John-poage 118 of thesis by Lowell Whiteside...I think a few sentences of quote is fair use:
" ....."in the layered, heterogenous, spheroidal earth, considerable variations in amplitude occur."
"....at 17 deg, 34.5 deg, 49.5 deg, 59.5 deg, 70.5deg), body waves are as large or larger than they are at 10 deg (Fig. 1, Kaila and Sarkar, 1975). These amplifications are caused by the combined amplitudes of several waves relfecting off internal discontinuities and reaching the surface simultaneously at cusps."
"There is also amplification of waves diffracting off the inner and outer core. These diffracted waves reach the surface at approximately 103 deg and 144 deg from their source."
etc etc.
Chris