740 



[chap. 22 



2000-d.b surface, reached a value of 8.45 dynamic centimeters. Even more 

 spectacular was a 14.5 dynamic centimeter variation with time noted at 

 Snellius Station 253a. The variation demonstrated the importance of internal 

 waves in the calculation of currents. 



Thus it is evident that erroneous conclusions from hydrographic observations 

 can be drawn unless the effect of internal waves on the distribution of tempera- 

 ture, salinity, currents, etc., is considered. Defant (1950) pointed out that 

 certain time spacings of hydrographic stations would reduce the relative errors 

 caused by internal waves, and developed an equation of time spacing between 

 observations when the period of the internal tide is known. 



HIGH TIDE 



HIGH TIDE 



LUNAR HOURS 

 1200 



1.24 

 1.25 



Fig. 11. Variations of dynamic-height anomaUes (0/305 dynamic meters) jilotted with 

 reference to lunar time, showing the calculated diurnal {ADo^) ; semi-diurnal {AD\2) ; 

 and the resultant {AD2^ + AD12) lunar cycles. 



f. Sound transmission 



Internal waves affect the transmission of sound through water. Sound is 

 refracted by the vertical (and horizontal) sound velocity gradients, which, in 

 turn, depend on the strength of the thermocline and the angle at which the 

 sound rays intersect it. 



With an undulating thermocline, caused by internal waves, the sound rays 

 intersect at different angles. The refraction and sound-focusing effects can be 

 calculated by applying Snell's law, but it is a very tedious process. However, a 

 theoretical sound transmission problem was solved by means of a high-speed 

 UNI VAC computer (Lee, 1961). 



In this problem, a three-layered ocean was used for a theoretical two- 

 dimensional study of an underwater sound-intensity field in the presence of an 

 internal wave. The internal wave (heavy lines in Figs. 12 and 13) and the 

 sound- velocity structure were idealized to simplify machine computation, but 

 both were representative of summer conditions off the southern California 

 coast. Sound travels at a constant speed in the top layer. The sound velocity 

 gradient in the second and third layers was — 4.8 ft sec~i ft^^ and — 0.6 ft sec~i 

 ft~i, respectively. 



