Another small anomaly in Figure 4.21 is the minimum in the propagation loss for 

 the Lands End summer profile near 800 Hz. This irregularity arises from the sound speed 

 profile which has near isovelocity water in the top 20 meters. Rays vertexing at this depth 

 have extra long loop lengths and thus reflect from the bottom less often and suffer less 

 bottom loss. When the mode structure provides a mode which corresponds to this effect, 

 this mode has smaller loss. This happens near 800 Hz. 



4.2.2 Propagation Loss Comparisons Including the Korea Strait and the Straits of Sicily 



Figure 4.24 compares the propagation losses calculated for the winter profiles for 

 the Korea Strait and the Straits of Sicily (two water depths) at the 50 km range with an 

 average loss curve for the sand sediment areas of Figure 4.20. The region between 200 and 

 600 Hz on the Korea Strait curve is dotted in to indicate the probable losses there. A 

 similar comparison for the summer profiles is shown in Figure 4.25 with the Korea Strait 

 and Lands End omitted. The propagation losses for the 165 m Straits of Sicily, winter 

 profile, are nearly the same as the average losses for the previously noted sand areas and are 

 about 10 dB lower than those for the summer profiles except in the region of 600 to 650 

 Hz. The summer profile losses there show a shift in the loss curve in the amount of 4-5 dB 

 less loss. This arises from the nature of the bottom reflection loss dependence upon fre- 

 quency. At 600 Hz, the magnitude of the bottom loss increases suddenly at about 7.4 

 degrees, while at 650 Hz a similar increase is at about 9.9 degrees. This causes the anomaly 

 in the propagation loss at that point. Beyond 700 Hz, the loss resumes an increase in line 

 with other curves in that region. At this time we are not aware of the physical mechanism 

 involved. 



The most distinct feature of Figure 4.25 is the high loss shown for the 500 m depth, 

 Straits of Sicily. The profile is downward refracting to a slow bottom. These conditions 

 lead to high propagation losses. This case is discussed further in the next section (4.2.3). 



4.2.3 Effects of Sediment Attenuation 



The sediment models for the Straits of Sicily, as mentioned previously, were taken 

 from Reference 13. In that report, Matthews gives two models (Geoacoustical Models IIA 

 and IIB) for water depths in excess of 500 m which differ only in the attenuation coeffi- 

 cient. We have used the "high attenuation" model (IIB) in Table 4.13. The attenuation 

 coefficients differ by a factor of 6 at the top of the first sediment layer. Table 4.13 com- 

 pares the attenuation of the two models. 



Attenuation K, in (dB/kin)/Hz 

 Depth, 



m Model IIA Model IIB 



.0045 .0270 



250 .0170 .0345 



500 .0160 .0235 



Table 4.13. Attenuation coefficients. 



59 



