SECT. 3] DEEP- CURRENT MEASUREMENTS 299 



engineering formulae. While aluminum is not the only material with these two 

 properties, it is the easiest to machine and to obtain. 



In the present pinger circuits, no back discharge can occur through the gas 

 trigger tube and the length of the pulse is governed by the mechanical resonance 

 of the ring itself. The output is in the vicinity of 80 dB/(j.b at 1 m. Higher 

 efficiency and longer pulses may be obtained from tuned oscillator circuits. 



Equally important in increasing listening range is a properly designed 

 listening system. While it Mould appear that a filter sharply tuned to the pinger 

 frequency would be best, the relatively short (1-2 msec) pulse from the pinger 

 places an upper limit of about 1000 c/s on the band width. Another important 

 feature of the system is that it has strong rejection in the lower frequencies, since 

 it is here that most of the ship noises occur. The receiving hydrophones should 

 have a sensitivity of about — 75 dB referred to 1 fxb/V in order to get down to 

 ambient noise on a quiet day. Ranges of 3 to 4 miles are standard with the 

 present system and ranges of 6 and 7 miles have been obtained on quiet days. 

 For the shallower pingers (less than 700 m), near-surface refraction of sound 

 by sea-water can limit the range to an abrupt 2 miles or less. 



Positions of the pingers are obtained by taking a series of three or more cross- 

 bearings. Under most conditions the bearing accuracy is about + 3 degrees. 

 This means that the accuracy of the fixes is limited by the accuracy of the 

 navigation. In limited areas, Loran and Decca are available. In other areas 

 anchored buoys and bottom topography have been successfully used. 



At great depths the gradient of the density of sea-water is almost solely a 

 function of the pressure and the pinger may be expected to follow an isobar 

 fairly closely. By fortuitous circumstances the temperature coefficients of 

 water and aluminum are quite similar so that at shallower depths, where the 

 properties of the water are changed more rapidly by mixing, the pinger may be 

 expected to follow some isopleth which is a function largely of pressure and 

 salinity. Since the motion of the pinger will be affected by the turbulent mixing- 

 motions themselves, exact solution of its path in terms of the changing 

 properties should not be anticipated. 



Other devices using similar principles can be imagined. For example, an 

 enclosed volume of fresh water will have the same coefficients of temperature 

 and pressure and could be ballasted to follow a given isohaline. 



While to date pingers have been used only for measuring currents, the same 

 principle can be used to float a variety of sensing elements. The measurements 

 could either be telemetered to the surface acoustically or a release mechanism 

 could be used to bring internally recorded data to the surface. 



3. Measurements 



A number of measurements with pingers have been reported in the literature 

 and they suggest that deep currents are appreciably faster than had heretofore 

 been suspected. It is, however, measurements just completed in the spring 

 and summer of 1960 that form the main body of deep-current measurements 



