SECT. 1] 



SEISMIC REFRACTION AND REFLECTION MEASUREMENTS 



13 



interface results in downward refraction owing to the fact that the com- 

 pressional wave velocity, a4, in Layer 4 is higher than the shear-wave velocity, 

 185, in Layer 5 ; hence there can be no arrivals at the surface in this case. It is 



HYDROPHONE 



DISTANCE (KM) 

 10 15 



_1 I I _L 



SHOT 



Fig. 8. Ray diagram and time-distance graph for commonly observed arrivals in oceanic 

 areas. 



theoretically possible to receive an arrival which was first transformed from 

 compression to shear at the 3-4 interface, then was re -transformed to com- 

 pressional and refracted horizontally at the 4-5 interface. Such an arrival has 

 not been identified, however, very possibly owing to insufficient energy transfer 



