6354 HYDROGRAPHIC MANUAL PaGE 592 



in which the terms p, q, r, and s, stated in known values with a equaling the angle BAC (see fig. 132), 

 are as follows: 



C 

 V 



-Ci) 





c 



-a: 

 {^-(t)i-<--ay] 



cot oc 



In solving for v, the sign of the term to the right of the ± sign must be chosen to give an apparent 

 horizontal velocity in approximate agreement with the estimated value. If the wrong sign is used, 

 the error will be obvious, in most cases, from the value obtained. 



It is obvious that there must be no doubt regarding the accuracy of the elapsed 

 times used in the computation, and conditions of temperature, salinity, and depth 

 should be similar along the three paths of travel; otherwise an erroneous value for the 

 velocity of sound will result. Computations should be made for several different 

 positions and if a reasonable check is obtained, the results should be averaged for use. 



6354. Effect of Current on Velocity of Sound 



If sound travels through water which is moving as a mass, the velocity of sound 

 will be increased or decreased by an amount depending on the velocity and direction 

 of the movement. A current of 1 knot whose direction is the same as the propagation 

 of sound, increases the apparent horizontal velocity of sound by a little more than 0.5 

 meter per second, and of course decreases it by an equal amount if the duections are 

 opposite. Any other current will increase or decrease the velocity by an amount 

 depending on the strength of the current and the relation between its direction and 

 the direction of propagation. Ocean currents are generally of moderate strength so 

 their effect on the velocity of soimd is not large; furthermore, sufficient information 

 is seldom available to correct for it; therefore the influence of current on the velocity 

 of sound is usually disregarded in R.A.R. 



The abnormal temperature and salinity conditions caused by turbidence near the 

 edges of an ocean current, and in eddies generally, have a pronounced effect on the 

 propagation of sound directed through them. The soimd wave may be completely 

 dispersed at such places, so that no sound is received beyond them. This effect has been 

 repeatedly observed at the edge of the Gulf Stream in R.A.R. surveys off the South 

 Atlantic Coast. 



636. Methods of Applying Velocity Data 



Corrections to echo soundings are computed from theoretical velocities based on 

 depths, and average temperatures and salinities. The method of determining these 

 corrections is explained in 561. 



