FACTORS AFFECTING DEEP-WATER TRANSMISSION 



91 



H'lvr tlic lu)ii/,()ii(:il lines sliowii in the figure. Under 

 .such tenipertiture eondition.s a shadow zone i.s pro- 

 dieted at a eortain hmitinp; range. 



In praetico, the oeean is never stratified in plane 

 parallel laj-ers, each of uniform temperature. In- 

 stead, an exact teniperature-depth record might be 



TEMPERATURE IN DEGREES F 



Figure 5. Temperature conditions at beginning and 

 end of transmission run. 



similar to that shown in Figure 7. Plots of tempera- 

 ture against range at different depths would be curves 

 similar to the wavy curves also shown. This "tem- 

 perature microstructure" must be taken into account 

 in any explanation of observed underwater sound 

 transmission. 



The evidence available on thermal microstructure 

 is very limited. Measurements on a surface .ship are 

 difficult to interpret because of the rise and fall of the 

 measuring instrument through the water. Some of 

 this vertical motion arises from the roll and pitch of 

 the measuring ship, and some from the distortion of 

 the temperature-depth pattern by the surface waves. 

 For this reason, a very small-scale microstructure is 

 difficult to measure from a surface ship, although 

 changes over a hundred yards or so can usually be 

 disentangled from the more rapid changes resulting 

 from roll and pitch. Measurements from a submarine 

 show conclusively the presence of complicated ther- 

 mal microstructure.^ In Figure 11 of reference 6, 

 fluctuations of the vertical gradient are shown which 

 amount to about 0.020 degree per ft over patches 

 about 100 yd long. This result was obtained with 



large temperature gradients present near the surface. 

 When the bathythermograph shows mixed water to 

 more than 100 ft, the microstructure is much less 

 marked. 



60 70 

 TEMPERATURE F 



VERTICAL SECTION OF OCEAN 



Figure 6. Temperature distribution in ideal ocean. 



A general theory of underwater sound transmission 

 which takes microstructure into account has not yet 

 been formulated. However, certain general results 

 seem apparent. These temperature fluctuations are 

 usually fairly small compared to the smoothed gradi- 

 ent, and on the whole the actual temperature-depth 



65 F 

 60 F 



RANGE 



60 70 



VERTICAL SECTION OF OCEAN 

 TEMPERATURE F 



Figure 7. Temperature distribution in actual ocean. 



SOURCE 



MICROSTRUCTURE 



MICROSTRUCTURE 



ABSENT 

 PRESENT 



Figure 8. 

 ture. 



Distortion of sound beam by microstruc- 



pattern portrayed in Figure 7 does correspond to the 

 ideal pattern shown in Figure 6. Thus some corre- 

 spondence may be expected between observed sound 

 transmission data and predictions based on the 

 •smoothed temperature-depth pattern. 



