86 



RAITT 



[OHAP. 6 



as a first arrival are : Layer 2, 7 km to 12 km ; Layer 3, 12 km to 36 km ; Layer 

 4, >36 km. 



In actual practice the time-distance plots differ from the idealized model of 

 Fig. 1 for various reasons : 



1. Layer velocities and thicknesses differ somewhat from the assumed 

 values. 



2. Layering is not horizontal. 



3. The velocity within one layer may vary. 



4. A layer may be prevented from observation as a fii'st arrival by prior 

 arrivals from another layer. 



5 10 15 



Direct wafer wave travel time (sec) 



20 



Fig. 2. Travel-time plot for Challenger Station CR9 near the Hawaiian Islands. Layer 

 2 velocity = 4.30 km/sec; Layer 3 velocity = 6.58 km/sec. (After Gaskell, Hill and 

 Swallow, 1958.) 



^ 10 



0) 



E 



5 10 



Direct water wave travel time (sec) 



Fig. 3. Travel-time plot of Challenger Station 32 in the Atlantic near the British Isles. 

 Layer 2 velocity = 4.36 km/sec; Layer 3 velocity = 7.15 km/sec. (After Gaskell, Hill 

 and Swallow, 1958.) 



Nevertheless, widely spaced deep-sea seismic-refraction stations usually 

 have a strong similarity to the model. Studies at widely separated stations in 

 the Atlantic and Pacific Oceans have shown the presence of all of the foin- 

 layers. In the Indian Ocean the few observations of Gaskell, Hill and Swallow 

 (1958) on H.M.S. Challenger did not reach Layer 4, but Layers 1, 2 and 3 were 

 observed. 



These seismic-refraction studies are not yet as widely distributed over the 

 world oceans as might be desired. They tend to be concentrated in areas of 

 good weather, or in regions close to the operating bases of the investigating 



