Guided wave attenuation in laterally varying media

Coupled mode techniques for guided wave propagation are extended to 2-D stochastic heterogeneity superimposed on a stratified medium. This approach requires the variations to be smoothly varying and of modest size (less than ±2 per cent). By averaging over an ensemble of statistically similar models, coupled equations for the modal energy transport can be generated. The intermode coupling depends on the horizontal correlation functions for the heterogeneity in the crust and mantle, and the integrated effect of the vertical variations in velocity and the modal eigenfunctions. For a particular stochastic model, the attenuation of a single mode as a function of distance can be calculated as a superposition of intrinsic attenuation and scattering loss by energy transfer to other modes of propagation. These statistical estimates of attenuation can be compared with observations of regional phases travelling over a variety of paths in a single region. For Lg and Sn phases, intermode scattering may represent up to 30 per cent of the apparent loss.