SCATTERING 



483 



gral vanishes. The integral of Vx over the yz plane is 

 simply the net rate at which the fluid is flowing across 

 this plane. At any time, the total amount of fluid 

 passing through the yz plane in one direction must be 

 just equal to the amount of fluid passing through in 

 the other direction, and the net flow vanishes. Thus, 

 a random distribution of velocity does not contribute 

 to backward scattering of sound. However, sound 

 may be scattered in other directions, as indicated in 

 reference 3. 



Measurements at San Diego ^ and at Orlando ^- * 

 are consistent with the result that the sound scat- 



tered backward from velocity microstructure is very 

 weak. At San Diego attempts were made to obtain 

 echoes from imderwater vortex rings, while at Or- 

 lando a mechanical device was used to produce 

 turbulent water in the path of a sound beam and at- 

 tempts were made to measure the reflected sound. In 

 both cases, no reflected sound could be observed. Al- 

 though the data do not exclude the possibility that 

 weak echoes may have been present, the combination 

 of measurements and theory point to the conclusion 

 that backward scattering of sound from velocity 

 microstructure may be practically neglected. 



