Page 529 echo sounding 5611 



To correct any sounding it is necessary to know the average velocity of sound from 

 surface to bottom and the caHbration velocity of the echo-sounding instrument. From 

 these a factor may be derived by which to multiply a registered depth to find the cor- 

 rection to be added to or subtracted from it. This factor is found from the formida: 



Actual mean velocity— calibration velocity 



/ — : ^= ± factor 



calibration velocity 



Theoreticall}^, for each sounding the velocity of sound should be known at every 

 depth from surface to bottom in order to have a correct mean velocity. Practically, 

 of course, it is necessary to use average velocities based on average conditions of tem- 

 perature and salinity for certain periods of time and for certain areas. The extent of 

 the area and the time through which average conditions may be assumed depend on 

 the stability of the physical conditions during the period of the survey. 



There are several methods by which a correction for velocity may be made. The 

 universal one, which is applicable in any case, is the algebraic method outlined in 5611, 

 by which velocity corrections for soundings of various depths are derived algebraically, 

 and are applied to the recorded soundings algebraically. Other methods of more limited 

 application, which may be used in special circumstances, are described in 5616, 5617, 

 and 562. 



5611. Velocity Correction by Algebraic Method 



Since the temperature and salinity of the water vary irregularly with depth, and 

 since it is necessary to determine a correction for velocity for the entire depth, the 

 conditions at various depths must be considered and at some point in the computations 

 averages must be derived applicable to the entire depth, or the corrections may be 

 found by summation. The temperatures and salinities from surface to bottom may 

 be averaged to find average conditions for the entire column of water, from which one 

 velocity may be derived; or velocities from surface to bottom may be averaged to 

 find an average velocity for the entire depth; or the column of water from surface to 

 bottom may be considered layer by layer and a correction for each depth layer de- 

 termined, the summation of which will give the correction applicable to a given depth. 

 The last has been adopted by the Coast and Geodetic Survey as the most practicable 

 means of deriving the corrections. It can be accomplished either numerically or 

 graphically, the two methods being described in 5613 and 5615 respectively. 



The various steps to be performed to determine velocity corrections by the alge- 

 braic method are as follows: 



(1) After all of the serial temperatures and scattered temperature and salinity observations have 

 been plotted on Form J3-1528-5, in accordance with 6342, a study must be made of the results to 

 determine how these can be grouped best by area and by time to permit mean curves to be drawn. 

 These mean regional temperature and salinity curves are made in accordance with 5612, each being 

 plotted on Form B-1528-5. 



(2) After the mean regional temperature and salinity curves are available, a study of these must 

 be made to select the depth layers for which the physical conditions must be considered. Layers 

 must be selected of such thicknesses that the conditions at the midpoint may be considered applicable 

 to the entire layer. Usually it will be found necessary to adopt layers of less thickness near the 

 surface, but as the depth increases and the physical conditions change less relatively with depth, 

 thicker layers may be adopted. 



(3) The temperatures and salinities scaled from the mean regional curves for the midpoints of the 

 adopted layers are then used to derive, numerically or graphically, the velocity correction for each 



465382—44 35 



