DIFFRACTED SEISMIC WAVES 



101 



edge— the rectilinear diffracting edge — will then pass through the origin 

 of the co-ordinates normally to the plane of drawing. 



We shall now examine the case when i\ is greater than ly, that is, when 

 the shear wave passes from the medium with the higher velocity into the 

 medium with the louver velocity. At a mom.ent of time ^ = 0, let the system 

 consisting of the head ^\■ave and the shear wave reach the vertical contact 



Fig. 1. Fronts and hodographs of a diffracted and a head wave in a case where a shear 

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



edge 0. According to Huygens's principle, this edge is, as it were, the 

 source of a diffracted wave travelling in all directions. In Fig. 1, (a) shows 

 the position of the wave fronts at a moment of time ^ > 0, when the wave 

 has already encovmtered the contact edge. The front of the diffracted wave 

 in the Vq medium will be the semicircle KCDEF with radius VqI; in the fj 

 medium it will be the quarter circle GA with radius v\t; and in the fg medium 

 it will be the quarter circle MB with radius v^t. 



The shear wave is partly reflected from the vertical contact and partly 

 refracted by it. The reflected shear wave causes a head wave EG on the 

 path FG, while the refracted shear wave will pull the head wave CB on 



