An Improved Vicinity Ray Tracing in Two-Dimensional Laterally Inhomogeneous Velocity Structure

Abstract

We have developed a modified vicinity ray tracing (VRT) method that has improved the travel time approximations and strengthened the theoretical base of the original one. The vicinity ray tracing (Kim and Cormier, 1990) method is a high-frequency asymptotic procedure to compute the wavefield in laterally inhomogeneous velocity structures. However, in calculating travel times the original VRT method only considers the central rays on the far side of the station but misses those on the side near the source. The modified version extends the travel time approximation to the rays on both sides of the station. This is facilitated by assuming that the ray paths near the surface in the vicinity of the station are parallel to each other. The modified approach has been examined with a variety of velocity models. For the model with a low velocity layer, the synthetic seismograms demonstrate that the dependence of the amplitudes on the source-receiver distance in the shadow zone is as clear as that illustrated using a classical Gaussian Beam Method. When strong lateral heterogeneity and thus multiple caustic regions are present, the synthetic wave field is still shown to be stable. As another improvement, the calculation of wavefront parameters is furnished in kinematic ray tracing system without invoking Kim and Cormier's dynamic VRT system. This alternative procedure can save large computational time without sacrificing the accuracy in synthetic seismograms.