Effects of Surface Scattering in Waveform Inversion

Abstract

In full waveform inversion of seismic body waves, the free surface is often ignored on grounds of computational efficiency. We investigate the effect of this simplification for highly irregular topography by means of a synthetic example. Our test model and data conform to a long-offset survey of the upper crust in terms of size and frequency. Random fractal variations are superimposed on a background model. We compute synthetic data for this model and different topographies, and we invert it neglecting the free surface. The resulting waveform models are relatively similar and, for the most part, show a high degree of correlation with the true model. The inversion of the irregular-topography data produces a few strong artifacts at shallow depths, but only a minor decrease in overall resolution. However, both waveform models fail to image below a strong shallow velocity contrast. The results suggest that in this part of the model the incapacity to properly reproduce the reverberations from that contrast without free surface derails both inversions.