The solid inner core of the Earth consists of heavy minerals Fe and Ni with a fraction of light elements such as O,S,Si.During the inner core formation,these light elements escape and rise up through the outer core as the result of buoyancy,but their existence is still being debated.Previous studies have presented seismological evidence for lowered wave speed beneath the core-mantle boundary (CMB) relative to PREM,suggesting light elements there,but counter argument also exists.In this study,we use traveltime measurements from recorded and modeled SmKS waves to investigate the effect of the velocity under the CMB on the differential traveltimes between SKKS and S3KS waves.A dataset of high-quality measurements of S3KS-SKKS differential traveltimes has been established from teleseismic records of globally distributed deep earthquakes.We compare the observed differential times with predictions by the direct-solution method in a suite of one-dimensional (1D) models with different structural profiles under the CMB to examine the effect of lowered velocity at the top of the outer core.We also carry out a Bayesian inversion for the 1D structure beneath the CMB using this dataset of the S3KS-SKKS differential traveltimes.