XI. BATHYMETRIC MEASUREMENTS 



Bathy metric Section 

 Survey Branch 



A. INTRODUCTION 



The hydrographic survey programs of the Hydrographic Office 

 include the measurement of depths of water in open ocean areas as well 

 as in areas near the coast. The depths measured range from practi- 

 cally zero to about 6,000 fathoms. Although depths generally are 

 expressed in fathoms, in shallow water it is customary to use feet and 

 fractions of feet for greater definition. In many foreign countries other 

 unit systems are used, the most common being the metric system. 

 In coastal waters less than 100 fathoms, soundings are reduced for 

 tidal height. The datum used for these reductions varies in different 

 parts of the world. On the Atlantic coast of the United States, the datum 

 is Mean Low Water, whereas on the Pacific Coast Mean Lower Low 

 Water is used. 



The echo sounding machine has almost completely replaced the 

 older methods of obtaining soundings. This instrument consists of four 

 parts: (1) A transmitter which generates an electrical impulse which is 

 transmitted to (2) a transducer, which translates the electrical impulse 

 into a sonic impulse atthe same frequency either by means of the piezo- 

 electric effect of quartz or other crystals or the magnetostriction 

 effect of a nickel alloy embedded in a ceramic rod. The sonic impulse 

 travels through the water to the bottom, or any other reflecting surface, 

 and an echo is returned to the transducer where it is transformed 

 again into an electrical impulse and passed to (3) a regular radio 

 frequency receiver where the signal is detected, amplified, and sent 

 to (4) a recorder where the travel time from the initial impulse to 

 the return of the echo is measured and displayed. Most American- 

 built instruments are calibrated for an assumed velocity of sound in 

 sea water of 4,800 feet per second. The actual velocity varies with the 

 temperature, pressure, and salinity of the water in the column being 

 sounded. Since the actual velocity is normally faster than the calibra- 

 tion velocity, the sounding indicates slightly less water than actually 

 is there, thus providing a safety factor. In effect, a repeatable depth 

 rather than a true depth is indicated, since any ship using an echo 

 sounder with the same calibration will obtain the same depth at a 

 given location without the necessity of computation. For this reason, 

 H. O. charts are published with soundings uncorrected for sound 

 velocity. 



XI- 1 



