566 SEISMIC METHODS 



terms of ratios R = z/x, 



D.G. = 



[Chap. 9 



2Rd sin tp -\- I 



and 



U.G. 



W{2RdY + ^Rd sin <p-\-\ 



— .Ru sin ^ + 1 



vV(2/2„)2- 412„sinv+ 1' 



> (9-79e) 



Fig. 9-88. Time gradients for dipping bed. 



With these equations the diagram shown in Fig. 9-88 has been con- 

 structed. For high ratios of zjx the gradients become negative for com- 

 paratively small dip angles, that is, the reflection impulses travel back- 

 ward in the seismogram. For low angles of dip, travel times of dipping 

 beds arc virtually identical with those from horizontal beds; dip shooting 

 becomes applicable if dips exceed about 100 feet to the mile. However, 

 as a means of checking correlations, it is used with lesser angles of dip. 



