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Acoustics offers an alternative scheme which does not require a cable. 

 Two hydrophones suspended from two ships would be placed on the bottom, on a 

 line in the direction of the current to be observed. By means of a radio-link, 

 underwater shots would be fired simultaneously (or at a known interval) at the 

 two hydrophone locations. The difference in travel time in the two directions 

 would give the current velocity. For example, if the ships are one mile apart 

 the difference in arrival time is approximately 0.7 milliseconds for each knot of 

 current. Time differences at this range can readily be measured to better than 

 100 microseconds. If sound transmission paths via several bottom-surface 

 reflections are utilized for the time measurement, the observed current velocity 

 would represent a form of average current from surface to bottom. This ob- 

 servation would probably be most suitable for calculation of the total mass flow. 

 Other sound transmission paths could be selected for additional ocean current 

 information. 



Contribution to the study of ocean waves might be made by applying 

 acoustic techniques. Substitution of an inverted echo sounder for the conven- 

 tional wave meter has already been successfully tried. However, this kind of 

 measurement yields only the time dependent displacement of the sea surface 

 overhead. More important would be an observation of the instantaneous spatial 

 distribution of the waves. Eckart has shown that the spatial distribution of the 

 waves may be characterized statistically by the spatial auto-correlation function 

 of the sea surface. This characteristic function can be obtained in principle 

 by echo-ranging on the sea surface. Choice of echo-ranging frequency would 

 be determined by the period of surface waves of interest. Thus, if surface 

 waves of length 10 feet are of interest, the echo-ranging frequency should be in 

 the neighborhood of 500 cycles. 



