RAPID MULTI-WAVE-TYPE RAY-TRACING IN COMPLEX 2-D AND 3-D ISOTROPIC MEDIA

We describe a method for seismic ray tracing in complex 2-D and 3-D he terogeneous isotropic media. Both the interface geometry and the horiz ontal velocity field are parameterized by using nonuniformly spaced cu bic B-spline nodes. In particular, we present a modified cubic B-splin e method for modeling discontinuities. This model parameterization sch eme facilitates tomographic inversion of traveltimes and can represent any realistic geological structure that is continuously varying with embedded discontinuities. Many wave types, such as transmitted, reflec ted, refracted, mode-converted, diffracted, and turning waves, are des irable for complex geological models, We select rays of various wave t ypes by choice of initial paths and refine them by applying Fermat's m inimum time principle. Since the raypath is piecewise linear between i nterfaces, we solve a system of linearized equations by iteration to p roduce the actual raypath between source and receiver. The method is f ast and unifies the ray-tracing procedure for various wave types and m ultiple paths between a source-receiver pair. Ray amplitudes can be al so evaluated by existing analytical formulae. Several examples show th at the method is computationally accurate and effective. The method is practicable for multiphase traveltime tomography and 3-D ray-trace se ismic modeling.