Tunnel Seismic Investigations - 3D Imaging

The project OnSITE (Online Seismic Imaging for Tunnel Excavation in hard rock) aims at developing an integrated approach for seismic imaging around the tunnel and prediction ahead of the tunnel face during construction work. We present the recent results of our subproject which is concerned with the determination of the spatial locations of faults, fractures and heterogeneities 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 application of the mentioned imaging techniques to data from the Gotthard base tunnel show 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 in the stacked section as well as in single receiver gathers.