This work, was done in the mid-70's with Don Helmberger, while I was a post-doc at Cal Tech. Perhaps the most significant result of this work was that the apparent arrival times of horizontally-polarized ScS were systematically earlier than the arrival times of vertically-polarized ScS for at least one group of paths in our study. We interpreted that observation as being caused by a positive velocity gradient in the lower mantle, just above the core. More recently, researchers have observed the same phenomena, but interpret it as being due to anisotropy.

Mitchell, B.J. and D.V. Helmberger, Shear velocities at the base of the mantle from observations of S and ScS, J. Geophys. Res., 78, 6009-6020, 1973.