82 EWINC; AND XAFK [OHAP. 5 



Sub-bottom reflections on standard echo-sounding records have been dis- 

 cussed by Hersey and Rutstein (1958), Heezen, Tharp and Ewing (1959), 

 Worzel (1959) and Ewing, Luskin, Roberts and Hirshman (1960). These 

 studies Imve shown that, in many areas, penetration of the sediments to depths 

 greater than 100 ft is achieved, even at the relatively high frequencies employed 

 in the sounding equipment (12 kc/s). The sub-bottom echoes are often stronger 

 than the bottom reflection. In certain areas, particularly on moderate topo- 

 graphic highs, these sub-bottom interfaces can be traced for many miles. As 

 in Shor's results, the depth to the sub-bottom reflector is usually correlated 

 with topography ; the upper layer thins or ])inches out on hills and thickens in 

 valleys. In abyssal plains, the sub-bottom reflectors are usually not as con- 

 tinuous as on the rises, as might be expected when one considers that the 

 plains are subjected to turbidity current deposition and the rises only to pelagic 

 sedimentation. 



Eff"orts to sample specific sub-bottom interfaces with coring apparatus have 

 been made on two occasions. Worzel identified a prominent reflector in the 

 southeastern Pacific as a widespread layer of white volcanic ash. Ewing et al. 

 (1960) found that strong reflectors on the outer rise of the Puerto Rico 

 Trench correlated with increases in rigidity, presumably associated with 

 increased carbonate content. 



Continuous profiling of sub-bottom reflections in shallow-water areas is 

 described by Hersey in Chapter 4. This recent development has made it possible 

 to obtain detailed surveys of sediment structure to depths of thousands of feet 

 in some areas and ofifers great advances in the study of marine stratigraphy. 



5. Summary 



Measurements of velocity gradients by variable-range reflection studies give 

 average values between 0.9 and 1.4 sec~i for the upper 0.2 to 0.4 km of sediments 

 in deep-water areas. With the evidence cited previously for low velocities 

 immediately below the water-sediment interface, we can consider that the 

 velocity versus depth relationship most common in deep-water sediments is 

 approximately described by a parabolic or exponential function in which the 

 upper sediments have velocities equal to or slightly lower than that in the 

 water. As greater depth below the sea floor is reached, the high initial gradient 

 diminishes rapidly and may become insignificant at depth in some areas. 

 There is some indication that the high gradients persist to some depth 

 at which de-watering is largely achieved. Below this depth compaction would 

 be retarded, resulting in lower gradients. Such behavior would be consistent 

 with observations of porosity versus pressure of the kind summarized by 

 Hamilton (1959) on the consolidation of sediments. Porosity decreases rapidly 

 at first, then more slowly with increasing pressure, and would be expected to 

 result in the variable gradients of the type observed. 



As shown by Nafe and Drake (1957), gradients in shallow-water sediments 

 generally do not vary as markedly with depth as do those in deep water. 



