6351 HYDROGKAPHIC MANUAL * PaGE 590 



varying with distance, depth of water, and character of velocity gradient, than they 

 would be if the sound had traveled by a direct horizontal path. The apparent horizon- 

 tal velocities needed for plotting are lower than the corresponding theoretical velocities, 

 the difference generally increasing as the depths and the distances from the R.A.R. 

 stations increase 



6351. Apparent Horizontal Velocity From Tests 



Where apparent horizontal velocity is to be used in plotting, the values should be 

 derived from distances slightly longer than the R.A.R. distances involved, if practi- 

 cable. Under unfavorable conditions it may be necessary to determine apparent hori- 

 zontal velocity for various distances from each R.A.R. station, using values varying 

 with distance when plotting. 



Apparent horizontal velocity tests should be made under carefully selected condi- 

 tions. Where practicable, the temperature and salinity conditions should be normal for 

 the period and area, and the depths should be average for the area. If the tests are made 

 between two buoys, the distance between them should have been measured with taut 

 wire and the tests should be made when there is a strong steady current to make the 

 buoys lead approximately the same amount and direction from their respective anchors. 

 A period of slack current should not be selected, for then the buoys may be leading in 

 different directions, thus introducing an error into 'the horizontal distance between 

 them. Wliere sextants are used to fix the positions of the bombs for tests, only strong 

 fixes using accurately located shore stations should be used. A series of three to five 

 bombs, each located in this manner by an independent sextant fix, should be used to 

 obtain an average velocity for each distance for which tests are made. The sextant fix, 

 with a check angle if possible, should be marked at the instant the bomb strikes the water 

 and as near to this point as practicable; or the distance and bearing between the observa- 

 tion station and the point where the bomb strikes the water should be measured and 

 allowed for in plotting or computing the position. Bombs used in tests should be of the 

 same size as would be used at comparable distances in R.A.R. Results of tests that are 

 in any way questionable should be rejected and not considered in computing the veloci- 

 ties. The distances from the bomb positions to the R.A.R. stations may be either 

 computed, or scaled graphically on an accurate projection. 



An excellent opportunity is afforded to determine the apparent horizontal velocity 

 during a taut-wire measurement of the distance between two buoys, at least one of which 

 is a sono-radio buoy. Test bombs should be fired at frequent and regular intervals, 

 the taut-wire sheave being read at each bomb to determine an accurate distance to 

 the R.A.R. station. Such a test discloses any variation in apparent horizontal velocity 

 due to distance or depth, and the data obtained may be used to plot R.A.R., provided 

 the tests are based on sufficiently long distances. 



To correlate the apparent horizontal velocity with the theoretical velocity and to 

 provide adequate data for studies of the propagation of sound in sea water, a serial 

 temperature should be observed in the locality immediately before, or after, each test 

 or series of tests. If the test is in conjunction with a taut-wire measurement, a serial 

 temperature should be observed near one end before starting the measurement, another 

 near the other end after the completion of the measurement, and if practicable, a third 

 about halfway between, if the distance exceeds 10 miles. 



