561 HYDROGRAPHIC MANUAL PaGE 528 



and harbor surveys, where the plane of reference is known in advance and communica- 

 tion can be maintained between the tide station and the survey party, the necessary 

 corrections are sometimes applied as the soundings are taken, and there are echo- 

 sounding instruments with an adjustment by which the correction for tide can be applied 

 instrumentally (see 529). Also, in the case of soundings recorded graphically, it is 

 possible to apply the correction for tide mechanicaPy or graphically at the time the 

 depths are scaled (see 562). 



The correction for the variation in the velocity of sound is, perhaps, the most 

 important correction to be made to echo soundings. In the Coast and Geodetic Survey 

 all echo-sounding instruments are operated at an instrument speed selected to register 

 soundings based on a chosen velocity of sound, known as the calibration velocity. 

 Whenever the actual mean velocity of sound from surface to bottom differs from this 

 calibration velocity, a correction must be applied. This correction is usually made 

 by algebraic addition to soundings recorded in the Sounding Record, although it can 

 be made by instrumental adjustment (see 5616), assuming an advance knowledge of 

 the velocity of sound, or it can be made mechanically or graphically to soundings 

 scaled from fathograms (see 562). The methods of deriving and applying corrections 

 for the velocity of sound are described in 561. 



Echo soundings registered on some echo-sounding instruments do not represent 

 vertical depths in areas of irregular submarine relief and, theoretically, so-called cor- 

 rections for slope should be made. It is not practical to do this with any great preci- 

 sion in most cases, and there are other reasons why it should not be done. (See 563.) 



561. Velocity Corrections 



In echo sounding the sound wave passes vertically downward through a column of 

 water in which the velocity of sound differs at different depths, and since the true depth 

 is a product of velocity and time, the average velocity from surface to bottom must be 

 known for each sounding. The velocity of sound in sea water depends on the tempera- 

 ture, salinity, and pressure (see section 63), and the velocity of sound used in echo 

 sounding is usually calculated from these physical characteristics. 



The hydrostatic pressure increases in direct proportion to depth, and the tempera- 

 ture and salinity of the water usually decrease with depth, but not necessarily propor- 

 tionally. The result is that the velocity of sound, by which an echo sounding must 

 be computed, is seldom uniform from surface to bottom. Hence, the average velocity 

 of sound from surface to bottom will be different for echo soundings of different depths. 

 Furthermore, due to regional and seasonal changes in the physical conditions, the aver- 

 age velocity from surface to bottom for the same depth changes from place to place and 

 from time to time. 



Because of the theoretically linear relationship which exists between the travel 

 time required for an acoustic wave to travel between two points, and the distance 

 between them, echo-sounding instruments may be calibrated in units of depth. Those 

 used by the Coast and Geodetic Survey are driven by motors operated at a constant 

 speed for a pre-selected velocity of sound, usually 820 fathoms per second (see 555). 

 This is known as the calibration velocity. Therefore, whenever the average velocity of 

 sound from surface to bottom differs from the calibration velocity of the echo-sounding 

 instrument, the registered depths must be corrected to what they would have been on 

 an instrument calibrated for this actual velocity. This correction is known as the 

 velocity correction. 



