Wave Field Modeling And Elastic Wave Pre-stack Depth Migration Based On Screen Method

Screen propagator is a kind of one-way propagator, with which both forward propagation and primary reflection could be calculated. In wave filed modeling, besides modeling different types of wave simultaneously, it could just model one of that, such as primary reflection or multiple of compressional wave, shear wave or converted wave, as well as primary wave. Compared with two-way method, such as finite-difference and finite-element, this one-way method is very efficient. In seismic migration, screen propagator neglects back-scattering, but can correctly handle focusing, defocusing, diffraction, etc. It can be used to seismic migration of large velocity contrast media after the wide angle correction.In this paper, the equation of scattering wave in heterogeneous media is deduced based on perturbation theory, and several approximations of resolving scattering wave equation and their comparisons are introduced: Born approximation, Rytov approximation and De Wolf approximation. De Wolf approximation is a kind of multiple forward-scattering single back-scattering (MFSB) approximation. Two domain thin-slab formulas of scattering wave in acoustic media and elastic media are deduced based on renormalized MFSB. Under the condition of small angle approximation, thin-slab formula is reduced to screen formula, which improves calculating efficiency remarkably. The screen propagator with one-order correction can calculate scattering wave with moderate scattering angle. Primary reflection in acoustic media and elastic media are modeled with one-way screen propagator, and some problems such as reflective boundary, source loading are also discussed briefly.Extrapolation operator that adapts to large velocity contrast media and accurate imaging condition are keys of migration in complex media. In this paper, according to wide angle correction to phase screen propagator of acoustic wave equation, elastic screen propagator is corrected with Pade expand. Model testing confirms that Pade screen is powerful for acutely lateral velocity variation. Vector imaging condition is used though calculating polarized vector, and projecting component to polarized direction to image. It makes the information carried in wave field used efficiently and resolves the polarization issue of P-SV in imaging, which allows partial images from different sources can be stacked to generate a final image. Finally, elastic Pade screen propagator and vector imaging condition are tested with model data, and the migration result is ideal.