PRACTICAL PROBLEMS IN THE DIRECT 

 MEASUREMENT OF OCEAN CURRENTS 



R. G. PAQUETTE 



Defense Research Laboratories 



General Motors Corporation 



Santa Barbara, California 



ABSTRACT 



The motion of water in the sea is a complex 

 of turbulent, oscillatory, sporadic and steady 

 movements with wide scales of size, velocity and 

 period. All of these vary with wind, season and 

 year and generally decrease in magnitude from the 

 surface downward. Attention may he concentrated 

 on various aspects of the motion by taking mea- 

 surements which are time-integrated with appro- 

 priate time constants. In many situations the 

 integrated mean vector is much smaller than peaks 

 of the instantaneous velocities and may have 

 little correlation with the instantaneous direc- 

 tion. Attempts to deduce mean velocities from 

 measurements having too short a duration may 

 lead to large errors. The current meter of most 

 general utility would he one which can be kept 

 in place continuously to record continually on 

 as short a time scale as is feasible. 



Current meters are subject to many errors due 

 to undesired motions of the flexible supporting 

 cable and the supporting platform. These errors 

 are described and estimated. Existing current 

 meters are of severely limited value for any but 

 the grossest measurements when the suspensions 

 must be long, relatively flexible or disturbed 

 by waves. Periodic stray motions could be 

 removed by vector-integration either in the meter 

 or at a later date on the record, but with con- 

 sequent serious deterioration in the representa- 

 tion of real changes of similar period. Small 

 systematic errors accumulate to such important 

 magnitudes in long integrations that meters hav- 

 ing accuracy, rapid response and faithful inte- 

 grating characteristics in a high degree are 

 required. 



INTRODUCTION 



The direct measurement of ocean currents 

 appears simple, perhaps because current meters 

 are relatively simple and because current meters 

 are described in text books and in the litera- 

 ture well dissociated from any mention of prac- 

 tical field problems. It is to dispel this 

 fallacious idea of simplicity in the minds of 

 the many new instrument designers entering the 

 field of oceanography that this paper is written. 

 It will be shown that the measurements of cur- 

 rents by means of conventional meters suspended 

 from ships, buoys or other platforms are 



seriously distorted in many ways due to the stray 

 motions induced by the platform and supporting 

 cable as well as the imperfect response of the 

 meter to the resulting transients and to the real 

 transients which exist in the sea. Some of the 

 information to be presented has appeared inci- 

 dentally and in a qualitative way mostly in scat- 

 tered papers describing successful series of 

 current measurements at sea. It is hoped that 

 the present more unified and semi-quantitative 

 discussion will be a stimulus to the design of 

 equipment and techniques which will be useful in 

 the era of unattended instruments which is now 

 beginning. 



The discussion will treat specifically those 

 current meters whose velocity element is a rotor 

 but it may be applied with little modification to 

 other types, such as those having pressure plates, 

 tilting bodies or acoustic paths as sensors. 

 The emphasis will be on depths and velocities 

 typical of the open sea but will apply also to 

 estuarine motions in which the problem has not 

 become grossly simplified by the shallowness of 

 the water. 



The problem of current measurement cannot be 

 attacked in the abstract. Instead it must be 

 faced in relation to conditions which exist in 

 the sea and to that aspect of the complex motion 

 which it is desired to study. For this reason 

 the discussion will begin with a general descrip- 

 tion of the types of motion in the sea in relation 

 to the portion of the motion which the experi- 

 menter may wish to extract . 



DISCUSSION 



Types of Motion in the Sea 



The net motion of water in the sea may be 

 considered as a system of average established 

 currents upon which are superimposed transients 

 that may be slowly changing and with a size scale 

 of thousands of miles . At the other extreme is 

 the random motion of turbulence of a size scale 

 smaller than we care to deal with now and a time 

 scale correspondingly short. There are motions 

 which are periodic, such as those of tidal 

 period, inertial rotations and currents associ- 

 ated with surface and internal waves . Others are 

 more or less random, such as the wind driven 

 transients and the various scales of turbulence. 



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