82 



S. D. Shushakov 



In general, multiple waves reflected from a thin layer must be character- 

 ized by complex oscillations consisting of vibrations reflected from the base 

 and top of that layer. Reflections from thin layers have been described 

 in works by Gurvich (^) Lyamshev <^) and Ivakin <'>. Fig. 5 shows theoretical 

 hodographs of such waves. At short distances from the source, waves reflected 

 irom the top and base of the layer may pile up one upon another. As the 



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200 



Fig. 5. Theoretical hodographs of multiple reflections. 



distance from the source increases, the phase shifts between the component 

 oscillations change, and head waves appear, superimposed on waves reflected 

 from the base of the layer. At great distances from the source the phase 

 shifts between the oscillations of some reflected waves increase with distance, 

 but the phase shifts between the oscillations of the head waves and of the 

 waves reflected from the base of the layer diminish. Many of these phase 

 shifts increase with the number of multiples. As the number of multiples 

 increases, or as distance from the source increases, the complex oscillations 

 resolve into groups of simpler componental oscillations which are separated 

 in time. 



Under real conditions it is difficult to distinguish between the multiple 

 and single reflections from the shape of the oscillations. The problem has 



