Sound velocity in the Arctic has previously been assumed to be a 

 continuously increasing function of depth. Under these conditions all sound 

 rays from a source undergo continuous upward refraction eventually reflecting 

 from the water-ice interface leading to a surface soiind channel axis. It is 

 well established that low frequency sound propagates over long ranges with 

 very little attenuation in sound channels. However, in the Arctic as the 

 frequency increases the attenuation increases very rapidly, and is abnormally 

 high compared to the open ocean attenuation above a hundred hertz (Urick, 

 1967). The high attenuation at higher frequencies has been attributed to 

 scattering losses at the ice-water interface. 



Up to the time of our measurements the existence of steps in the Arctic 

 sound velocity structure had not been considered. Urick (1967) points out 

 that the most significant effect of microstructure at short range and high 

 frequency is the focusing and defocusing of the sound , whereas for long 

 ranges and low frequencies forward scattering becomes the dominant process 

 causing sound level fluctuations. However, these conclusions have been reached 

 by considering the thermal microstructure in the sea to consist of patches more 

 or less randomly distributed throughout the water column. Our measurements 

 indicate that an important part of the microstructure, at least in the Arctic, 

 consists of quasi- stable layers that may extend over large horizontal di- 

 mensions. Therefore, the influence of the microstructure in the Arctic 

 might be quite different from its influence in open waters. 



Another aspect of the step structure in the Arctic that differs from the 

 lower latitude open oceans is that the steps are most prominent below the 

 sound channel axis, whereas at lower latitudes the steps are above the 

 sound channel axis. For a near surface source, many of the energy con- 

 taining rays arrive at the depth of the stepped zone at near grazing angles. 



The Rayleigh reflection coefficient of a single step is: 



where pj and p^ are the densities and C1/C2 the sound velocity ratio across 

 a finite discontinuity. is the angle of incidence. Density and sound 

 velocity contracts across the layer boundaries cannot be calculated without 

 knowing the salinity. High resultion salinity measurements are not available 

 to go with our temperature measurements at this time. An estimate of the 



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