726 



EXPLORATION GEOPHYSICS 



It is to be noted that the value of JVc obtained by Equation 81 depends 

 directly on the value of Vu, and therefore uncertainties in the unconsoli- 

 dated layer velocity will introduce corresponding errors in the determina- 

 tion of the thickness of the low velocity layer. A check on the value of F^ 

 may be secured by frequent direct measurements by shooting a charge at 

 various depths and recording the travel time to the surface. See Figure 438. 

 When the time is plotted as a function of depth, the velocities are obtained 

 from the reciprocal slopes of the time depth curve. Often these plots show a 

 sharp break at the bottom of the low velocity layer as shown by Figure 438. 

 At other locations the time-depth plots will show a gradual transition 



rather than a sharp discontinuity of the 

 velocity at the base of the low velocity 

 zone. This is shown in Figure 444. In 

 this case an average value of the ve- 

 locity used is equal to the depth at 

 which the velocity first reaches Vc, di- 

 vided by the travel-time to that depth. 

 In practice a continuous check of 

 the value of Vu is maintained by a study 

 of the up-hole times recorded at each 

 shot. This procedure also gives information as to whether or not the 

 explosive charge has floated up the hole and serves as a means of explaining 

 certain types of seismograms obtained in field practice. 



The value of Vc has been found to vary within certain limits. This 

 velocity can be checked by a study of the travel-time distance plot of the 

 instrument spreads. However, upon an inspection of Equation 81 it will 

 be noted that small uncertainties in the value of Fc do not result in large 

 percentage errors in the value of JVc, the thickness of the low velocity layer 

 at station C. 



Some systems of computation do not make use of the thickness of the 

 low velocity layer, but instead make the correction for the delay in the low 

 velocity layer by use of the time Twc, the time necessary for the wave to 

 pass through this layer. An expression for this quantity Two may be 

 written from inspection of Equation 81. 



Fig. 444. — Graph showing velocity increasing 

 with depth. 



Vu 



— I Wr, — 



T 



PQO 



+ TsjRO — 



Vc) 



(83) 



In making corrections for the delay caused by the presence of the low 

 velocity layer, it should always be borne in mind that the wave paths of 

 the F-wave and the reflected waves through the low velocity layer are 

 different. This fact constitutes an inherent source of error when the low 

 velocity correction is made from the F-wave alone. Although a reliable 

 correction can usually be made, there are occasions, particularly when the 

 low velocity layer is thick and the ray paths have a large angle with the 

 vertical, when the uncertainty is large. 



