INTRODUCTION TO SONAR 



- '^ 



NO SIGNAL 



NEGATIVE GRADIENT 

 SHORT RANGES 



71.87 

 Figure 5-13. — Sound pulse travel through negative gradient. 



in coastal waters, this condition may be found 

 anywhere. In figure 5-14 a positive gradient is 

 shown to exist down to a certain depth, beyond 

 which a negative gradient is formed. Depending 

 on the depth of the start (top) of the negative 

 gradient, such a trace can also mean trouble 

 for the Sonar Technician. As a result of this 

 condition, the shipboard sonar operator observes 



unusually long ranges to targets near the surface. 

 Little of the pulse's power penetrates the nega- 

 tive gradient, however. The reason is that the 

 higher temperatures in the lower part of the 

 positive gradient bend the pulse back up toward 

 the surface of the sea from which it is reflected 

 downward again, and so on, forming a surface 

 channel. 



POSITIVE GRADIENT 



UNUSUALLY LONG 



RANGES 



71.86 

 Figure 5-14. — Sound pulse travel through positive gradient. 



74 



