Chap. 9] 



SEISMIC METHODS 



547 



the travel time ti = Z cotan i/Yi + s cos ^/va + z cotan i/Y^ ; with Co = 

 Z cotan i/Yi as shot point constant, C = cotan i/v^ as depth point constant, 



fc = Co + '-^^ + C'z 



V2 



(9-67o) 



^— 



Trai/e/ Time 

 second} 

 -0.5 



-0.7 



=:i 



T^575^555^^J55^7n^^^''7:5J577^'^^^57?^j:^^ 



A?^ 



/4^ 



JKAV 



40,m 



(o) 



/<i;a!%7 





?5^557!^^5^5J^5?55SP55:j55iJ^155^^^?;;55:^7^J5^^ 



Fig. 9-74. (a) Longitudinal, and (6) transverse profiles, with travel times and lime- 

 stone profile (arc method). (After Jones.) 



Since cos <p "^ \ for small dips, the depth at any point 



c V " V2 " ^V' 



2 = ^, U2 - — 



(9-676) 



which, strictly speaking, is depth normal to formation but for small dips 

 may be taken as vertical depth. With t' ^ h — s/v2 , eq. (9-676) may 

 be written 



z = ^, (f - Co) 



(9-67c) 



