5. HYDROGRAPHY 



183 



Table 11. — Example of velocity correction computations 



(Computed for echo soundings taken with an instrument calibrated for a velocity of 



sound of 820 fathoms per second.) 



(a) Starting at a depth which approxi- 

 mates the draft of the transducers (2 fath- 

 oms in the example) or at a depth fixed by 

 bar check, divide the first hundred fathoms 

 into 5-fathom layers and enter in column 

 (A) the mid-depth of each layer. Starting 

 with 100 fathoms enter the mid-depth for 

 each 20-fathom layer from 100 to 200 fath- 

 oms. For depths greater than 200 fathoms 

 enter the mid-depth of 200-fathom layers. 



(b) Scale the temperature and salinity 

 values from the mean regional curves for 

 the mid-depth of each layer and enter them 

 in columns (B) and (C) respectively. 



(c) Derive the velocity for each tempera- 

 ture and salinity and depth and enter the 

 velocities in column (D). The velocities may 

 be found from the tables in 7-28 or from 

 the diagrams in 7-29. 



(d) Derive the factors corresponding to 

 each layer velocity and enter them in column 

 (E). Table 17 gives these correction fac- 

 tors for velocities of sound to the nearest 

 meter for instruments operating at three 

 calibration velocities. For an instrument 

 operated at any other calibrated velocity of 

 sound, the factor can be obtained from the 

 formula : 



-= ± factor 



in which A = actual mean velocity and C = 

 calibration velocity. 



(e) Multiply each correction factor in 

 (E) by the layer thickness in fathoms to 

 derive the correction applicable to this depth 

 interval. Enter these products in column 

 (F). 



