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HYDROGRAPHIC MANUAL 



Page 532 



Table 20.- — Example of velocity correction compulations 

 [Computed for echo soundings taken with an instrument calibrated for a velocity of sound of 820 fathoms per second. 



The procedure, referring to table 20, is as follows: 



(1) The transmitted sound originates at, and the echo returns to, an acoustic unit which is some 

 distance below the surface of the water. For survey ships, for velocity correction purposes, this may 

 be assumed to be 2 fathoms. (For auxiliary vessels and launches the acoustic units are at other depths, 

 which should be used.) Starting then at 2 fathoms below the surface, divide the first hundred fath- 

 oms of depth into 5-fathom layers and enter in. column (A) the mid-depth of each 5-fathom layer. For 

 example, the first entry would be 4.5 fathoms, or the mid-depth between 2 and 7 fathoms, and similarly 

 the entries for the following 5-fathom layers are 9.5, 14.5 fathoms, etc. Starting with 100 fathoms, 

 enter the mid-depth for each 20-fathom layer for depths from 100 to 200 fathoms, disregarding now the 

 depth of the acoustic unit below the surface, as for example, 110, 130 fathoms, etc. For depths deeper 

 than 200 fathoms enter the mid-depth for each 200-fathom layer, as for example, 300, 500 fathoms, etc. 



An adjustment is required at the deepest 5-fathom layer, below which the depth of the acoustic 

 unit is ignored. Note that the 5-fathom layer whose mid-depth is 94.5 fathoms extends from 92 to 

 97 fathoms. This leaves a layer of only 3 fathoms whose mid-depth is 98.5. This adjustment must 

 be carried over into column (G). (See (6) below.) 



(2) Obtain the temperature and salinity values from the mean regional curves for the mid-depth 

 of each layer in column (A) and enter them in columns (B) and (C) respectively. 



