HISTORICAL BACKGROUND 



Early mariners probed the sea bottom in shallow 

 nearshore waters and brought up samples of sand, clay, 

 and rock. Their major interest was in making safe passages 

 across the oceans and in and out of harbors and channels. 

 The shortness of their sounding lines prevented them from 

 reaching the deeper sea floor surfaces. Consequently little 

 was learned about the detailed microcharacter of the deep 

 sea floor. On the basis of meager knowledge and scattered 

 samples, many thought of the sea floor as an extremely 

 flat, monotonous plain composed of a soft, clayey upper 

 surface that blended gradually upward into seawater. In 

 recent times, sonic depth finders, deep-sea cameras, 

 modern sampling equipment, acoustic probes, and manned 

 diving submersibles have revealed the structure and char- 

 acter of what is referred to as microrelief at the sea floor. 



Major underwater topographic relief features have 

 now been located and mapped. Distinct provinces encompass- 

 ing similar features have been recognized and delineated. ' 2 

 Intermediate bottom features are now being studied to make 

 our knowledge of the sea floor more comprehensive and to 

 provide a basis for the examination of microroughness and 

 internal structures. 3 It has been shown, for example, that 

 abyssal hills cover 80 to 85 percent of the Pacific floor 

 alone while, in other areas, bottom environments vary from 

 smooth plains to steep eroded escarpments, mountain ranges, 

 and fracture zones of irregular roughness. 



For many years the very small vertical relief on the 

 sea floor was not thought to be significant to mankind. Re- 

 cent advances in methods of propagating and receiving 

 underwater sound signals, however, have shown that the 

 roughness formed by natural chemical, physical, and bio- 

 logical forces at the sea floor has sufficient height and 

 extent to affect the reflection and scattering of certain 

 sound frequencies therefrom. Thus, Urick made back- 

 scattering studies of the bottom in 1954 with a tiltable trans- 

 ducer at several locations in a harbor on the East Coast. 



Superscript numbers denote references in the list at the 

 end of this report. 



