Comparison of Different One-Way Propagators for Wave Forward Propagation in Heterogeneous Crustal Wave Guides

Crustal wave guides are usually heterogeneous on many scales, some of which can be investigated for regional phases by one-way approximation methods. The advantage of one-way propagation methods is the greater saving of computing time and memory, often up to several orders, than the full-waveform numerical meth-ods. By slicing a half-space crustal wave guide into a number of slabs perpendicular to the propagation direction, Wu et al. (2000) formulate a wide-angle pseudoscreen method for the SH half-space problem, which leads to a generalized screen propagator (GSP) for simulating Lg propagation. In this article we introduce a broadband con-stant-coefficient propagator for the SH half-space problem, which accounts for wide angles in large-contrast media while allowing implementation using Fourier trans-forms alone. Particular attention is paid to the first-order separation-of-variables screen propagator (SVSP1) that significantly improves the split-step Fourier (SSF) method for large lateral variations at the cost of one more Fourier transform in each slab. Advancing wave fields by SVSP1 is actually a linear interpolation in the wave-number domain between two split-step terms. We benchmark the SSF, GSP, and SVSP1 synthetic seismograms against the full-waveform boundary-element synthet-ics for flat, belling, and necking crustal wave guides, which shows that the SVSP1 method can model the Lg phase and the mantle wave (head wave) quite well in both travel time, energy, and waveform for most common mantle velocity perturbations. These numerical comparisons also demonstrate some limitations (especially in wave-form) of the one-way propagation methods to model the Lg code attributed by for-ward scatterings as a result of lateral irregularities of the Moho.