Seismic prediction ahead of tunnel construction using tunnel surface-waves

To increase safety and efficiency of tunnel constructions, online seismic exploration ahead of a tunnel can become a valuable tool. We developed a new forward looking seismic imaging technique to e.g. determine weak and water bearing zones ahead of the constructions. Our approach is based on the excitation and registration of tunnel surface-waves (TS-waves). These waves are excited at the tunnel face behind the cutter head of a tunnel boring machine and travel into drilling direction. Arriving at the front face they generate body-waves (mainly S-waves = ”RS”-waves) propagating further ahead. Reflected S-waves are back-converted into tunnel surface-waves (”RSSR”-waves) and can be recorded by geophones mounted on the tunnel wall. Using 3D Finite Difference modeling, an analytical solution of the wave equation in cylindrical coordinates and field data acquired at the Gotthard massive (Switzerland) we investigated the propagation characteristics of tunnel surface waves in terms of dispersion and polarization. Understanding the excitation and propagation of TS-waves is the key for developing processing and imaging techniques for our seismic look ahead prediction in tunnel constructions.