A three-dimensional robust seismic ray tracer for volcanic regions

Seismic velocity structure of volcanic region is highly heterogeneous so that seismic ray tracer used in this field should be robust for the velocity heterogeneity. From this view point a three-dimensional robust seismic ray tracer, effective in any complicated velocity structure, is developed by using a hybrid scheme of the shortest path calculation and the downhill simplex optimization method. The node configuration necessary for the three-dimensional shortest path calculation is newly presented. Validity and efficiency of the calculation are examined by synthetic tests. A travel time accuracy of less than 0.1%, and a rms ray path error of 0.05 km are achieved. Calculations for the synthetic velocity models and checkerboard testing show the effectiveness of this ray tracer in practical situations. The present ray tracer is recommended to be used in travel time tomography in highly heterogeneous velocity structure such as volcanic regions.