DIFFRACTED SEISMIC WAVES 105 



character which is pecuhar to a diffracted wave and distinguishes it from all 

 other types of seismic waves. 



We may further note that in overtaking systems the tangential points 

 of the hodographs of the diffracted and the head waves also correspond to 

 a single marker peg on the profile. 



FIELD OBSERVATIONS 



The field observations were made in a region ^vhere the geological cross- 

 section included primarily aranaccous and sandstone deposits of Cretaceous, 

 Ternary and Quaternary ages. 



The Cretaceous deposits lie directly on a Paleozoic basement at a depth 

 of about 1000 m. Ihe top of this basement contains rocks of different 

 lithological varieties bedded in the form of a block of steeply dipping strata. 

 Diffraction of vertical head waves ^vas observed to occur from the contact 

 edges between these strata, which have different velocities. 



The observations were made mth a standard detector whose response 

 curve had its maximum at /^ax ^ ^"^ ^h- 



Kinematic Properties. 



Three groups of waves -were recorded in the covirse of seismic observations 

 in the sector mentioned: A, B and C. Group A consisted of refracted waves 

 with low apparent velocities (1200-1600 m/sec); group 5 of refracted waves 

 with high apparent velocities (5000-6500 m/sec) ; group C of diffracted 

 waves. 



In this paper we shall treat in detail only the characteristics of the 

 diffracted waves. The seismograms given below show diffracted ^vaves 

 recorded after the refracted waves ^sith high apparent velocities. The hodo- 

 graphs of these waves invariably have an almost hyperbolic shape. 



On the seismograms in Fig. 3, group A refracted waves, a group B 

 refracted wave and a group C diffracted wave can be distinguished. The 

 diffracted wave t(^, which has a distinctly curvilinear axis of co-phasality, was 

 followed over a distance of 450 m. 



Another example of a diffracted wave trace is given in Fig. 4. Here the 

 first waves to arrive are the B group waves t^, t^ and t^. A diffracted 

 wave tf-. is recorded behind the wave t^. This can be seen on the seismogram 

 at a distance of 5100 rn from the shot point and followed up to a distance 

 of 5520 m, where it is already converging -with the wave t^. The diffracted 

 wave has a distinctly curvilinear axis of co-phasality; its intensity is roughly 

 the same as that of the wave t^. 



