134 



DEEP-WATER TRANSMISSIOIN 



BT INFORMATION 



4890 4940 4990 



SOUND VELOCITY IN FT PER SECOND 



4000 

 RANGE IN yAROS 



6000 



Figure 54. Peaked transmission anomaly possibly resulting from sound channel ray diagram. 



mal irregularities will reduce this theoretical inten- 

 sity, sound intensities considerably above normal 

 would be expected at certain ranges if a sound chan- 

 nel were produced by a slight temperature gradient 

 lying above rigorously isothermal water. 



The conditions for a sound channel, when no posi- 

 tive temperature or salinity gradients are present, are 

 thus rather critical. There must be a temperature 

 gradient at the surface which extends somewhat be- 

 low the projector depth. Below this must be a layer 

 of completely isothermal water a hundred feet or 

 more in depth. The temperature difference between 

 the isothermal layer and projector depth must be not 

 less than about 0.1 F but not greater than about 

 0.3 F, the exact limits depending on the surface tem- 

 perature as well as on the depth of the layer. Thus, 

 even if no thermal microstructure were ever present, 

 sound channels of this type would normally be quite 

 transitory, appearing during the development of sur- 

 face heating in deep isothermal water; they would be 

 expected to become prominent when the temperature 

 difference between the projector and the isothermal 

 layer increased to 0.1 F, and to disappear as the 

 gradient extended downward and the temperature 



at projector depth gradually increased by another 

 one or two tenths of a degree. 



Sound transmission measurements at the UCDWR 

 laboratory in San Diego show transitory effects simi- 

 lar to those which may be expected to result from 

 sound channels. Because of the difficulty in reading 

 the bathythermograph slide accurately to 0. 1 F, and 

 because of the high variability of thermal conditions 

 in space as well as in time, it is not possible to predict 

 from the bathythermograms exactly when a sound 

 channel may be present. However, marked peaks in 

 the measured transmission anomalies are occasionally 

 found when thermal conditions are appropriate. 



A good example of the type of effect that can be 

 observed is shown in Figure 54. As shown in the ac- 

 companying temperature-depth record, a sharp nega- 

 tive gradient extends from about 15 ft to the surface 

 while below this a nearly isothermal layer extends 

 down to 100 ft. A careful reading of the trace indi- 

 cated a slight negative gradient in this layer (about 

 0.3 F in 100 ft) giving a constant sound velocity be- 

 low the projector. Moreover, the sharp surface gradi- 

 ent did not extend below the projector depth. Thus, 

 the ray diagram in Figure 54 does not show a sound 



