The effects of near-source heterogeneity on shear-wave evolution

Abstract

The Born and Rytov approximation, radiative transfer theory, and other related techniques are commonly used to model features of wave propagation through heterogeneous geologic media such as scattering, attenuation, and pulse-broadening. However, due to the underlying assumptions about the scattering direction and the reference Green’s function, these methods overlook important features of the wavefield such as mode conversion and near-field term coupling. These effects are particularly important within the predicted S-wave nodes of a seismic source, so we analyzed the problem of wave propagation beneath a vertical-point force on the surface of a heterogeneous, elastic half space. To do this, we generated a suite of 3D synthetic heterogeneous geologic models using fractal statistics and simulated the wave propagation using the finite-difference method. We derived an estimate for the effective source radiation patterns, and we used these to compare the results of the models. Our numerical results showed that, due to a combination of mode conversion and near-source coupling effects, S-wave energy on the order of 10% of the P-wave energy is generated within the shear-radiation node. In some cases, this S-wave energy may occur as a coherent pulse and may be used to enhance seismic imaging.