means of an evolving series of geological or geophysical instruments of in- 

 creasing precision and discrimination. These methods are providing increas- 

 ingly accurate data on bottom topography and roughness which aid in un- 

 derstanding reflection loss at the bottom as a function of composition, rough- 

 ness, frequency, and grazing angle. 



Ambient noise is also a primary concern. Because sonar systems must detect 

 their target signal against a background of noise, it is necessary that the 

 noise field be as well understood as transmission and scattering phenomena. 

 Present investigations are concerned with identifying the possible sources of 

 the noise, mesisuring their source strength, and statistically describing their 

 fluctuation in time. In addition, measurements of ambient noise as a function 

 of depth and of both horizontal and vertical directionality are important 

 aspects of the program. Interim models for the prediction of ambient noise 

 are being developed, validated, and improved as new understanding of the 

 physical processes is developed. 



Major experiments were conducted during the year in the Mediterranean 

 Sea and in the North Atlantic, North Pacific, and Indian Oceans to measure 

 biological reverberation, bottom loss, scattering layers, ambient noise, and 

 other characteristics. An acoustic transmission study — the first of its kind — 

 between Cobb Seamount and another undersea mountain off the Pacific 

 coast gave particular attention to the effect on propagation of the water mass 

 microstructure. Six east and gulf coast studies were made using the stable 

 buoy platform SPAR (Seagoing Platform for Acoustic Research) and an 

 AGOR (Auxiliary General-Oceanographic Research) ship to support de- 

 velopment of models to predict the environmental effects on weapon system 

 performance. 



During 1971, an instrument to record noise level at several successive 

 depths beneath a ship was developed. It free-falls to preset depths, hover- 

 ing for a period of time at each before sinking to the next. At the end of 

 the cycle, the instrument becomes buoyant and returns to the surface for 

 recovery by the launching ship. 



USNS Hayes (AGOR-16), the world's largest catamaran research ship, 

 was accepted last year. The 3,100-ton Hayes will give Navy a major new 

 capability to handle heavy research equipment over the side when outfitting 

 and shakedown are completed in 1972. Hayes replaced the USNS Gibhs, a 

 World War II conversion which will see continued service as a hydro- 

 graphic and oceanographic survey ship in the navy of a European ally. 



The Navy's contract research program, carried out primarily through 

 academic institutions, continues to achieve goals which hold potential for 

 enhanced naval operations. Among these are developments applicable to the 

 coastal zone such as remote-sensing reconnaissance methods ; two- and three- 

 dimensional mathematical simulation of river mouth processes; computer 

 models of beach water-table fluctuations, wave characteristics based on hind- 

 casting and coastal conditions, and the effect on effluents of sea water and 

 freshwater density differences; and new concepts of shallow water wind- 

 driven currents, the action of a meandering current in the surf zone, and 

 the relation between longshore currents and barometric pressure. 



Research on the Arctic generated new knowledge of the relation of river 

 ice breakup to fresh-water flooding under the ice; the exchange of water 



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