86 



S. D. Shushakov 



are not so evident and some singles can be distinguished. Bortfeld^^^) 

 observed the same thing in north-western Germany. Under certain conditions 

 then, mukiple reflections can be more intensive than single ones. 



Some investigators, comparing the intensities of multiple reflections of 

 different types, point out that full-path echoes are the more intensive <^^). 

 Such waves can in fact be distinguished by their records in the Krasnodar 

 country, in the Ukraine and in many other regions. This can be explained 

 to some extent by the fact that such waves are more noticeable owing to the 

 periodic repetition of their records. Bortfeld, however (^-^J, demonstrates 



Fig. 7. Asymmetrical paths of wave propagation. 



that under certain conditions interbed echoes can be more intensive than 

 fuU-path ones. The reason is that an interbed echo does not have a single 

 path along which it is propagated, but when the reflecting boundaries are 

 bedded horizontally, simultaneously arrives at the observation point and 

 there causes co-phasally accumulated vibrations (Fig. 7). With increased 

 angles of gradient of the reflecting boundaries or with increased differences 

 between these angles, the phase displacements of the component vibrations 

 increase. As a result of such interference some multiple waves can be 

 distinguished while others are to some extent suppressed. 



The intensity of different types of multiple waves and their dependence 

 on seismological conditions can be estimated by using tables compiled 

 according to the method of contour integrals (^''\ The complicated relation- 

 ships obtained by this method have been simpUfied by K. I. Ogurtsov 

 for the following simple conditions : an excitant force applied to a free surface ; 

 a lower reflecting boundary which is the surface of a solid elastic halfspace 

 covered by a medium in which no transverse waves are excited, while the 

 upper reflecting boundary is the surface of this medium parallel to the lower 



