We investigated applying tomographic techniques to amplitude-ratio data to quantify regional phase path effects for use in source discrimination studies.
Our technique solves for resolvable combinations of relative attenuation, source-generation, site and spreading terms. First difference regularization was used to remove singularities and reduce noise effects.
The technique was applied to a data set of 1488, 1.0 Hz, Pg/Lg amplitude ratios from 13 stations for paths inside a 30° by 40° box covering western China and surrounding regions. Tomography reduced variance 60%, relative to the power law correction traditionally applied to amplitude ratios. Pg/Lg attenuation varied with geologic region, with low values in Tibet, intermediate in basins and high for platforms and older crust. Relative spreading was consistent with expected values for Pg and Lg. Relative site terms were similar to one another, while some tradeoff with attenuation was evident.
Residuals followed systematic trends with distance, resulting from the evolution from direct to coda phases, focusing, model tradeoff or measurement effects. Examination of the residuals using a kriging interpolator showed coherent geographical variations, indicating unmodeled path effects.
The residual patterns often follow geological boundaries, which could result from attenuating zones or blockages that are too thin to be resolved or that have anisotropic effect on regional phases. The interpolated residuals can be combined with predictions of the tomographic model to account for path effects in discrimination studies, on a station by station basis.