over the Andaman-Nicobar Ridge, and then deepens slightly inside the Andaman 

 Basin. Axial velocities vary from less than 1490 m/sec over the North Wharton 

 Basin to nearly 1495 m/sec in the Andaman Basin. The very narrow deep sound 

 channel north of the Andaman-Nicobar Ridge is caused by a combination of 

 relatively large RSIW concentrations and local modification of lEW. At about 

 2° S. latitude, sound velocity perturbations above and below deep axial depth 

 merge with the deep sound channel (mixing of RSIW and BIW). 



In summary, unmixed RSIW is the main factor controlling deep sound 

 channel structure in the North Indian Ocean. In the southern Arabian Basin, 

 Bay of Bengal, and throughout most of the Somali, Mid-Indian, and North 

 Wharton Basins, the deep sound channel is very broad with an axis between 1200 

 and 1600 meters at velocities less than 1495 m/sec (Figures B-13, B-16, B-19, 

 B-22 and B-24, respectively). In these same regions, concentrations of unmixed 

 RSIW are less than 15%, However, in the Gulf of Aden, northern Arabian Basin, 

 and Arabian Sea, the deep sound channel has a narrower structure with an axis 

 deeper than 1700 meters at velocities greater than 1495 m/sec (Figures B-7, B-8, 

 and B-9, respectively) . In the Gulf of Aden and northern Arabian Basin, RSIW 

 concentrations vary between 15% and 35%. In the Arabian Sea, salinities above 

 1500 meters are greater than that for 15% RSIW (due to mixing of RSIW with PGIW 

 and high salinity ArSW). Therefore, relatively high salinities below 1000 meters 

 (often associated with high RSIW concentrations) result in a deeper and narrower 

 sound channel, while relatively low salinities below 1000 meters (often associated 

 with low RSIW concentrations) result in a somewhat shoaler and broader sound 

 channel . An analogous situation occurs in the North Atlantic Ocean in the 

 presence of varying concentrations of high salinity MIW (Fenner and Bucca, Nov 

 1971). Very low salinities below 1000 meters (associated with BIW) result in a 

 shoaler and narrower deep sound channel (Figure B-36). 



DEEP SOUND VELOCITY PROFILES 



Figure 8 shows the location of the six physiographic basins in the North 

 Indian Ocean that have somewhat different velocity structures below 2000 meters. 

 Typical sound velocities for each of these basins are given at 500-meter intervals 

 on Table I. The concept of typical velocity curves for physiographic basins was 

 initially shown for the North Atlantic Ocean by Moore, November 1965. Deep 

 temperature and salinity profiles for major ocean basins (including the Somali, 

 Madagascar-Mascarene, Mid-Indian, and Wharton Basins) have been discussed by 

 Olson, June 1968. These profiles can be used with the equation of Wilson, 1960 

 to obtain deep sound velocity profiles. 



Table I shows deep velocity variations of less than 3.0 m/sec below 2000 

 meters between the five major North Indian Basins. However, in the Andaman 



