SSP) that employs a vibrational source and few one-component receivers, oriented parallel to the tunnel and mounted on the cutting wheel of a tunnel boring machine (tbm). Thousands of seismic traces are automatically acquired while drilling and go through an automatic complex seismic processing sequence which yields a new 3-D model of the ground ahead about every five metres of advance. A system for hard rock exploration is Tunnel Seismic Prediction (TSP; Sattel et al., 1992).

2 SEISMIC PARAMETERS VIA ACTIVE SOURCES

The primary quantity recorded in seismology is amplitude as function of time. Further quantities derive from these registrations. The bulk of seismic energy recorded in tunnel seismology using active sources may often comprise direct waves, tunnel surface waves, and waves guided through the excavation damaged zone around the tunnel (Kneib & Leykam, 2004). These wave modes are sensitive to local heterogeneities up to fewmetres away from the tunnel. But they as well as undesired background noise should be suppressed prior to reflection imaging. The extraction of reflected waves can make use of the fact that the reflection traveltime reduces as the tunnel advances and the acquisition equipment approaches the reflector. The pressure wave and shear wave reflections can be extracted from the seismic raw data via multi-channel dip filtering and by covariance-based methods to separate different modes of particle movement. Amplitude loss due to geometrical spreading and attenuation must be compensated.

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