SEISMIC METHODS 



755 



Vertical Section in Which the Velocity Increases Continuously 



with Depth. — When the velocity 

 the path of the refracted ray has 

 the form shown in Figure 459. 

 Due to continued refraction, the 

 angle made by the ray with the 

 normal at any point along its path 

 becomes increasingly greater until 

 at some depth H the angle becomes 

 ecjual to 90°, where the ray path 

 becomes horizontal. Thereafter, the 

 effect produced by the continuous 

 variation of velocity is reversed and 

 the ray gradually swings upward 

 and returns to the surface along a 

 path RS which is symmetrical to 

 the path OR. 



To obtain the travel-time over 

 the path ORS, assume that the 

 velocity function may be written in 

 the form V = V(h) where V(h) is 

 continuous. The travel-time T and 



increases continuously with depth, 



Fig. 459. — Travel-time curve for a formation 

 in which the velocity increases linearly with 

 depth. (Dotted line C is the locus of the centers 

 of curvature of the ray paths.) 



