DIFFRACTED SEISMIC WAVES 



103 



words, the shear wave passes from the lower -velocity medium to the 

 higher -velocity medium. 



By a similar process of reasoning we obtain the arrangement of wave fronts 

 for some moment of time « > in the plane xy (Fig. 2, d) and the position 

 of the wave hodographs in the plane x, t (Fig. 2, h). 



Fig. 2. Fronts and hodographs of a diffracted and a head wave in a case where a shear 

 wave passes from a lower- to a higher-velocity medium. 



On the sector of the profile 



Ax — H{tan oci — tan x^ — Hvq 



]Jv^ - V^ Y^ - Vq^ 



the hodographs of ti{x) and ^2^^) intersect, while the hodograph of the 

 diffracted wave tQ{x) touches these hodographs at the points M(the beginning 

 of the hodograph of t^^ix)) and M (the end of the hodograph of ti{x)). The 

 hodographs of tQ{x), ti{x) and t2{x) form a kind of loop between the points 

 L, M, N. This case differs from the previous one in that the diffracted wave 

 does not emerge into the first onsets. 



Combining both the cases we have examined, we conclude that the minimum 

 on the direct and inverse hodographs corresponds to one and the same marker 

 peg on the profile ; it lies above the diffracting edge. Here we have a diagnostic 



