Hybrid shortest path and ray bending method for traveltime and raypath calculations

The shortest path method (SPM) is a robust ray-tracing technique that is pa rticularly useful in 3-D tomographic studies because the method is well sui ted for a strongly heterogeneous seismic velocity structure. We test the ac curacy of its traveltime calculations with a seismic velocity structure for which the nearly exact solution is easily found by conventional ray shooti ng. The errors in the 3-D SPM solution are strongly dependent on the choice of search directions in the "forward star," and these errors appear to acc umulate with traveled distance. We investigate whether these traveltime err ors can be removed most efficiently by an SPM calculation on a finer grid o r by additional ray bending. Testing the hybrid scheme on a realistic ray-t racing example, we find that in an efficient mix ray banding and SPM accoun t for roughly equal amounts of computation time. The hybrid method proves t o be an order of magnitude more efficient than SPM without ray bending in o ur example. We advocate the hybrid ray-tracing technique, which offers an e fficient approach to find raypaths and traveltimes for large seismic refrac tion studies with high accuracy.