Three-dimensional seismic imaging of tunnels

Abstract

We present the recent results of our work which is concerned with the determination of the spatial locations of faults, fractures and heterogeneities around a tunnel by advanced seismic imaging techniques. So far we have extended existing 2D imaging techniques to 3D. We have implemented 3D versions of “Kirchhoff prestack depth migration” (KPSDM), “Fresnel Volume Migration” (FVM) and “Reflection-Image-Spectroscopy” (RIS). The 3D P- and S-velocity models required for the calculation of the Green's functions have been generated by using the 2D first-break tomography velocity models and rotating them around the tunnel axis. The 3D images were obtained by stacking the migrated data taking into account either the true phase or the absolute value. The application of the mentioned imaging techniques to data from the Gotthard Base Tunnel (Piora adit) shows significant improvements compared to standard (KPSDM) processing. The 3D RIS approach suppresses scattering effects in the low frequency band and increases resolution in the high frequency band so that some of the geological structures are much better visible. The 3D FVM technique uses slowness and polarization based estimates of the emergence angles at the receivers and restricts the imaging to the region around the actual reflection or diffraction point. We observe less spatial ambiguity and a higher resolution of most structures. The integration of both approaches (RIS and FVM) exploits their advantages and delivers an even more pronounced and clear image of the tunnel environment.