516 



SEISMIC METHODS 



[Chap. 9 



In the simple case illustrated in Fig. 9-52/^ the first part of the curve 

 is given by ti = s/V] , Then follows, past the intercept Xi , the travel 

 time ti for a single horizontal layer. This is s/v2 + 2d/vi cos i. Beyond 

 the edge of the step the rays travel through the low-velocity medium only. 

 Their path becomes increasmgly horizontal with increasing distance so 



l^ • SOOO m-a*c 



Fig. 9-52. Travel-time curve for deep- Fig. 9-53. Reverse travel-time curve for 

 seated fault block. deep-seated fault block. 



that the travel-time curve past x^ rapidly approaches the slope corre- 

 sponding to the overburden velocity Vi . Then the travel time 



U = 



Vi cos % 



e — d tan i . s/d^ + (s — e)^ 



V2 



Vi 



or 



^ e . d cos I . -y/d^ + (s — eY 

 13 = — -\ -\ 



V2 Vi Vi 



(9-48a) 



For greater distances the last term approaches s — e/vi , so that ds/dts — 

 Vi . The distance of the scarp from the shot point is given by 



e = X2 — d tan i; 



(9-486) 



d is calculated from Xi and i follows from Vi and V2 . This formula will 

 give only an approximate value, for X2 cannot be accurately determined. 



" Travel-time curves in these figures were calculated for long-range shots to 

 determine regional basement structure. 



