Chap. 9] 



SEISMIC METHODS 



521 



comparatively simple, inasmuch as the velocity in the red sandstone was 

 almost identical with the velocity in the overburden. The profile shot 

 down the fault from A is almost the same as the ideal profile of Fig. 9-57. 

 However, in the second part of the travel-time curve the velocity is not a 

 true but an apparent velocity, since the profile is shot down the dip. 

 Therefore, the velocity in the last part of the travel-time curve does not 

 coincide with that in the second part. Furthermore, the third part of 

 the curve is not curved as required by the theory but is straight. 



B and C represent the up-fault and also the up-dip profiles, as far as the 

 boundary between overburden and limestone to the east of the fault is 

 concerned. This explains the negative apparent velocity in profile C, 

 second part of the curve. Profile D was shot in the strike, along the line 



rdown<f/p(^d) 



Fig. 9-60. Schematic up- and down-dip travel-time curves. 



indicated in the figure. The travel-time curve is that of a single layer; 

 the calculated depth was 245 feet, the actual depth 257 feet. Considering 

 that the dip is 10° and that the depth calculated from the refraction 

 profile is the oblique depth, the discrepancy of 12 feet is reduced to 8 feet. 

 9. Single dipping layer. In the cases previously considered, the travel- 

 time curves gave, with a few exceptions, the true velocities of the sub- 

 surface formations. For dipping layers this is no longer true; the slope 

 of the travel-time curve depends upon the dip. Hence, it no longer repre- 

 sents the true underlayer velocity but an apparent velocity which depends 

 on both the dip and the velocity ratio. As seen in Fig. 9-60, the first 

 part of the travel-time curve always corresponds to Vi . If the bed dips 

 away from the origin, the seismic ray travels a greater distance through 

 the upper medium. This results in increased travel time, decreased ap- 



