INTRODUCTION 



Internal waves affect the propagation of underwater 

 sound and therefore also the U. S. Navy operational use of 

 sonar through the phenomena of refraction and scattering. 

 Refraction of the sound rays by internal waves can result 

 in sonar bearing errors and in fluctuations of bearing 

 determinations. Fluctuation in sound intensity occurs when 

 multipath interference is present, particularly when one 

 path passes through moving internal waves. The apparent 

 sound attenuation may be aggravated by loss of energy from 

 the beam by scattering from intervening internal waves. 

 Some sound may be scattered back to the source by internal 

 waves to increase the reverberation background of the 

 sonar. 



Background 



Figure 1 shows the distribution of published papers 

 on internal waves as a function of time. The graph shows 

 only casual interest in internal waves after the first paper 

 was written by G. G. Stokes in 1847. Attention in- 

 creased around the year 1900, and peaks occurred circa 

 1910 and in the 1930's. The decline in the number of 

 published papers after 1938 was apparently due to WWII. 

 The last year in which no paper was published was 1944, 

 and from then until now the number has steadily increased. 

 Figure 1 includes papers through 1962, and the bibliography 

 is comprehensive through June 1963. 



Observational data on internal waves have been 

 accumulating for the past 6 to 70 years. Most of these 

 data were confined to the Scandinavian countries until 

 about 1925. Until 1950, few of the observations on internal 

 waves had statistical significance. Since that time, three 

 unaliased spectra have been published to which statistical 

 significance can be assigned. These data were observed in 

 the Baltic Sea, off Bermuda, and off the California coast. 

 All were taken from fixed points and consequently do not 

 permit any study of geographical changes. Data taken from 

 towed thermistor chains on the east and west coasts of the 



