Rayleigh-to-shear wave conversion at the tunnel face — From 3D-FD modeling to ahead-of-drill exploration

Abstract

For safe tunnel excavation, it is important to predict lithologic and structural heterogeneities ahead of construction. Conventional tunnel seismic prediction systems utilize body waves ͑P-and S-waves͒ that are directly generated at the tunnel walls or near the cutter head of the tunnel boring machine ͑TBM͒. We propose a new prediction strategy that has been discovered by 3D elastic finite-difference ͑FD͒ modeling: Rayleigh waves arriving at the front face of the tunnel are converted into high-amplitude S-waves propagating further ahead. Reflected or backscattered S-waves are converted back into Rayleigh waves which can be recorded along the sidewalls. We name these waves RSSR waves. In our approach, the front face acts as an S-wave transceiver. One technical advantage is that both the sources and the receivers may be placed behind the cutter head of the TBM. The modeling reveals that the RSSR waves exhibit significantly higher amplitudes than the directly reflected body waves. The excavation damage zone causes dispersion of the RSSR wave leading to multimodal reflection response. For the detection of geologic interfaces ahead, RSSR waves recorded along the sidewalls are corrected for dispersion and stacked. From the arrival times, the distance to the S-S reflection point can be estimated. A recurrent application, while the tunnel approaches the interface, allows one to quantify the orientation of the reflecting interfaces as well. Our approach has been verified successfully in a field experiment at the Piora adit of the Gotthard base tunnel. The distance to the Piora fault zone estimated from stacked RSSR events agrees well with the information obtained by geologic surveying and exploratory drilling.

Detection of Geological Structures Ahead of the Tunnel Construction Using Tunnel Surface-waves

“…In addition to the calculated distances (the tunnel face positions plus the offsets), we also plotted the determined formation S-wave velocity. The distance between the tunnel face and the Piora basin is stable at about 50 m for each window, and corresponds very well with both the rock quality index (RQD, Figure 4b) and imaging results from a previous publication (Bohlen et al, 2007). Also, the formation S-wave velocities of about 3100 m/s obtained from each processing loop agree with results from previous surveys (Borm et al, 2003).…”

“…Also, the formation S-wave velocities of about 3100 m/s obtained from each processing loop agree with results from previous surveys (Borm et al, 2003). Bohlen et al,2007).…”

“…According to its wave path, as both a direct TS-wave, converted S-wave, reflected S-wave and back-converted TS-wave, we call these signals TSST-waves, even though TS-S-S-TS-wave would be the more precise term. At different tunnel construction sites the look-ahead prediction using TSST-waves has proven its capability of imaging fault zones, lithological interfaces and other geological structures (Bohlen et al, 2007, Lüth et al, 2006. …”

“…We recently introduced a seismic tunnel look-ahead prediction method using tunnel surface-waves (Bohlen et al, 2007, Jetschny et al, 2009. Without interference in the tunnel construction, tunnel surface-waves (TS-waves) are excited behind the TBM and travel along the tunnel wall in drilling direction (see Figure 1).…”

Seismic investigations of the Piora Basin using S-wave conversions at the tunnel face of the Piora adit (Gotthard Base Tunnel)

“…Numerical simulations of a tunnel seismic experiment using the method of finite-difference modelling [9] showed that only little P-wave energy is gererated and most of it is directed sidewards to the tunnel. P-waves are therefore of limited use for looking ahead when they are not generated at the tunnel face or away from the tunnel wall.…”