INTRODUCTION 



The performance of any system whereby information is 

 transmitted from one place to another, using radiated energy as 

 a carrier, is limited by the characteristics of the transmitting 

 medium. When measurements are made of the intensity of under- 

 water sound in the ocean, the results are often highly variable. 



Factors contributing to this variability include divergence or 

 partial convergence caused by refraction, destructive and con- 

 structive interference associated with multipath propagation 

 caused by reflections from the surface and bottom of the sea, and 

 diffraction and scattering caused by inhomogeneities of the water 

 medium. When there are present additional inhomogeneities such 

 as suspended particles, thermal cells, regions of turbulence, or 

 temperature variations caused by internal waves, an additional 

 variation in intensity occurs. 



Multipath interference arises particularly when a trans- 

 mission path passes through a gradient. Refraction of sound rays 

 by gradients can produce sonar bearing errors and fluctuations in 

 bearing measurements. Although salinity and pressure gradients 

 contribute to this effects, temperature changes normally have the 

 greatest influence on sound velocity and, hence, are the most 

 important source of multipath interference. Therefore, know- 

 ledge of temperature gradients is vitally important in the success- 

 ful use of sonar by the U. S. Navy. This report covers an in- 

 vestigation of a relation existing between the strengths of vertical - 

 and horizontal -temperature gradients and a dominant frequency 

 of oscillation in the thermocline. 



PREVIOUS STUDIES 



Information on gross physical features of the ocean including 

 temperature gradients is readily available in oceanographic 

 atlases. These atlases are generally derived in one of two ways: 

 first, from an accumulation of data over many years from in- 

 dependently conducted studies; and, second, from a concentrated 



