OCEANOGRAPHY IN THE UNITED STATES 285 



sound in tlie ocean and the ocean bottom. In some cases the studies are 

 directed toward a more complete understanding of the mechanisms 

 of sound propagation ; in others the known facts of sound transmission 

 are used as tools to study the structure of the ocean bottom. 



The most pressing problem in the underwater acoustics field today 

 is the detection of submersibles. The state of this art at the present 

 seems to indicate that only by overall study and detailed understanding 

 of sound transmission in the ocean will the ranges of effective detection 

 be substantially increased. Advanced schemes and novel ideas to solve 

 the problem are always being presented, but whether they will work 

 or not depends upon the accurate prediction of sound transmission. 

 The size of the oceans automatically implies that any detection method 

 will be very expensive and we can ill afford to invest in systems which 

 are not based on careful experimental measurements made under the 

 actual working conditions. Therefore, our efforts have been to make 

 measurements of sound transmission in various typical parts of the 

 ocean during different seasons of the year. These measurements are 

 related to such controlling factors as the water temperature structure 

 and bottom topography and analyzed in terms of these factors. The 

 ultimate aim, of course, is to be able to predict sound transmission 

 conditions anywhere at any time with reasonable accuracy. One can 

 scarcely work in such an undertaking without thinking of particular 

 techniques or methods for detecting submarines. As an institution of 

 oceanography it is hardly in our realm to pursue such ideas to a final 

 completed weapon, but we do follow such ideas through feasibility 

 studies or interest the proper people into doing the same. 



Sound is a wave disturbance that is propagated with different 

 velocities in different materials and is analogous to light undergoing 

 reflection and refraction. These characteristics make sound a most 

 useful tool for studying the structure of the ocean bottoms. The 

 answer to the song "How Deep Is the Ocean" is being answered by 

 just such means. A sound pulse travels from a ship to the bottom, is 

 reflected, and returns; the travel time is a measure of the depth. 

 Instruments working on this principle are so highly developed that 

 the major problem is knowing the ship's position accurately enough 

 to match the precision in the depth measurement. These instruments 

 do more than just measure the depth of the water; they show the 

 presence of the deep scattering layers, the presence of fish and whales, 

 and under certain conditions enough penetration of the bottom occurs 

 that reflections from subbottom layers are detected. We have devel- 

 oped an instrument for the specific purpose of detecting these sub- 

 bottom layers (the sound spectrum of the source is different from 

 that of a conventional echo sounder) , and it is fascinating to see the 

 layers beneath the sea bottom appear on the record in their many 

 forms. Unfortunately such reflection techniques do not give the 

 complete answer as to depth and thickness of the layers. Sound 

 refraction work in which the source and receiver are separated by 

 appreciable distances permit us, however, to infer layer thickness 

 and sound velocities. Laboratory measurements and field measure- 

 ments correlated with coring and dredging relate the sound velocities 



