108 T. I. Oblogina 



was used in the work and the extremely low intensity of these waves was 

 noted. 



We must also emphasize that the dominant frequency of the diffracted 

 waves is lower than that of the head wave. Therefore an apparatus for which 

 the response curves have a steep left-hand slope and which is designed to 

 record high frequencies produces marked distortions in tracing diffracted 

 waves. 



Dynamic Characteristics — Examining the diffracted waves on the 

 seismograms we can see that the amplitudes of the vertical component 

 of displacement of a diffracted wave are different at different points on 



Fig. 6. Seismogram showing growth of diffiacted wave amplitude near point of 

 contact with refracted wave. 



the profile, and that a diffracted v,'ave has its greatest intensity in the 

 neighbourhood of the point at which is tangential to a refracted wave. Figure 6 

 shews a seismogram on Avhich a refracted wave is recorded in the first onsets 

 and a diffracted wave in the last onsets.. As we see, the amplitudes of the 

 diffracted wave increase along the profile as the wave approaches the 

 point at which it is tangential to touch the refracted wave. 



From analysis of the seismograms it follows that in the vicinity of a point 

 of tangentionality the phase of the diffracted wave changes. As a rule this gives 



