5148 HYDEOGRAPHIC MANUAL PaGE 446 



ment. During rough weather when the vessel is pitching or rolling violently the num- 

 ber and intensity of strays on the registering device may be sufficient to obscure all 

 soundings. Strays caused by the mption of the vessel usually come in groups, between 

 which it is usually possible to detect a few true soundings. The acoustic noises origi- 

 nating in the ship, which cause strays, generally come from some parts of the machinery, 

 such as the main engines, pumps, clapper valves, or auxiliary equipment, but they may 

 also be caused by chipping or similar work performed on some part of the ship's hull. 

 Electric noises, generated either in the ship or in the echo-sounding equipment, may 

 have such a periodicity of recurrence that tfiey appear as echoes on the registering device. 

 When the echo is weak, as it may be in deep water, strays are more bothersome, since 

 their intensity level is then relatively higher. It is necessary in this case to increase the 

 gain of the amplifier to build up the weak echo, but the spurious noises are thereby also 

 amplified in the same proportion. 



5148. Velocity of Sound 



The accuracy of echo soundings is directly dependent on an accurate knowledge of 

 the velocity of sound in the water. This velocity is subject to regional, seasonal, and 

 diurnal variations. Echo-sounding instruments are operated for a certain assumed 

 velocity of sound, known as the calibration velocity, and any depth measurement is, 

 therefore, in error by an amount directly proportional to the variation of the actual from 

 the assumed velocity. This variation is a matter of concern in hydrographic surveying. 

 Correction must be made for it, and an unknown or unexpected variation may introduce 

 errors greater than the allowable limits. (See 561.) Systematic measurements of the 

 physical characteristics of the water are made to determine the exact velocity so that the 

 measured depths may be corrected to be within at least one-half percent of the true 

 depth. 



The velocity of sound in sea water, averaged from surface to bottom, in any of the 

 waters of the earth is usually within a range of 4,560 feet (760 fathoms) to 5,100 feet 

 (850 fathoms) per second. In the areas usually surveyed by the Coast and Geodetic 

 Survey the average velocity has been found to be about 4,920 feet (820 fathoms) per 

 second. If an average velocity of 820 fathoms per second is assumed, the maximum 

 error of uncorrected echo soundings in any locality will be 8 percent. Generally the 

 actual velocity does not vary materially from the assumed average and uncorrected 

 echo soundings are, therefore, sufficiently accurate for use in navigation. (See also 

 section 63.) 



515. General Classification of Echo-Sounding Instruments 



Echo-sounding instruments may be classified in various ways; for example, accord- 

 ing to their use, the frequency of the sound utilized (sonic or supersonic), or the type of 

 registering device used. Or, in order to give a more complete description of the type 

 of instrument, combinations of these classifications are employed; for example, a 

 particular instrument may be called a shoal-water, graphic-recording, supersonic, echo- 

 sounding instrument. Classifications as to use may be further subdivided according to 

 the service for which the instrument is intended or the depth range of the instrument. 



Except for some specialized uses, most echo-sounding instruments are intended for 

 use in either navigation or hydrographic surveying. This does not necessarily mean 

 that an instrument designed for one purpose cannot be used for two or more purposes. 



