Top: standard deviation of the measured travel-time shifts (square root of the diagonal elements of Λ) as a function of distance. The zigzag might be due to the discreteness of the windows that isolate the single-skip wavelet in the cross-covariance function when fitting the travel-time shifts, particularly for short-distance cases in which the slope of the single-skip ridge in the time-distance diagram is steepest. Bottom: weighting function, w(Δ), as a function of distance. The corresponding radii of the lower turning points from the ray approximation are indicated at the top. The weighted mean distance (vertical dashed line) is about 14.6°, which corresponds to a lower turning point of ∼0.91 R_⊙.