density and sound velocity contrasts across a boundary can be made by 

 assuming a one-to-one correspondence between salinity steps and temperature 

 stepSj that is, an equal number of temperature and salinity steps in a given 

 depth interval. For the sake of calculation, take the 300-350 meter interval 

 shown in Figure 3. There are eight temperature steps in this interval,averag- 

 ing 0. 027° C withaverage layer thickness of 6. 25 meters. Assuming a linear 

 salinity gradient of 2 x 10" %o/meter (Dunbar and Harding, 1968), eight 

 salinity steps of 0. 012 and 6. 25 naeter thickness would be required. Using 

 a temperature step of 0. 027° C and a salinity step of 0. 012%o in Wilson's 

 equation to calculate the sound velocity contrast and Ekman' s equation to 

 compute the density contrast, the Rayleigh reflectivity can be computed for 

 a single layer to be approximately 5 x 10"^ for normal incidence. While the 

 reflectivity is small for normal incidence, it must approach unity at the 

 critical angle (Brekhovskikh, I960). However, the critical angle is very 

 near zero for such small contrasts. 



The acoustic significance of the step structure increases when we go 

 from an isolated layer to many layers. Brekhovskikh treats the reflection 

 of a sound wave fronn a multi-layered medium and the calculations should be 

 made when better estimates of the sound velocity and density contrasts 

 are possible. Some additional factors must be recognized for the multi- 

 layered problem. First, the wavelength of the acoustic wave becomes 

 important in determining the interference pattern. Secondly, the steps we 

 measured do not appear as finite discontinuities. The boundary thickness 

 appears to be about 20 era. Officer (1958) shows that the reflection across 

 a boundary of finite extent must depend on the wave length. 



Thus, the significance of the layered structure in the Arctic is probably 

 small for low frequency long-range propagation, but will increase as frequency 

 increases. While the Rayleigh reflectivity at normal incidence is small, 

 the reflectivity increases to unity as the grazing angle is approached. Cer- 

 tainly the multi-layered reflection problem must be considered as well as 

 the finite thickness of the boiindaries. 



One of the chief aspects of the layering in the Arctic will be a redis- 

 tribution of the energy from the continuous profile case. An interesting 

 aspect to consider is the influence of the layers on the fluctuation of sound 

 intensity between a fixed source and receiver. In the ice-free oceans, the 

 intensity between a fixed source and receiver is observed to vary significantly 

 with time due to the variability in the sea. Yet our measurements indicate 

 that the layers in the Arctic are more stable than those in ice-free areas. 

 Therefore, the induced variability should be less. Since we do not have a 

 good picture of the internal wave structure in the Arctic, the significance of 

 irregularities in the depth of a layer cannot be specified. 



464 



