Characteristic or representative profiles were chosen only for those sites where 

 propagation loss calculations were made — the shallow Bering Sea, Bass Strait and North 

 Coast of Brazil are not in this set. The characteristic winter profiles chosen for each site 

 were ones with positive gradients from surface to bottom. Representative summer profiles 

 are of the non-positive gradient (downward refracting) type. Non-positive gradient pro- 

 files at East of Singapore occurred in only 40 percent of the summer observations but the 

 characteristic profile was chosen to be non-positive to contrast with the winter profile. 



4. ACOUSTIC RESULTS 



4.1 Propagation Loss Calculations 



4.1.1 North Sea 



The computational models for the water column (Table 4.1) and for the sediments 



(Table 4.2) for the North Sea site are as follows: 



Winter 



Inte 



imediate 



Summer 



Depth, 



Sound Speed, 



Depth, 



Sound Speed, 



Depth, 



Sound Speed, 



m 



m/s 



m 



m/s 



m 



m/s 







1475.8 







1507 







1501.5 



10 



1476.7 



30 



1503 



20 



1501.1 



100 



1478.4 



75 



1480.5 



30 



1492.7 







100 



1477.6 



50 

 100 



1482.5 

 1479.5 



Table 4.1 . Sound speed in the water column. 







Sound 



Sound 











Bottom 



Sediment 



Speed 



Speed 



Gradient 



Absorption 



Absorption 





Layer 



Depth, 



Top, 



Bottom, 



Top, 



Top, 



Bottom, 



Density, 



g/cm^ 



Number 



m 



m/s 



m/s 



s-1 



(dB/km)/Hz 



(dB/km)/Hz 



1 



0.0 



1760.0 



1778.0 



8.87 



0.47 



0.42 



2.05 



2 



2.0 



1700.0 



2000.0 



0.24 



0.12 



0.07 



1.74 



3 



1000.0 



2000.0 



2300.0 



0.24 



0.07 



0.02 



2.1 



4 



2000.0 



2540.0 





-0.1 



0.02 





2.2 



Table 4.2. Sediment parameters used in computations. North Sea site. 



Sediment sound speeds given here are for the winter profile. Those for the other 

 seasonal profiles will be adjusted to keep the ratio of surface sediment to bottom water 

 constant (1.19 for the North Sea site) . Density in the water was assumed to be 1.03 and 

 a constant value in each sediment layer. 



Propagation loss as a function of frequency is shown in Figure 4.1 for the winter 

 profile (positive gradient). Figure 4.2 for the summer profile and Figure 4.3 for the inter- 

 mediate profile (downward refracting). The source depth was 25 m and the receiver depth 

 was optimized. The concept of optimum receiver depth is illustrated in Figure 4.4. In this 

 figure the depth functions of propagation loss are shown for the frequencies of 150, 300 



39 



