P-SV resonances in irregular low-velocity surface layers

Abstract

Near-surface geological properties can have a large effect on the character of seismic arrivals. While it has long been recognized that a large velocity contrast between the surface layer and the substrate can cause reverberation of the seismic signal, the effects of small scale lateral heterogeneity on the wave field propagation have, to our knowledge, not been treated in detail. We show that lateral heterogeneity in the surface region can strongly alter the character of arriving seismic signals by causing a resonant coupling to the surface modes of the medium. We describe the effect using propagator matrix formalism and the Born approximation. We demonstrate the effect using finite difference simulations of P-SV wave propagation for a model containing a low-velocity surface layer having roughness along its interface with an elastic half-space. These simulations show that when a planar pulse is incident upon the low-velocity layer, the roughness induces a strong resonant coupling to Rayleigh modes, a condition which cannot occur for a laterally invariant medium. The resonant coupling causes a long coda following the direct seismic arrival. Frequency-wavenumber analysis of the simulated seismic arrivals recorded over the rough low-velocity layer shows that the scattered energy is distributed in FK space principally along the dispersion curves of the surface modes of the average smooth low-velocity layer. The amplitude of the simulated signal is modulated as a function of frequency according to reverberations in a smooth surface layer. Both the concentration of energy along the Rayleigh dispersion curves in FK space and the frequency modulation are predicted by the propagator matrix-Born scattering analysis.