Abstract:

In the field of exploration geophysics, the software packages that can fast and efficiently simulate 2D and 3D seismic full-wavefield, including simultaneously simulate P-wave, S-wave, and surface wave  are not very much.  In the structure mechanics, however, many sophisticated commercial software packages based on the finite element method including ANSYS and PLAXIS for  multi-wave and multi-component have been employed for the solutions of the elastic constitutive equations.  Compared to  the complete elastic wave equations, the elastic ones  include a damping term,  which is a firstorder differential term time. Therefore, by  adjusting  the parameter of  the medium, that is, the Rayleigh coefficient to make the damping to zero, the elastic constitutive equations will degenerate into full elastic wave equation. The wavefields propagating in  the rocks of a  tunnel are free of the disturbance of low velocity resulting from  the cover layer in the conventional seismic exploration. We simulated the wavefields of the tunnel seismic prediction  using the ANSYS software, and researched the  dispersion, damping and absorbing conditions etc. in the numerical modeling. For medium  absorption,  we compared time records and snapshots of the wavefields in the numerical simulation for the cases with and without the damping. In the tunnel seismic prediction, setting the damping to  zero is a reasonable assumption. By comparing  the relation of different grid length and dispersion in the numerical example, we find that the dispersion disappears when the mesh size is smaller than the wavelength of 1/π. By introducing  a viscoelastic boundary condition deduced from the wave equation,  boundary reflections can be effectively absorbed. Finally, an numerical example of the tunnel  seismic prediction shows that the  usage of the ANSYS software can effectively simulate the propagation of  waves in the complicated geological conditions.

Key words: numerical modeling, ANSYS, FEM, tunnel seismic prediction, seismic wavefield, damping, dispersion, boundary condition