MELLEN: SOUND PROPAGATION IN A RANDOM MEDIUM 



changed to 3 m, as predicted from Rayleigh scatter in Baffin Bay, 

 the result is 0.03 dB/km and agrees very well. It thus appears 

 that while most of the variability of scattering loss depends on the 

 strength of the channel, the smaller scale size is responsible for 

 the large value for Baffin Bay. 



Although the values of scattering loss may or may not be impor- 

 tant to a sonar problem since it can be very small, the information 

 about what is happening to signal coherence within the channel cer- 

 tainly should be valuable. For example, we can see in Figure 15 the 

 effect of scatter on 400 Hz signal fluctuation for two hydrophones 

 separated by 100 m. In this experiment done by Stanford (1974) in 

 Bermuda, the time fluctuations are quite incoherent and seem to have 

 two scales, the longer one probably related to internal waves and a 

 shorter scale that may be related to turbulence. 



The spectrum of the time fluctuations (Figure 16) definitely 

 shows a break, above 10 cycles/hour which varies with the seasonal 

 thermocline. The latter scale size compares to that associated with 

 scatter loss if the ocean currents are one- or two- tenths of a knot. 



The effect of scatter on spatial coherence is also important. 



Kennedy (1969) at Bermuda varied the vertical separation of two 



hydrophones and measured the CW signal correlation between them. 



The correlation distance appears to be close to our estimated value 



based on a = 15 m (Figure 17) . 

 o 



SUMMARY 



Our results are summarized in Figure 18; we have observed both 

 the Thorp (boron) relaxation and also what we believe to be forward 

 scatter loss in a number of sound channels throughout the world. The 

 coefficient of the Thorp term is unity in the North Atlantic water 



406 



