B. METHODS OF CURRENT MEASUREMENTS 



Existing methods of current measurements include indirect meas- 

 urements of density distributions from which currents may be deter- 

 mined, as well as direct measurements of flow. Density distributions 

 in the ocean are related to currents in the same manner that isobaric 

 weather charts are related to winds and do not involve any actual 

 measurements of flow. Density distributions are determined from 

 accurate measurements of temperature, salinity, and depth. These 

 parameters have been discussed in Sections III, IV, and V, respec- 

 tively, of this report. 



Direct measurements of currents may be accomplished by two 

 basic methods: (1) The drift method, the release of freely drifting 

 devices at a given time and place and their subsequent tracking or 

 recovery at some new time and place; and (2) the flow method, the 

 measurement of flow past a point that is fixed geographically. 



1. Drift methods 



Accuracies obtainable by this method are determined by the 

 navigational accuracy with which positions of the drifting device may 

 be fixed, as well as by the ability of the device to be unaffected by 

 influences other than the current under measurement. The primary 

 advantage of this method is that anchoring of the ship is not required. 



Drifting devices include woodchips, bottles, cards, confetti, dyes, 

 etc., as well as more sophisticated devices such as drogues, telemeter- 

 ing buoys, and neutral buoyancy floats. Since all of the surface devices 

 are at least partially exposed to the winds and extend to some depth 

 below the water, they may not truly measure surface drift, and data 

 must be viewed with caution. Another technique involving drift is the 

 lowering of current meters from a drifting ship. Frequent, accurate 

 positioning is necessary in this method to correct for the movement 

 of the ship. A difficulty with freely drifting devices is that instantaneous 

 measurements are not possible, and only time-averaged currents may 

 be obtained. 



Subsurface currents may be measured by the drift method with 

 drogues suspended at any given depth on a wire between a surface buoy 

 and a weight. The Hydrographic Office has made occasional use of this 

 method to obtain currents as deep as 3,000 feet. The complete device 

 is made at negligible cost from a surplus parachute, piano wire, a 

 weight, and a float. Although the system does not lend itself to automa- 

 tion, certain improvements in buoy design and location techniques 



VI- 3 



