94 



RAITT 



Table I (contd.) 



[chap. 6 



( ) Assumed velocity 



« Recalculated by Ewing, Sutton and Officer (1954). 



* Anomalous value omitted from averages and standard deviations. 



Much of the seismic-refraction work at sea emphasizes attaining maximum 

 penetration in the crust and mantle. For this, large shots are needed and 

 large shot spacing is usually adequate for delineation of first arrivals of Layer 3 

 and Layer 4. However, a shot spacing adequate for the long segments of first 

 arrivals of Layers 3 and 4 is not adequate to reveal Layer 2, which requires 

 close spacing of shots at a distance near the critical distance of first emergence 

 of the refracted wave. 



In the Pacific (Tables III, IV and V) Layer 2 is ])resent nearly everywhere. 

 In the Atlantic (Tables I and II), however, Layer 2 is found at less than one- 

 third of the stations. It is not clear that these results prove a significant dif- 

 ference between the Atlantic and Pacific Oceans in the occurrence of Layer 2, 

 for it may be masked more frequently in the Atlantic by a greater thickness of 

 sediment. It is also possible that Layer 2 has escaped detection in the Atlantic 

 by other causes, such as an unfavorable distribution of shots. It is likely that 

 more observations of Layer 2 would have been made if shot jjatterns had been 

 fired siiecifically to observe Layer 2, or better yet, if it had been possible to 

 place hydrophones and shots on the bottom in the manner described by J. I. 

 Ewing in Chapter 1. The Cambridge method, using radio-sonobuoys and a 

 very close spacing of shot distance in the critical range, favors the ob- 

 servation of Layer 2. The Cambridge group finds Layer 2 about as frequently 



