A NUMERICAL FREE-SURFACE CONDITION FOR ELASTIC VISCOELASTIC FINITE-DIFFERENCE MODELING IN THE PRESENCE OF TOPOGRAPHY/

An accurate free-surface boundary condition is important for solving a wide variety of seismic modeling problems. In particular, for earthqu ake site studies or shallow environmental investigations the surface o f the earth may have a significant impact on the outcome of simulation s. Computations based on several elastic/viscoelastic flat horizontal free-surface conditions are compared and benchmarked against an analyt ical solution. An accurate and simple condition is found and then gene ralized to allow for irregular free surfaces. This new method is simpl e to implement in conventional staggered finite-difference schemes, is computationally efficient and enables modeling of highly irregular to pography. The accuracy of the method is investigated and criteria for sampling of the wavefield are derived.