Chap. 9] 



SEISMIC METHODS 



519 



the section is split up into individ- 

 ual layers as discussed in the 

 preceding chapter. Data pertain- 

 ing to the normal section may 

 be obtained from off-dome travel- 

 time curves. 



Figs. 9-54, 9-55, and 9-56 are 

 examples of the problem treated 

 here, shooting downward from a 

 faulted block, from the edge of a 

 salt dome, and from a limestone 

 ridge. 



When faults, scarps, terraces, and 

 the like, have shapes indicated in 

 Figs. 9-57 and 9-58, additional 

 branches appear at the end of the 

 travel-time curve because the faulted- 

 down portions of the high-speed 

 media are within range. Down scarp, the last part of the travel- time 

 curve is given by 



Fig. 9-57. Travel-time curve for deep- 

 seated escarpment. 



U 



Vi cos I 



-. (c/i + di) + 



s — e — di tan i 



V2 



+ 



-\/{e — di tan lY -{■ (dj — diY 



V2 



(9-49a) 



If the displacement of the fault is small, the sloping path from the point 

 of incidence to the bottom of the scarp may be assumed to be equal to the 

 horizontal path along its surface, so that 



. 2d\ cos I . s , (di — di) cos i 



U = -r — + : 



Vi V2 Vi 



(9^96) 



By differentiation it follows that this part of the travel-time curve has the 

 same slope as the second part. By subtracting ti from U ; we obtain At 

 (see Fig, 9-57), which gives the height (dz — di) of the scarp: 



d2 — di = AtVi cos i. 



(9-49c) 



In the reverse direction of shooting, the rays emerging from the top 

 surface on the right are no longer parallel because of the different angles 

 of incidence from below. This causes a deviation of the last part (^4) of 



3* I. Roman, A.I.M.E. Geophys. Pros., 493 (1934). 



