MOORING BUOY 



-SUBSURFACE BUOY 



-CURRENT METER 



CURRENT METER 



Figure M-IS. Geociyne current meter array. 



alcohol and dry; then rinse the instrument 

 frame with fresh water and dry and reassemble 

 the components, close the meter and plac« the 

 meter in the shipping case for return to the 

 Naval Oceanographic Office. 



M-21 Geodyne Current Data Record Anal- 

 ysis. — Geodyne current data record films are 

 returned to the Xaval Oceanographic Office for 

 developing and analysis. Automatic data-proc- 

 essing equijjment is used to decode the binary 

 record, and the computer analysis yields data 

 printout sheets, strip charts, histograms, and 

 direction and speed versus time plots. In addi- 

 tion, sj^ectrum analysis, harmonic analysis, and 

 frequency-distribution analysis are performed 

 by computer. 



M-22 The Geomagnetic Electrokinetograph 

 (GEK). — The GEK is a shipboard current 

 measuring device designed to record the electri- 

 cal potential developed by the movement of an 

 electrical cable and an electrolyte (sea water) 

 through the earth's magnetic field (fig. M-20). 



Change 1—1970 



The GEK measures the net current (i.e., the 

 surface current minus the average currents to 

 the bottom). 



The essential physical equipment constitut- 

 ing the instrument is: 



1. A matched pair of electrodes mounted 100 

 meters apart on a two-conductor cable long 

 enough (ordinarily two or three times the 

 length of the ship) to stream them astern, away 

 from the magnetic and electrochemical influ- 

 ences of the ship. 



2. A recording potentiometer assembly to 

 which the cable is connected. 



3. A gyrocompass repeater, mounted above 

 or close to the recorder assembly. 



With the above equipment, observations of 

 the potential difference developed in the cable 

 are made when the ship is underway. These 

 potential differences result from the athwart- 

 ship motion both of the cable and of the water 

 through the earth's magnetic field. They are 

 rigidly related to the set and drift of the ship 

 and thus of the trailing cable. The potential 

 difference changes sign when currents set the 

 ship to port or starboard. The magnitude of the 

 potential difference depends on the rate of drift 

 normal to the course, on the length of cable 

 between electrodes, on the local strength of the 

 vertical component of the earth's magnetic field, 

 and on the vertical distribution of water veloci- 

 ties at the location. Through measurements of 

 the potential differences on two courses nearly 

 at right angles, the drift or component veloci- 

 ties in these two directions are determined. The 

 vector sum or resultant of these velocities is the 

 net current vector for that locality. 



NOTE: Near the magnetic equator where the 

 vertical component of the earth's magnetic field 

 is very small, small vertical water motions may 

 interact with the horizontal component of the 

 earth's magnetic field to produce large fictitious 

 GEK signals. If measurement errors from this 

 source are to be kept below 10 percent, it is ad- 

 visable not to rely on GEK measurements made 

 within approximately 200 miles of the magnetic 

 equator. 



M-23 GEK Models. — Navy survey ships 

 have two models of the GEK in use. The earlier 

 type was developed and constructed at 

 tlie Woods Hole Oceanographic Institution 

 (WHOI) and is referred to as the WHOI 

 model. The later type is called GEK Model V. 

 Although the two models operate in almost the 

 same manner, there are minor differences in loca- 

 tions and arrangements of oj^erating switches 

 and dials. The GEK model V (fig. M-21) is 

 more compact than the earlier WHOI model. 

 Both use Speedomax type-G recorders. Opera- 

 tion and maintenance manuals for this type 

 recorder are supplied by the manufacturer. 



M-17 



