GlobalPandPPtraveltime tomography: rays versus waves

Abstract

SUMMARY This paper presents a comparison of ray-theoretical and finite-frequency traveltime tomography for compressional waves. Our data set consists of 86 405 long-period P and PP‐P traveltimes measured by cross-correlation. The traveltime of a finite-frequency wave is sensitive to anomalies in a hollow banana-shaped region surrounding the unperturbed ray path, with the sensitivity being zero on the ray. Because of the minimax nature of the surface-reflected PP wave, its sensitivity is more complicated. We compute the 3-D traveltime sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation. We compare tomographic models with the same χ 2 fit for both ray theory and finite-frequency analysis. Depending on the depth and size of the anomaly, the amplitudes of the velocity perturbations in the finite-frequency tomographic images are 30‐50 per cent larger than in the corresponding ray-theoretical images, demonstrating that wave front healing cannot be neglected when interpreting long-period seismic waves. The images obtained provide clear evidence that a limited number of hotspots are fed by plumes originating in the lower mantle.