REFRACTED AND REFLECTED LONGITUDINAL WAVES 



35 



For four -layered media with layer velocities which increase with depth, 

 that is Vq p < ^-'i p < ^^2 p "< %,p' ^^'^ ^^^ ^^^® show that as the difference 

 in the values of V2 „ and v^ decreases the intensity of head waves 

 P0P1P2P3P2P1P0 increases. If in a four -layered medium one or two of the 

 layers have a high speed, that is if the medium is characterized by one of 

 the ratios: 



^0, p < ^1, p > ^2, p < ^3, p' 



^0, p > ^1, p > ^2, p < ^3, p' 



^0,p>''l,p<i2,p<%,p 



Fig. 9. The increase in intensity of primary waves /head ^ three-layered media as 



the sharpness of the interface of layers diminishes at which the primary wave 



originates. 



%P 



(but the highest velocity in the overburden does not exceed 0.9 v^^ p) we 

 can then also show as the ratio v^ „/% p increases within the limits 0.3—0.9 

 the intensity of the head waves P^P-J^^^zP^PiPq^ although it increases, 

 does not do so by a factor of more than 0.7. 



The greater the number of interfaces with v^ p < ^j+i p which separate 

 the intermediate boundary under consideration from the refracting boundary 



