Page 575 radio acoustic ranging 631 



631. Practical Use of Velocity of Sound 



The computation of velocity corrections for echo sounding and of distances in 

 R.A.R. surveys are impossible unless the velocity of sound at the time of such surveys 

 is known with reasonable accuracy. In most water areas the velocity is variable from 

 surface to bottom and, principally because of the seasonal variation in water* tempera- 

 ture and salinity, the velocity varies seasonally. The temperature and salinity of the 

 water must be measured at various depths at well-distributed points in the project area 

 and at frequent intervals, so that average values of the velocity of sound may be deter- 

 mined with sufficient accuracy. 



6311. Velocity Corrections to Echo Soundings 



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. These 

 instruments are described in chapter 5. In most instruments, the motion of the depth- 

 registering device is regulated to give correct measurement for an assumed velocity of 

 sound in water, known as the calibration velocity. If the actual velocity at any time 

 differs from the calibration velocity, 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, its magnitude varying with depth as well 

 as with the divergence of the actual velocity from the calibration velocity. A velocity 

 correction is based on the mean of all the velocities from the surface to the bottom for 

 that depth. In order to determine these mean velocities, the instantaneous velocity 

 in each of the depth layers to the greatest depths and in all parts of the project area 

 must be known. (See also 561.) 



6312. Use in Radio Acoustic Ranging 



A knowledge of the velocity of sound is required in R.A.R. surveys in order to 

 reduce the time intervals to linear distances. Sound is rarely propagated in a straight 

 horizontal line through sea water — under the physical conditions usually prevailing 

 it is refracted and reflected tln-ough a longer path so that the actual elapsed time is 

 longer than it would be if the path were a straight line. It is the apparent horizontal 

 velocity that is wanted in R.A.R. , but the problem of determining it under all conditions 

 from data easily observed is indeed complicated, if not impossible. Some method of 

 reducing, to a horizontal basis, the measured times of sound travel along the refracted 

 and reflected path is needed. 



6313. Accuracy of Velocity Determination 



For use in correcting echo soundings the velocity of sound must be known with 

 sufficient accuracy to ensure that no sounding will be in error from this cause alone by 

 as much as one-half percent of the depth. Therefore the mean velocity of sound must 

 be known for each sounding with an error not greater than 7.5 meters per second Of 

 the three characteristics of sea water affecting the velocity of sound, temperature is the 

 most variable, and to satisfy the above requirements the mean temperature of the 

 vertical column of water at each sounding should be known within approximately 

 2° C. The frequency with which serial temperature observations must be made to 

 attain this accuracy is discussed in 6322. 



