Chav. 9] 



SEISMIC METHODS 



561 



or 



, 2 . ,_ikx 

 < = z- sin n r— . 

 k 2vo 



(9-73a) 



By introducing the following dimensionless quantities: t = k^/2; d = 

 ka:/2vo ; v = (vo + k/i)/vo = v^/vo, the travel time may be written 



X = cos K 



f-^^] 



(9-736) 



200 



400 



600 



800 



1000 



1200 



Fttt 



Fig. 9-85. Reflection travel-time curves for three depths. 



A graph of reflection travel-time curves for a linear increase of velocity 

 with depth is shown in Fig. 9-84. Fig. 9-85 gives travel-time curves for 

 three reflections originating in a Cretaceous section in Canada. These 

 curves become flatter with depth, which is caused by both the increase 

 in depth and the increase in average velocity with depth. 



