13. DEEP-CURRENT MEASUREMENTS USING NEUTRALLY 

 BUOYANT FLOATS 



G. H. VOLKMANN 



1. Introduction 



Our knowledge of the patterns of circulation in the deep oceans has come 

 largely through tracing particular properties such as temperature, salinity and 

 oxygen as they are modified with increasing distance from their source. Water 

 of Antarctic origin can be traced to the area south of the Grand Banks in the 

 Atlantic. The lack of a source of deep or bottom water in the North Pacific may 

 be deduced. Wiist (1935) has used a more refined technique to trace quantita- 

 tively the spreading of Mediterranean water into the North Atlantic (and more 

 recently, the ultimate source area of this water within the Mediterranean 

 itself). This method remains the most powerful one for the deduction of the 

 general features of circulation. 



It is, however, not appropriate for the deduction of current velocities and 

 transports since they will be dependent on the mixing rates along the path of 

 the current, and the process of mixing in the ocean is, at best, incompletely 

 understood. 



With few exceptions the velocities and transports associated with the circula- 

 tion in the ocean have been deduced from the geostrophic equation, in which 

 the horizontal pressure gradients associated with a variable density field are 

 balanced by the Coriolis force. 



The most satisfactory method of measuring currents is to observe them 

 directly. However, this is a time-consuming process and, until recently, devices 

 for such measurements have been somewhat marginal. Pillsbury's measure- 

 ments in the Straits of Florida are the outstanding example of this technique 

 and also give a fair indication of the difficulties involved. 



In 1955 J. C. Swallow of the National Institute of Oceanography developed 

 an instrument for the measurement of deep ocean currents. Coincident with 

 this has been a renewed interest in the deep circulation of the oceans. Swallow 

 and others are applying this device to the problem, and a picture of the motions 

 at depth is slowly emerging. 



2. The Pinger 



Swallow's instrument, called the pinger, consists of a small acoustic source 

 enclosed in a pressure case which can be set to float with neutral buoyancy at 

 a predetermined depth without attachment to surface or bottom. Being 

 naturally buoyant at that depth it will move more or less with the current at 

 that depth and a series of fixes made by taking bearings on its acoustic source 

 will determine its track. 



The problem may be divided into three parts: (1) the construction and 

 calibration of the pinger itself, (2) the design of a properly matched listening 

 system, and (3) navigation. 

 [MS received November, 1960] 297 



