113 



This particular phenomenon is found only occasionally. Even at the lo- 

 cation of the above observations, it was quite localized. Equipment failure pre- 

 cluded delineation of its boundaries; but it was observed for a distance of 7000 

 yards along the ship's course, and it was not found at other positions nearby. 

 Some areas display the phenomenon; others do not, apparently. On rare oc- 

 casions, the interference pattern has been almost circular in shape, indicating 

 a severe distortion of the layer from a plane configuration. 



Significant to the purposes of this Symposium is the fact that here is a 

 tool which infers some physical properties of the sea, responsible for the con- 

 figurations of scatterers which do not appear to be completely explained by bio- 

 logical mechanisms alone. 



The measurement of small current velocities existent in the ocean, 

 would appear to be most reliably accomplished by a method utilizing "observa- 

 tion at a distance". Such observation could be made with a high frequency echo- 

 ranging set which senses current velocity as a shift in frequency of sound re- 

 turned by scatterers within the current. Tests have shown the feasibility of 

 principle, but the packaging and the recording portrayal of the information for 

 general application as an oceanographic instrument will require considerable 

 engineering. Such a device, with little increase in complexity of the basic elec- 

 tronic assembly, could be constructed to give the three coordinate components 

 of the velocity. It appears that measurement of current velocity as low as 0.1 

 knot would be entirely feasible. 



The cross-correlator, a device applicable for precise determination of 

 the time difference of arrival of two signals derived from a broadband frequency 

 source, can be used in the study of underwater acoustic propagation to determine 

 likely transmission paths. The method is most applicable to the case in which 

 there are just several preferred paths. For a continuum of path lengths in 

 which all paths share the energy more or less equally, the method would be of 

 no value except to indicate the existence of such a condition. 



Study of the acoustic properties of the ocean bottom, located near the 

 water-bottom interface, can be aided by the use of techniques and devices de- 

 veloped for logging oil wells. Such properties as acoustic velocity, density, 

 and porosity might be so investigated. 



DISCUSSION: H.E. Nash 



While the oceanographer and the underwater sound research worker 

 have common interests in both underwater sound and in the ocean, they face 

 these common problems from somewhat different points of view. The oceanog- 

 rapher is interested in underwater sound primarily as a tool which may be uti- 

 lized to gain greater insight and more detailed knowledge of the ocean, including 

 its boundaries and the life within it. The underwater sound researcher is inter- 

 ested primarily in the manner in which the ocean, its structure, its boundaries, 

 and the life within it affect his ability to transmit and receive intelligence by 

 means of underwater sound. 



The general groupings chosen by Dr. Hersey for the framework of his 

 presentation, i.e.: passive listening, acoustic ranging, sound transmission 

 studies and echo ranging and echo sounding, when coupled with applicable labor- 

 atory techniques for nneasurement of the physical properties of the ocean define 

 rather completely the areas of common interest. Like Dr. Hersey I shall con- 

 fine my remarks to instrumentation for field observations. 



