182 



HYDROGRAPHIC MANUAL 



raphy of the area. A minimum of one serial 

 temperature shall be observed in the deepest 

 part of the area surveyed each month. 



There are several methods by which a cor- 

 rection for velocity may be determined. 

 These are described in the following sections 

 and any one of them may be used as may 

 be appropriate for the depth. 



5-115 Velocity correction by bar check. 



— For soundings in shoal water, the bar 

 check can be used to derive corrections ap- 

 plicable to the various depths (see 3-109). 

 Only bar checks taken under the most favor- 

 able conditions are considered sufficiently 

 accurate for this purpose. The reliability of 

 bar check results is influenced by the size of 

 the reflecting surface of the bar, the fre- 

 quency and beam width of the transducer, 

 and condition of the sea. Bar checks of 

 sufficient accuracy can seldom be obtained at 

 depths greater than 60 feet. Where the maxi- 

 mum depth does not exceed 120 feet, the 

 correction curve can be extended to this 

 depth from simultaneous comparisons with 

 accurate leadline or wire soundings. 



Data obtained from bar checks and verti- 

 cal cast comparisons shall be used to draw a 

 correction curve. The hydrographer must 

 use his judgment as to whether a number of 

 bar checks should be averaged over a con- 

 siderable period of time, and when a new 

 correction curve should be drawn. Correc- 

 tions which differ more than 0.4 foot shall 

 not be averaged. 



It should be emphasized that this method 

 is not to be used unless exceptionally good 

 results are obtained from bar checks and 

 they are consistent enough to give reason- 

 able assurance that the greatest part of the 

 difference is due to the velocity of sound. 

 Velocity corrections must not be extrapolated 

 from a correction curve so obtained for 

 depths greater than one-third more than 

 the range of the bar checks (see 5-110) . 



5-116 Vekicity correction by algebraic 

 method. — In the algebraic method of com- 

 puting velocity corrections, the column of 

 water from surface to bottom is considered 

 layer by layer and a correction for each 



depth layer determined. The summation of 

 layer corrections will give the correction ap- 

 plicable to a given depth. The corrections 

 may be derived numerically or graphically 

 by procedures described in the following 

 paragraphs. 



All serial temperature and salinity ob- 

 servations are plotted on Form 121 and the 

 curves drawn. Salinity in a project area will 

 seldom change by an amount sufficient to 

 affect the corrections appreciably. Therefore 

 it is usually practicable to draw an average 

 salinity curve for the entire season or project. 

 This will not be true where large amounts of 

 fresh water are discharged by rivers and 

 under such circumstances a series of regional 

 curves will be required. The temperature 

 curves must be studied to determine how 

 these can best be grouped by area or by time 

 to permit mean curves to be drawn. The 

 temperatures and salinities scaled from the 

 mean regional or period curves for the mid- 

 points of the adopted layers are used to de- 

 rive the velocity corrections either numer- 

 ically or graphically. It must be borne in 

 mind that, to comply with requirements, the 

 average temperature used to compute a cor- 

 rection must be within 2° C of the actual 

 mean temperature. 



5-117 Numerical determination of veloci- 

 ty corrections. — The standard form of ve- 

 locity correction computations by the nu- 

 merical method is illustrated in Table 11 

 with columns identified by the letters (A) 

 to (H) for reference purposes only. It should 

 not be assumed that all depth layers, for the 

 purpose of determining velocity corrections, 

 need to be chosen as shown in this example, 

 but from past experience 5-fathom layers in 

 the upper hundred fathoms, 20-fathom layers 

 in the second hundred fathoms, and 200- 

 fathom layers in greater depths have usually 

 been found satisfactory. Where the change 

 in temperature is regular with respect to 

 depth and not too great, thicker layers may 

 give sufficient accuracy. 



The procedure, referring to Table 11, is 

 as follows : 



