and 600 Hz. The minimum loss is about 105 dB at 150 Hz, 89 dB at 600 Hz and 76 dB 

 at 300 Hz. These are the values of propagation losses at 1 50, 300 and 600 Hz that were 

 plotted in Figure 4.2 for the 100 km curve. 



From Figure 4.4 we note that the minimum loss at 150 Hz occurs at a receiver 

 depth of about 65 m, while at 300 Hz and 600 Hz the minimum loss is near the source 

 depth at 25 m. In Figure 4.5 propagation loss versus receiver depth is given for the same 

 three frequencies but for a source depth of 90 m. The minimum losses occur at receiver 

 depths which are conditionally similar to those in Figure 4.5 (65 or 70 m at 150 Hz and 

 near the source depth (90 m) at 300 and 600 Hz), but the propagation is much better for 

 the 90 m source. Propagation losses for these frequencies are 5-20 dB less. The 90 m 

 source is obviously more efficient in exciting those modes which propagate best by succes- 

 sive bottom bounces from this highly reflecting bottom at low frequencies. 



Optimum frequencies of propagation for the North Sea site are about 300 Hz for 

 both the winter and summer conditions. The optimum frequency of propagation is near 

 275 Hz for the intermediate profile. The low loss and sudden decrease in loss with fre- 

 quency near 250-300 Hz (Figures 4.2 and 4.3) are due to a corresponding variation in the 

 bottom reflection loss in the North Sea area. This is discussed in Section 4.2. 



20 





1 1 



1 I 



1 









FREQUENCY 150 Hz^^^ 



600 Hz :' 



300 Hz \. 





40 



~ 







-•\ 



~~ 



60 



- 







\ 





80 



nn 



- 



NORTH SEA SUMMER PROFILE 



90 m SOURCE DEPTH 

 100 km RANGE 



1 1 



1 ^^ 1 .. 



1 .^ 



\ 



130 



110 100 90 



PROPAGATION LOSS, dB 



Figure 4.5. Propagation loss vs receiver deptii for the frequencies of 150, 300 and 600 Hz; 

 North Sea summer profile, source depth 90 m. 



42 



