SEISMIC METHODS 



719 



depend on trend of data obtained from scattered up-hole shooting in 

 a given region. 



Figure 439 shows the ray paths for reflected waves and the paths for 

 first-break or first arrival-times. The shot usually is placed below the low 



W/'fiii^''!.VA>MSy//^ 



Fig. 439. — Ray paths for reflected and refracted rays. 1, first 

 arrival ray paths; 2, reflected ray paths. 



velocity layer. For the present investigation, it will be assumed that the 

 shot is placed at a short distance below the base of the low velocity zone 

 and that the ray paths for first ar- 

 rival waves are essentially horizon- 

 tal in that part of the paths which is 

 in the consolidated zone, i.e., below 

 the low velocity layer. Figure 440 

 shows the corresponding low velo- 

 city zone refraction diagram, where 

 first arrival-times are plotted versus 

 distance from shot-point. The slope 

 of the best-fitting straight line 

 through these points is the recipro- 

 cal of the velocity Vc in the top of 

 the consolidated section. 



Treating the velocity Vu in the low velocity layer as though it were 

 constant in the vertical direction, the angle of the first arrival ray at the 

 surface is given approximately by the critical angle equation 



Fig. 440. — Low velocity layer refraction 

 diagram. 



sm Bu = 



Vc 



where Vu,k is the average vertical velocity between the base of the low 



