BROWNING: ENVIRONMENTAL FACTORS AFFECTING LOW FREQUENCY 

 PROPAGATION IN THE OCEAN 



Figure 17 shows the KIWI-profile transmission loss at 25 Hz. 

 The axis is at 3,000 feet, along with source and receiver. We are 

 comparing the high-loss and the low-loss bottom. As you can see, 

 they fall right over each other. 



First we compared the two profiles at a nominal frequency to see 

 how the details compared. We next vary the frequency. Figure 18 

 represents the situation at 10 Hz, and you can start to see a 

 difference appear both in the general level and in the detail of the 

 propagation loss. They both go simultaneously because the minute the 

 energy is being absorbed, you are losing in the amount of energy 

 reflected. 



Figure 19 shows the results at 1 Hz for the KIWI profile. 

 Here there is really a significant difference between the high- and 

 low-loss bottoms. Again, from an experimental point of view, you 

 come to the question of what is significant. If you are talking 

 attenuation, a small difference in propagation loss is very signifi- 

 cant because attenuation is very small. It is a bit subjective where 

 you draw a line about whether there is a significant difference and 

 where there is not. 



We made a series of model runs with different bottom depths. 

 For a given frequency, if bottom depth is raised, then there will be 

 certain modes that cannot be contained that could be contained in 

 deeper water. Conversely, for a given bottom depth, that number of 

 trapped modes will vary with frequency (see Figure 20) . Figure 21 

 shows the KIWI-loss prediction at 25 Hz for bottom depths of 4500 

 and 4000 feet. 



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