The buoyant section containing the electronics withdraws the transducers 

 and a core sample and returns to the surface. 



4. Velocity Dispersion in Sea Water and the Pressure Coefficient 

 of Velocity 



The possibility of variations in the speed of sound in sea 

 water with frequency is a basic question in the use of velocimeters to 

 measure the speed. Sonar frequencies are typically less than 10 KHZ. 

 Velocimeters work at ultrasonic frequencies. The frequency in the pulse 

 is perhaps 3 MHZ. Velocity dispersion in sea water is always assumed 

 absent in this frequency range. No dispersion has been reported in 

 ultrasonic measurements and no dispersion has been observed in pure water. 

 There are theoretical reasons why dispersion in pure liquids can only occur 

 at very high frequencies (say greater than 50 MHZ) . It seems then that 

 the assumption is reasonable that the velocity measured by the velocimeter 

 is the same as for sonar frequencies or that the discrepancy is extremely 

 small. However, this is a vexed question which has not been totally answered, 

 primarily because of the difficulties of making velocity measurements at 

 long wave lengths. 



The indirect method of sound speed measurement using bathythermo- 

 graphs relates the pressure dependence of the speed with depth in the ocean. 

 The pressure dependence of the speed is determined in general from labora- 

 tory experiments at ultrasonic frequencies. 



There is some detailed discussion in the literature (Reference 17) 

 that the values of the depth term may be in error and there are certainly 

 discrepancies up to 3 meters per second between laboratory measurements by 

 Wilson (Reference 18) , the tables by Kuwahava (Reference 19) , and in situ 

 measurements (Reference 17) . 



17 



arthur ZD.lUttleJnr. 



