Figure 12. Computation time required for one travel-time calculation using the 3D raytracing technique of Podvin and Lecomte (1991) (open circles) and for 1000 travel-time calculations by searching previously prepared disk files as discussed in this study (solid circles). The study region of 230 km x 150 km x 17 km is modeled in terms of 3D models with cubic blocks of various sizes to test the dependence of computation time on block size.