INTRODUCTION TO SONAR 



71.31 

 Figure 4-17.— Negative thermal gradient tends 

 to bend sound downward. 



layer of water is isothermal; beneath this layer 

 the temperature decreases with depth. This 

 temperature change causes the transmitted sound 

 to split and bend upward in the isothermal 

 layer and downward below it. 



Don't forget: When no temperature difference 

 exists, the sound beam bends upward. When the 

 temperature changes with depth, the sound beam 

 bends away from the warmer water. 



Under ordinary conditions the sea has a 

 temperature structure similar to that in figure 

 4-19. This temperature structure consists of 

 three parts: a surface layer of varying thick- 

 ness, with uniform temperature (isothermal) or 

 a relatively slight temperature gradient; the 

 thermocline, a region of relatively rapid de- 

 crease in temperature; and the rest of the 

 ocean, with slowly decreasing temperature down 

 to the bottom. If this structure changes, the 

 path of a beam of sound through the water also 

 changes. 



Layer Depth 



thermal gradient. Here the effect of tempera- 

 ture greatly outweighs the effect of depth, and 

 the sound is refracted downward. Tills common 

 condition is illustrated in figure 4-17. 



If the temperature is the same throughout 

 the water, the temperature gradient is isothermal 

 (uniform temperature). In figure 4-18 the upper 



Layer depth is the depth from the surface 

 to the top of a sharp negative gradient. Under 

 positive gradient conditions the layer depth is 

 at the depth of maximum temperature. Above 

 layer depth, the temperature may be uniform. 

 If it is not uniform, a positive or weak negative 

 gradient may be present. 



SEA SURFACE 



71.32 

 Figure 4-18. — Sound wave splits when tempera- 

 ture is uniform at surface and cool at bottom. 



71.33 

 Figure 4-19. — Typical layers of the sea. 



48 



