SEISMIC METHODS 



763 



Oi' and O2'.) Obviously, this process can be continued for any number 

 of layers. 



The discussion given above is restricted to ideal cases only because 

 space does not permit coverage of field records. The solution of special 

 cases must depend primarily on the diligence of the computer and can 

 be learned only through experience. Usually each experienced operator 

 eventually develops his own computing technique. 



Fig. 461. — A, travel-time curves corresponding to a geo- 

 logical structure which consists of two inclined layers; B, 

 computed curves obtained when the three layer case is 

 "reduced" to a two layer case. Curve 1 is the travel-time 

 curve for down dip shooting (shot-point at Oi) ; Curve 2 

 is the travel-time curve for up dip shooting (shot-point at 

 O2). (After W. M. Rust, Jr.) 



Numerical Illustration 



The values of the apparent velocities obtained from the travel-time 

 curve are : 



Fi = 6000 ft./sec, ^ = 1.67x 10"* 



Fa' =8250 ft./sec. 

 F2"= 10,000 ft./sec. 



