2-D/3-D irregular shortest-path ray tracing for multiple arrivals and its applications

The purpose of this study is to introduce a multistage irregular shortest-path method (ISPM) for tracking multiple seismic arrivals including any combinations of transmissions, reflections (or refractions) and mode conversions in complex 2-D/3-D layered media, incorporating irregular interfaces (or subsurface in 3-D) and velocity discontinuities. The basic principle is to first divide the model into several different layers (using irregular cells near each interface, discontinuity and the Earth's undulating surface topography) and then to apply the multistage technique to trace the multiple arrivals. It is possible to realize the multiple arrival tracking with the multistage scheme because the multiple arrivals are just different combinations or conjugations of the simple incident, transmitted, reflected (or refracted) and mode converted waves via the velocity discontinuities and the interfaces. Benchmark tests against the popular multistage fast marching method (FMM) and the multistage MSPM (modified shortest path method) are undertaken to assess the solution accuracy and the computational efficiency. The results show that the multistage ISPM is advantageous over both the multistage FMM and the multistage MSPM in both solution accuracy and CPU time. Several examples (including the Marmousi model) are used to demonstrate the viability and versatility of the multistage ISPM in heterogeneous media, even in the presence of high-velocity contrasts involving interfaces of relatively high curvature. Applications to the seismological problems, such as traveltime tomography and earthquake location, indicate that it is possible to improve the spatial resolution in traveltime tomography and solution accuracy in earthquake location if later arrival information is combined with the first arrivals.