SECT. 1] THE UNCONSOLIDATED SEDIMENTS 81 



Hill (1952). The lowest was measured on cruise VEMA-15, Profile A of Fig. 2. 

 The low value of the gradient in this case is not unreasonable, because it is the 

 average gradient in approximately 1 km of sediments. The higher values come 

 from profiles in which only the ujjper 0.2-0.4 km was measured, and higher 

 gradients are expected near the sea floor. Hill's value of 2.5 sec~i was also 

 measured over a depth of ajDproximately 0.4 km. A possible alternative inter- 

 pretation of his results, allowing that the closer points represent reflections 

 instead of refractions, could be made which would give a lower value of gradient. 

 As shown in Figs. 2 and 4, if there is a sub-bottom reflector at a depth less than 

 that reached by the ray corresponding to the cusp, the reflection curve Rn 

 associated with it will join smoothly to the Rs curve, and it would be difficult 

 to distinguish between the two types of arrival. 



Table I 

 Velocity Gradients in the Sedimentary Layer 



4. Normal-Incidence Reflections 



In addition to variable-angle reflection and refraction measurements, a large 

 number of normal-incidence reflection measurements have been made. Only a 

 small percentage of these have been published (Hersey and Ewing, 1949; 

 Shor, 1959). Hersey and Ewing made certain classifications of reflection 

 records on the basis of the character of the bottom reflection and of the number 

 and type of sub-bottom reflections. They were able to show some correlation 

 between reflection types and physiographic provinces for certain areas of the 

 western North Atlantic. Shor's measurements in the Pacific covered the 

 boundary between the present-day clay deposition and carbonate dejaosition 

 areas and showed markedly thicker sedimentary cover in the latter (southern) 

 region. His results also showed greater accumulation of material in valleys than 

 on hills. 



4 — 9. Ill 



