508 



SEISMIC METHODS 



[Chap. 9 



equating (9-42) and (9-43), substituting x for s, and multiplying both 

 sides by Vi/cos i, 



2 \ cos 



1 1 2 y V2 + vi 



(9-44a) 



where C\ = 



1 i A-^M 



2 y V2 + Vi 



is a constant for any area as long as overburden 



and underlayer velocities remain constant. A number of travel-time 



Fig. 9-46. Strike travel-time curve on anticline of Masjid-I-Suleiman, Persia (after 



Rankine). 



curves for a single horizontal bed of varying thickness are given in Fig. 9-41. 

 Fig. 9-46 is a good example of a depth determination in the single-layer 

 case (limestone under shale on crest of anticline). 



From eq. (9-44) it is seen that the depth is always less than a:/2. The 

 break in the travel-time curve occurs at distances at least twice the depth 

 when the velocity contrast is great, but the break moves further out from 

 the shot point as the contrast is reduced. If q is refractive index and r 

 its reciprocal (ratio of velocity in lower and upper medium), 



X = 2d /</ ^-3-j and x = 2d \/\^ 



(9-446)" 



" Examples of depth calculations are given on p. 510. 



