Propagation effects of an underground excavation

Abstract

Induced seismicity associated with the horizontal excavation of a simple, cylindrical tunnel within a homogeneous, unfractured rock mass has been used to investigate the propagation effects associated with an underground excavation. The tunnel was found to significantly delay P-wave travel times due to diffraction effects, which were accounted for using a finite-difference transmission technique. Underground excavations will also result in amplitude changes associated with reflection, mode conversion and diffraction effects, which were qualitatively illustrated using an example of a finite-difference solution of the wave equation. Velocity inversion of the induced seismicity data indicated significant velocity changes in various regions around the tunnel, which could be validated with the known rock mass deformation. In the roof of the tunnel where notches formed (analogous to borehole breakouts), inversion results were able to resolve relatively high velocities in the highly stressed zone and decreased velocities delineating the excavation-induced fractured zone. In the region at the end of the tunnel, decreased velocities were found corresponding to where 3D numerical stress models predicted a tensile regime. This controlled case study illustrates how an underground excavation can significantly affect seismic propagation, and how images of the P-wave propagation velocity can be used to delineate induced changes in the rock mass properties.