Modeling of seismic wave propagation near the earth's surface

Abstract

Realistic modeling of seismic wave propagation in the vicinity of the earth's surface is complicated by large velocity contrasts, strong heterogeneity, severe attenuation, and topographic relief. To account for these complications, we employ a finite-difference solution of the 2D viscoelastic equations, a grid-refinement technique in the shallow parts of the model, and a generalized imaging condition to model free-surface topography. The grid-refinement approach allows us to vary the discretization of the model and the wavefiedl with respect to the velocity structure. Compared to a standard uniform finite-difference grid approach, this saves considerable amount of memory and computations; thus enables modeling of wave propagation through large portions of the earth's crust. Moreover, the decreasing accuracy of the finite-difference method near the irregular free surface is compensated by using a finer grid-spacing in this region. Numerical tests show that the method is reliable and accurate. By applying this modeling technique to several canonical models of the near-surface region and upper crust, we find that scattering and mode conversions from topographic relief, waveguide effects, and attenuation in the immediate subsurface tend to dominate the seismic coda at near lapse times.