CHAPTER 5 



BATHYTHERMOGRAPH 



The travel of sound waves in the sea was 

 discussed in the preceding chapter. To interpret 

 correctly the information displayed by sonar 

 equipment, Sonar Technicians must have a thor- 

 ough knowledge of the factors affecting sound 

 wave travel in water. Because thermal conditions 

 of the sea are of the utmost importance to Sonar 

 Technicians, a means must be available for 

 measuring water temperatures at various depths . 

 The measuring device used is the bathythermo- 

 graph, called BT, Use of the bathythermograph 

 enables the submarine hunter to determine the 

 layer depth and to estimate sonar detection 

 ranges at various depths. By determining layer 

 depth, an estimate can be made of the probable 

 depth the submarine captain will employ to 

 escape detection. The submarine captain uses 

 the BT to determine the best depth to avoid 

 detection. 



Much of the data used in conjunction with 

 BT information for determining sonar detection 

 ranges cannot be included in this text because 

 of the classified nature of the subject matter. 

 Sonar range prediction methods are presented 

 in other publications, such as Sonar Technician 

 G 3&2, NavPers 10131 and Sonar Technician 

 S 3&2, NavPers 10132. 



Before proceeding to our discussion of the 

 BT and its use, a brief review of the tempera- 

 ture effects on sound travel is appropriate. 



TEMPERATURE 



As explained in chapter 4, the conditions 

 that affect sound travel in water are pressure, 

 salinity, and temperature. Increases in pres- 

 sure speed up the velocity of sound, making 

 the speed of sound higher at extreme depths. 

 An increase in salinity alsoincreases the velocity 

 of sound. The effects of pressure and salinity 

 are not nearly as great, however, as are those 

 caused by changes in temperature — particularly 

 abrupt changes. Information obtained about the 



ocean temperature at a given time can be used 

 to predict what will happen to the transmitted 

 sound as it travels through the water. 



As a result of knowing the temperature of 

 the water at various depths, we can arrive at 

 fairly accurate conclusions regarding the maxi- 

 mum range at which asubmarine maybe detected, 

 as well as the most favorable depth for the 

 submarine to avoid detection. These two funda- 

 mentals are important considerations in anti- 

 submarine warfare. They influence the types of 

 screens used for convoys, and aid in determining 

 the spacing of ships in the screen. 



EFFECT OF TEMPERATURE 



The speed of sound through water increases 

 at the rate of between 4 and 8 feet per second 

 for each 1°F rise in temperature. This 4- to 

 8-fps change depends on the temperature range 

 of the water. At water temperatures in the 

 30° range, for instance, a 1° rise increases the 

 rate of travel differently than does a 1° rise in 

 the 70° range. 



Ships and submarines of the U. S. Navy operate 

 in all sea temperatures — from near freezing in 

 the polar regions to the upper 80s in the tropics. 

 To provide accurate range data, sonar equipment 

 must be adjusted to the changes in sound velocity. 



Under some conditions, a pulse transmitted 

 by sonar equipment may travel easily through 

 water that varies greatly in temperature. Under 

 different conditions, the pulse transmitted may 

 be unable to penetrate a 5° temperature change 

 layer at all, because it is reflected and scattered 

 instead. 



LAYERS 



A pulse of acoustical power may provide 

 sonar reception out to several thousand yai'ds 

 of range. The same pulse, if transmitted into a 

 layer of water (such as a sound channel that 

 tends to keep the pulse confined within it), may 



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