1. INTRODUCTION 



1 . 1 Intent of Report 



This report will give the results of a study of sound velocity profiles in shallow water 

 for a limited number of sites. The sites considered here are located on continental shelves 

 or shallow sea areas and with two exceptions are limited to water depths of 200 meters (m), 

 a nominal depth limit for shallow water. The relative percentage of surface duct versus 

 downward refractive conditions is determined for different seasons and various geographical 

 areas. Sediment models in certain areas allowed normal mode calculations to determine 

 optimum frequency domains for sound propagation at those sites. 



1.2 Background 



Ten shallow water stations were occupied during the FASOR cruises, to the south 

 and western Pacific and adjacent areas in the 1960s. Seven of those stations (Reference 1) 

 had sound speed profiles with significant surface ducts, i.e., positive gradient surface layers. 

 Only three of these stations had strong surface ducts, whereas three were downward refract- 

 ing. It is noteworthy that five stations with moderate to strong surface ducts were run in 

 May, June or July and he within 12 degrees of the equator. Although these data are limited, 

 they demonstrate that surface ducts may be more prevalent in shallow water then previously 

 considered. 



Propagation characteristics will differ markedly in shallow water with downward 

 refraction. In the surface duct case there is little interaction with the bottom and the opti- 

 mum frequency for sound propagation wih be relatively high, depending primarily on the 

 duct depth. For example, the optimum frequency for a 100-ft isothermal duct is approxi- 

 mately 2 kHz. In contrast, the bottom characteristics are the critical factors for the case of 

 downward refraction. Since at low frequencies the principal loss mechanism is the attenua- 

 tion of the sound waves in the bottom sediments and this attenuation is a linear function of 

 the frequency (Reference 2), the optimum frequency is expected to be quite low. 



In 1976 M. A. Pedersen and D. White (Reference 3) selected two of the FASOR 

 shaUow water stations for an optimum frequency study. Results for a downward refraction 

 profile were compared to optimum frequencies for the surface duct case. While their 

 results support the contention that optimum frequencies will be less for downward refrac- 

 tion cases, the problem of optimum frequencies for shallow water needs additional study. 

 Also for considerations of future shallow water sonar systems it would be valuable to 

 determine the geographical areas and times of year when surface ducts are prevalent in 

 shallow water areas. 



1 .3 Organization of Report 



The selection of the sites, the methods of assessment and classification of sound 

 speed profiles and the selection of representative profile types are discussed in Section 2. 

 Also an introduction is given to the propagation loss calculations and optimum frequency 

 determination. 



Propagation Losses and Reverberation from the Shallow-Water FASOR Areas with Comparisons to 

 Propagation Loss Models, J. A. Whitney, Naval Ocean Systems Center TR 400, March 1979. 

 Compressional-Wave Attenuation in Marine Sediments, E. L. Hamilton, Geophysics, Vol 37, No. 4, 

 August 1972. 

 Reference available to qualified requestors. 



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