106 T. I. Oblogina 



Specific observations showed that minima on the opposing hyperbolic 

 hodographs of the diffracted waves corresponded to one and the same marker 

 peg on the profile. Figure 5 shows two seismograms a and b, one of which 

 corresponds to the direct hodograph and the other to its inverse. On both 

 seismograms it can be seen that the minimum travel time of the diffracted 

 wave corresponds to the 5100 marker. 



Fig. 4. Example of a diffracted wave trace t^, t^, fg-group B refracted waves; 



f(;;— diffracted wave. 



Note the following point about the relationship between the intensity 

 of the recorded vertical component of the diffracted wave and the angle 

 at which this w^ave approaches the detectors. For a distance x along the 

 profile from the projection of the diffracting edge on to the plane of the 

 observations, when x is not greater than the depth H of the diffracting edge 

 {x < H), the angle of approach (p of the diffracted wave (that is, the angle 

 between the diffracted ray and the observation line) is not less than 45° 

 (9? ^45°). Within the range of such x distances therefore, depending on 

 the depth H, a fairly intensive vertical component is recorded; whereas 

 for X ^ H the horizontal component will be greater than the vertical. There 

 is a particularly marked variation in the approach angle when shallow 

 depths are being prospected. The only indication that diffracted waves 

 have been recorded during shallow depth prospecting which is to be found 

 in the literature is in Yepinat'yeva's paper (^). Ordinary seismic apparatus 



