Application of a three‐dimensional ray‐tracing technique to global P , PP and Pdiff traveltime tomography

Summary A 3‐D ray‐path tracing algorithm was successfully applied to global P ‐wave traveltime tomography. The inversion was conducted iteratively using the resultant P ‐wave velocity model as the initial model for the subsequent iteration. The LSQR method was adopted to solve a large and sparse system of equations. This iteratively linearized inversion with 3‐D ray tracing increased wave‐speed anomalies, located heterogeneities better and reduced smearing as compared to those derived from a conventional one‐step inversion using 1‐D ray tracing, although the general pattern of velocity anomalies was similar. A major difference was found in the lowermost mantle, where the departure of a ray path from the great circle path tends to be in general greatest. In particular, a pronounced high‐velocity anomaly develops beneath the Indian Ocean, a feature not obvious in the result of 1‐D inversion. The final P ‐wave velocity model was obtained by including reported PP and Pdiff traveltime data. The addition of the PP data sharpened the images and enhanced velocity anomalies in the upper mantle, especially at latitudes above 45° of the Northern Hemisphere. The addition of the Pdiff data sharpened and amplified velocity anomalies in the lowermost mantle in general.