SEISMIC METHODS 



765 



which are too thin do not show up in the travel-time curves. For example, 

 refracted waves traversing the structures a, b, c, and d (Figure 462B) 

 have the same travel-time curve (Figure 462 A) even though the geological 

 conditions are radically different, d, only, can be differentiated from the 

 others by use of a refraction shot in the reversed direction. (Figure 462B, 

 (c) shows the case of an intermediate thin layer. A layer of this thickness 

 would, in general, be detected.) 



I* /« TO to tt 

 X (Thousand /ke/) 



B 



'Mm'/Zli^Aii^ 



/I- 2970 f- /^ ^SOOOfAptr Sfco/fi/. 



WA!f/,iyAy//iir 



^2 ' /ZOOO /y-prf Sfconcf 



^//m/mmj% 



h'Z490ff. [/, =9000/?. per steoncf 



WV^WAV^VAW 



vWANVAWAWAW 



/)= 960ft. Vz-' llOOOff. p9r Mcond 



\/3'=-IZOOO /A p»r atcond 



.^/^/^w^- f,fyffefttn,0'3l7Of1. 





Fig. 462. — The same travel-time curve (portion ^) is obtained 

 for refracted waves traversing the structures a, b, c, d (portion 

 B). (After W. M. Rust, Jr.) 



An interesting example of the use of refracted waves other than first arrivals is 

 the earlier work of Ewing, Crary and Rutherford f on the Atlantic Coastal Plain. By 

 using the indications of the arrival of waves refracted through beds that were not 

 thick enough to give first arrivals, they were able to increase the accuracy of their 

 work and to discriminate between cases corresponding to (a) and (6) of Figure 462B. 



Stated briefly, the assumptions on which refraction calculations are 

 based are: 



1. The velocities in successive strata increase as the depth increases. 



t Bulletin of the Geological Society of America, Vol. 48 (1937) pp. 753-802. 



