6362 HYDROGRAPHIC MANUAL PaGE 594 



to a certain depth, perhaps arbitrarily chosen; or one of these to which an arbitrary 

 correction is apphed based on apparent horizontal velocities experimentally determined. 

 The cases that occur fall into five categories, depending on the type of velocity gradient. 

 These categories are described below, together with the procedure to be followed. It 

 is thus incumbent on the hydrographer to determine, in any given case, which procedure 

 should be used. 



Tlu'ough a study of the velocity curves obtained (see 6361(2)), the project area is 

 divided into sections, if necessary, with characteristics as nearly uniform as the available 

 information warrants. The category is next determined. If it is not obvious from the 

 velocity curves and the text in (a) to (e), into which category a sectional area belongs, 

 recourse must be had to the procedure in 6361(3). Thus the final criterion by which 

 procedure (a) to (e) is determined, is greatest practicable accuracy. The proper cate- 

 gory is selected by applying the following rules to the apparent horizontal velocities 

 determined by test. 



(a) Velocity nearly constant from surface to bottom. The apparent horizontal velocity determined 

 by test should almost equal the mean computed velocity from surface to bottom at the time and loca- 

 tion of test. Use mean computed velocity for -plotting. This condition is general in many ocean areas 

 during the winter and early spring. For moderate distances the sound wave will follow practically 

 a straight path from source to point of reception. Where the distance involved is long enough for 

 the bottom to obstruct this path, because of the curvature of the earth, the first sound wave received 

 will be one which has been reflected from the water surface — the reflection from the bottom being a 

 later arrival. It is under these conditions that R.A.R. will function most favorably and at maximum 

 distances. Any increase in the length of the path due to reflection of the sound wave will l)e so slight 

 that the sound wave can be assumed to travel a horizontal path. 



{b) Velocity increases moderately from surface downward to some depth at which it starts to decrease. 

 The apparent horizontal velocity determined by test should almost equal the mean theoretical velocity 

 at time of test from surface to the depth where the velocity starts to decrease. Use mean computed 

 velocity from surface to the depth where the velocity starts to decrease. The path of the sound wave will 

 be a succession of widely spaced reflections from the surface. The first sound impulse received will 

 never have been reflected from the bottom. The increased length of the path of the sound wave will 

 be so slight that error thus produced can be ignored and the sound wave can be assumed to travel a 

 liorizontal path. Where the R.A.R. distances are relatively short, the velocity based on the physical 

 conditions at or near the surface may have to be used, rather than the mean velocity. 



(c) Velocity increases rapidly from surface downward to some depth at which it starts to decrease. 

 The apparent horizontal velocity determined by test should be less than the mean theoretical velocity 

 at time of test computed from surface to the depth where the velocity starts to decrease. Use mean 

 computed velocity from surface to the depth where the velocity starts to decrease, corrected by the difference 

 determined by test. The sound wave is refracted sharply upward so that the path is a succession of 

 relatively closely spaced reflections from the surface. The increase in the length of the path is too 

 great to permit the use of mean computed velocity without correction. Furthermore, if the distance 

 is long, the sound wave reflected from the surface may be attenuated so that it is never received, in 

 which case the received sound wave is one reflected from the bottom. For this reason it may be prefer- 

 able to u.se apparent horizontal velocity if a sufficient number of adequately distributed te.sts can be 

 made. 



(d) Velocity decreases from the surface downward to a depth at which it starts to increase. The 

 apjiarent horizontal velocity determined by test should be appreciably less than the mean theoretical 

 velocity at time of test computed for a section of water from surface to bottom. This type of velocity 

 gradient is general during the summer months. Use apparent horizontal velocity, or mean computed 

 velocity from surface to bottom corrected by the difference deterjuined by test. Under such conditions the 

 sound wave is refracted sharply downward through the upper layer of water if the decrease in velocity 

 is rapid, and is reflected repeatedly between surface and bottom. The path of the sound wave is 

 appreciably longer than the horizontal distance and uncorrected computed velocities cannot be used. 



(e) Velocity computed for a selected depth. A comparison of the apparent horizontal velocity 

 determined by test discloses that it is in consistent agreement with the velocity computed for a certain 



