90 



DEEP-WATER TRANSMISSION 



than it is losing. Surface heating and marked temper- 

 ature gradients in the top 30 ft of the ocean are par- 

 ticularly marked on summer afternoons with calm 

 seas and cloudless skies. At night, or during periods of 

 high winds, gradients near the surface tend to dis- 

 appear. 



The ray diagram computed for a temperature 

 gradient extending up to the surface is shown in 

 Figure 4. The decrease of sound velocity with in- 



TEMPERATURE 



RANGE - 



PROJECTOR 



DEPTH 



Figure 4. Ray diagram for negative gradient extend- 

 ing to surface. 



creasing depth bends the entire sound beam down- 

 ward, as discussed in Chapter 3. Beyond a certain 

 limiting range, which increases with increasing depth, 

 no sound ray can penetrate, and a shadow zone of 

 complete silence should result. Observations of imder- 

 water sound transmission confirm the presence of this 

 shadow zone when the temperature gradient is .suf- 

 ficiently strong, about 1 degree or more in the top 

 30 ft. The only sound reaching such a shadow zone is 

 the scattered sound, which also produces reverbera- 

 tion back at the echo-ranging projector. When the 

 surface gradients are weak, however, other effects 

 become important, and shadow zones do not appear. 



In addition to these two basic but simplified tem- 

 perature-depth patterns, innumerable varieties of 

 intermediate situations occur. Complicated tempera- 

 ture structure is especially likely in the surface layer; 

 and the accurate computation of a ray diagram from 

 a temperature-depth record can be very laborious. 

 Since the observations usually do not confirm the 

 detailed predictions of the sunple theory, which is 

 based on small details of the temperature structure, 

 the computing of ray diagrams for these intermediate 

 cases is of limited usefulness. 



Sharp positive temperature gradients are extremely 

 rare in deep water. Such gradients are stable only 

 when accompanied by positive salinity gradients. 

 Salinity gradients may also affect sound velocity, 

 but their effect is usually negligible compared to that 

 of temperature gradients. Salinity gradients may be 

 appreciable in some near-shore areas, where large 

 rivers drain into the sea, and at the coastwise margins 



of the permanent ocean currents, such as the Gulf 

 Stream. In such regions, sharp positive temperature 

 gradients may occur. In the open ocean, however, 

 they are usually less than a few tenths of a degree in 

 30 ft. Because of the rarity of sharp positive gradients, 

 there is a complete absence of data on sound trans- 

 mission in deep water through regions of strong up- 

 ward refraction. 



5.1.3 Variability of Vertical 



Temperature Gradients 



The way in which ocean temperature changes with 

 depth is variable from time to time and from place to 

 place. Gradual changes from day to day and from one 

 geographical region to another have an important 

 effect on the performance of sonar gear. These changes 

 are discussed in detail in Volume 6 of Division 6, and 

 form the basis for the Sound- Ranging Charts'* and the 

 Submarine Supplements.' 



In some areas these changes are so rapid that they 

 greatly complicate the study of underwater sound 

 transmis.sion. In the coastal waters off San Diego, a 

 bathythermograph lowered at one end of a transmis- 

 sion run frequently showed marked differences from 

 the ba thy thermogram obtained at the other end, with 

 wholly different ray diagrams resulting. Two samples 

 of such records are presented in Figure 5. Some of this 

 variation represents a change with time, while much 

 of it arises from changes with location. In early com- 

 parisons between transmission data and the com- 

 puted range to the shadow boundarj^, an average was 

 taken of the ranges computed from several bathy- 

 thermograph records. More recently, a single bathy- 

 thermograph record taken on the recei\ing vessel has 

 been used at UCDWR in studying the relation be- 

 tween the transmitted sound intensity and the tem- 

 perature-depth record. 



Temperature Microstructure and Effects 



In addition to these large temperature changes 

 over several thousand yards, smaller changes take 

 place over much smaller distances. These changes 

 may affect the way in which the sound beam travels 

 through the water. In Chapter 3 the predictions of the 

 ray theory were discussed for a sound beam passing 

 through an ocean in which the sound velocity depends 

 only on depth, but decreases gradually with depth. 

 In such an ideal ocean an exact temperature-depth 

 record would be similar to that shown in Figure 6. A 

 plot of temperature against range at any depth would 



