Page 589 radio acoustic ranging 635 



salinity (6342), the velocities may be plotted with reference to depth on cross-section 

 paper and a graph may be drawn to represent the velocities at all points through a 

 vertical column of the water. In the same manner that temperatures and salinities are 

 combined, the Telocity curves may be combined and an average velocity curve drawn, 

 from which a mean velocity from surface to each depth may be determined. Velocity 

 gradients affect the propagation of sound in sea water to a great extent, and they are of 

 utmost importance in R. A. R. Even a graph of the temperatures plotted with reference 

 to depth will reveal the approximate character of the velocity gradient in the upper 

 layers, where temperature changes are relatively large. 



R.A.R. smooth sheets are frequently plotted by using velocities determined experi- 

 mentally; that is, by measuring the travel time of a subaqueous sound between two 

 points whose horizontal distance apart is known. The distance divided by the elapsed 

 time gives the velocity of forward propagation. Such a velocity is known as an 

 apparent horizontal velocity, because it may differ from the actual velocity of the sound 

 wave in the medium, due to the propagation path of the wave (see 623). It is, never- 

 theless, the value that is needed for plotting distances in R.A.R. Apparent horizontal 

 velocities are satisfactory for use in an area where the velocity increase with depth is 

 nearly uniform from surface to bottom; but where the velocity decreases appreciably 

 with depth, apparent horizontal velocity can be used with accuracy only for distances 

 and depths approximately the same as those where the test was made and for limited 

 periods of time in the same locality. 



Apparent horizontal velocity is frequently determined from the elapsed time 

 required for a subaqueous sound to travel between two buoys, the distance between 

 them having been measured by taut wire. In an area adjacent to prominent shore 

 signals or high mountains which have been accurately located, it is generally deter- 

 mined from elapsed times from R.A.R. stations to positions fixed by sextant angles, 

 the horizontal distances being determined graphically or by computation. An 

 apparent horizontal velocity determined between two buoy stations a sufficient dis- 

 tance apart is usually more reliable than a value based on distances depending on sextant 

 fixes, because the distance involved is usually more accurately known in the former 

 case. 



635. Determination of Velocity 



An accurate knowledge of the velocity of sound in sea water is obviously of vital 

 importance in R.A.R. and echo sounding, for without it accurate hydrographic surveys 

 using these methods of position determination and depth measurement are impossible. 

 Tables have been prepared from which the theoretical velocity at any depth may be 

 determined if the temperature and salinity of the water at that depth are known (see 

 6343) . Theoretical velocities are entirely satisfactory for use in computing corrections 

 to echo soundings, but for R.A.R. the path of the sound wave through the water must 

 also be taken into account before accurate positions can be determined. 



The path of the effective sound wave in R.A.R. varies with the character of the 

 velocity gradient, as explained in 6231, and at times and in many areas it is very difficult, 

 if not impossible, to determine velocities satisfactory for accurate plotting unless the 

 elapsed times are first corrected for the path of the sound wave. Because of the com- 

 plicated nature of the problem, no entirely satisfactory method of doing this has as yet 

 been devised. Regardless of the method used to plot R.A.R. distances on the smooth 

 sheet, the hydrographer must realize that the elapsed times are greater, by an amount 



