84 



S. D. Shush AKOV 



The shape of the muhiple wave, hke that of the single wave, maintains to 

 a certain distance from the source, but changes as the distance gi'ows further 

 stilh The point at which the shape of the osciUations changes from constant 

 to variable is the point of origua of the head waves. Beyond this point the 

 shape of the multiple wave may not be repeated as the number of multiples 

 increases at any one distance from the source. No such change in form is 

 iound in certain cases of modelhng multiple waves in thm reflecting layers. 



Water /7| a, H 



Fig. 6. Theoretical seismograms calculated for large difference between the wave 



resistances of the media separating the reflecting plane. 



^1=1.0, ai=1450m/sec, if=10cm, §2=2,47. ao=5490n/sec, 5=2700 m/sec. 



This may be because in these cases it is an interference wave consisting of 

 waves reflected from the roof and base of the layer which is recorded. If 

 the wave resistance diminishes as the wave passes through one of these 

 boundaries, there will be no points of origin of head waves and the correspond- 

 ing component Mill keep its form at any distance from the source. This 

 /Component, superimposed on a second component for wliich there are 

 points of origin of head waves, can cause the changes in its shape to be 

 lunnoticeable. Thus, in the case of thin layers it can happen that the changes 

 in shape wliich are chiefly observed are those caused by wave interference 

 of the kind described above. This may explain the fact that, as the distance 

 from the source increases, the single and multiple reflections from many 

 tliin layers are easier to follow than the reflections from tliick layers. 



The modelhng of multiple waves has also shown that the principal frequencies 

 become considerably higher as the thickness of the thin reflecting layer 



