The ak135 velocity model is designed to provide a good fit to a
wide variety of seismic phases and particularly for S represents an
improvement on the iasp91 model.
The details of the construction procedure are described in
Kennett, B.L.N. Engdahl, E.R. & Buland R., 1995.
Constraints on seismic velocities in the Earth from travel times, Geophys J Int, 122, 108-124
