SOUKD SCATTERING LAYERS IN THE ARCTIC OCEAN 

 OBSERVED FROM FLETCHER'S ICE ISLAND (T-3) 



by 



Kenneth Hunkins 



Senior Research Associate 



Lamont-Doherty Geological Observatory 



of Columbia University 



Palisades, N.Y., 10961* 



ABSTRACT 



Continuous observations of sound scattering layers have been 

 made in the Arctic Ocean from Fletcher's Ice Island (T-5) since 

 1963 with a 12 KHz sounder. A characteristic diffuse scattering 

 layer between the depths of 25 and 200 m. was observed in the sum- 

 mers of 1965, 1964, 1965, aiid 1968, but not during the summers of 

 1966 and 1967* Since the ice island drifts continually, the meas- 

 urements may indicate a spatial variation in this scattering layer 

 which is superimposed on a seasonal cycle. The layer also varies 

 divimally, particularly at the time of the equinoxes when the maxi- 

 mum light contrast between day and night occurs. 



Observations with a 100 KHz sounder in 196? and 1968 showed 

 that many individual scatterers are present in the upper layers 

 even when no diffuse layer is observed at 12 KHz. The 100 KHz 

 sounder was also able to resolve individual scatterers in the same 

 layer which appeared cloud -like on the 12 KHz. 



In addition the 100 KHz records show a persistent layer at a 

 depth of about 50 m. This thin scattering layer coincides with the 

 base of the homogeneous mixed layer. Tests are being made to de- 

 termine whether this layer is caused by specular reflection from 

 the density discontinuity or whether it is due to sound scattering 

 from organisms or debris at that level. 



Introduction 



Sound returns from within the water column are often recorded 

 with ships' depth so\mders. These early returns arriving before 

 the bottom echo are sound which has been either reflected or scat- 

 tered by objects within the water. If the object is much larger 

 than a wavelength, as for example a large fish, sound waves are 

 specularly reflected. If the object is of the order of a wave- 

 length or less in size, as may be the case for certain groups of 

 plankton or for air bubbles, sound waves are scattered in all di- 

 rections . 



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