100 T. I. Oblogina 



the isolation of useful ^vaves; the record becomes so difficult to decipher 

 that as a rule the individual waves cannot be distinguished. 



Of the three types of diffi-acting objects we have mentioned, the one which 

 has greatest practical importance is the diffi-acting edge. 



Comparatively little ^vork has been done on problems of diffi'action in 

 seismic prospecting. Most of the existing pajjers deal with particular 

 problems in the Idnematics of diffi'acted waves ^^' ^' ^' ^' ^K 0«ly in certain 

 experimental works do we find any indications of the relationship between 

 the amplitudes of diffi-acted waves and the ampUtudes of reflected and 

 refracted waves: some investigators stress the low intensity of diffracted 

 waves as compared with refracted waves (^), while others on the contrary 

 note that diffracted waves may have the same order of intensity as other 

 types of waves (^■'^» ^^K In essence, however, the dynamic properties of waves 

 in a case of diffraction have yet to be examined. 



This paper explains the kinematic basis for distinguishing diffi'acted waves 

 on seismograms, sets out recent experimental findings on the dynamic 

 properties of these waves and compares these findings with theoretical data. 



THE KINEMATIC PROBLEM OF DIFFRACTION FROM THE EDGE OF A VERTICAL 



CONTACT 



Let us imagine a combination of media consisting of a medium mth 

 a velocity Vq filling an upper half -plane, and media with velocities t\ and v^ 

 filling respectively the left-hand and right-hand quarters of a lower half -plane 

 (Fig. 1). Let the velocities Vq, v-^ and v^ be constants, and let Vq <,v-^'^V2. 

 We shall treat the case of f ^ < '^2 separately from the case of t\ > t'g. The Vq 

 medium is separated from the other two media by a horizontal interface; 

 these two in turn are separated from each other by a vertical boundary. 



Let a system consisting of a head and a shear wave move along the 

 horizontal interface from left to right. In Fig. 1 (a) the clashed line shows 

 the fronts of these waves at some moment of time f < 0, before diffraction 

 has occurred, and the direction of travel is indicated b}- arrows. It is required 

 to determine the kinematic pattern of Avave propagation after diffraction 

 occurs; that is, we have to find the position of the wave fronts in the plane xy 

 and the form of the wave hodographs* in the plane xt. 



We choose a system of co-ordinates xy such that its origin lies on a 

 diffracting edge, with the :;c-axis running along the horizontal interface and 

 the r-axis upwards towards the observation line (profile). The contact 



* A Russian term referring to distance-time curves. [Editor's footnote]. 



