FACTORS AKFKCTINC; DEEP-WATER TRANSMISSION 



S9 



tliau 1,500 fathoms is practically (Iccii-watcr trans- 

 mission under many conditions. 'I'hiis the study of 

 sound transmission in drop water is of considei'ahle 

 jjractical imi)ortance. 



5.1.2 Vertical Teniporatiire Structure 

 and Computed Ray Diagrams 



The temperature distribution in the ocean largely 

 determines the sound velocity distribution, which we 

 have seen is an important factor in sound intensity. 

 For this reason, measurement of ocean temperatures 

 at various depths has been an integral part of the re- 

 search on sound transmission and is also important 

 in the tactical use of sonar equipment. 



The temperature in the ocean is affected by the 

 absorption of radiation from the sun and sky, by the 

 cooling of the surface layer by evaporation, by dis- 

 placements due to currents and upwelling, and by the 

 addition of fresh water near shore. Usually a water 

 column in the deep sea can be divided into three 

 principal layers, shown by the sample temperature- 

 depth plots in Figure 2: (1) a relatively warm surface 

 layer, which is subject to daily and seasonal changes 

 in thickness and vertical temperature gradients, (2) a 

 layer of transition at mid-depths called the thermo- 

 cline, in which the temperature decrea.ses rapidly 

 with depth, and (3) the cold deep-water layer, in 

 which the temperature decreases only gradually with 

 depth. A detailed discussion of the temperature dis- 

 tribution in the ocean is given in Volume 6 of Divi- 

 sion 6. Here, only a few of the basic temperature- 

 depth patterns are described and their expected in- 

 fluence on underwater sound transmission briefly dis- 

 cussed. 



It will be pointed out in later sections that the 

 transmission loss is least and sound ranges are longest 

 when the surface layer is reasonably isothermal and 

 deeper than about 100 ft. Such deep isothermal layers 

 tend to occur when the water at the surface is losing 

 more heat than it is gaining, as in midwinter in the 

 high latitudes. The colder surface water will be 

 heavier than the water just beneath and will mix 

 with it. As a result, a surface layer of more or less 

 constant temperature will be formed. In midwinter 

 the isothei'mal surface layer is usually several hun- 

 dred feet deep, except in tropical waters, where this 

 depth varies from 50 to 500 ft depending on ocean 

 currents and other factors. In very high latitudes the 

 isothermal layer may extend down to the ocean bot- 

 tom in February or March. 



The ray diagram l'orsoun<l transmission in thcca.se 

 of an isothermal siulacc layer above a thermocline 

 has approximately the characteristic shape shown 

 in iMgure -i. .Vccording t(; the simple ray theory, the 

 .sound beam should split at the bottom of the isother- 

 mal layer, with the upper portion bending gradual!}' 



TEMPERATURE 



Fioi'RE 3. Ray diagram for isothermal water above 

 thermocline. 



back to the surface because of the effect of pressure 

 and the lower portion bending sharply down into the 

 thermocline. Temperature-depth patterns resulting 

 in such a predicted ray diagram are called "split- 

 beam" patterns. If the intensity of the sound field 

 were measured along the vertical line SS' in Figure 3, 

 the intensity immediately below the isothermal layer 

 should decrease .sharply with increasing depth. After 

 having reached a minimum, the sound field intensity 

 should begin to increase slowly with increasing depth 

 until finally the edge of the main sound beam is 

 reached. 



At longer ranges with split-beam patterns a meas- 

 urement of intensity, as along the vertical line RR' in 

 the diagram, should indicate a substantial amount of 

 sound in the isothermal layer, but in or below the 

 thermocline very little sound should appear. As 

 .shown in Section 5.3.2, these predictions of theory 

 for long range are not confirmed by the observations, 

 which show no clear trace of the predicted shadow 

 boundary below the layer. 



It may be pointed out that the shaded area in 

 Figure 3 is usually not a true shadow zone, even in 

 theory. The temperature-depth graph usually curves 

 continuously from the isothermal layer into the 

 thermocline, rather than breaking at the sharp angle 

 shown in Figure 3. As a result of this curvature, some 

 direct sound always theoretically penetrates the 

 "shadow zone" in this case, but this theoretical sound 

 is much weaker than the sound actually observed. 



Heating of the surface la3'er of the ocean some- 

 times produces a temperature gradient which extends 

 all the way to the sea surface. Such conditions are 

 most common at high latitudes during the summer 

 months, when the surface water is gaining more heat 



