Page 535 echo sounding " 5615 



5615. Graphic Determination of Velocity Corrections 



In the graphic method of determining velocity corrections, use is made of specially 

 prepared graphs called "Velocity correction graphs." By their use the velocity cor- 

 rection curves (5614) can be plotted directly from temperatures and salinities derived 

 for columns (B) and (C) of table 20, without recourse to any tables or arithmetic 

 computations. After facility in their use has been acquired, the method is very much 

 faster than that described in 5613. It is even possible to use temperatures and salinities 

 directly from the mean regional curves (5612), to find the corrections for plotting the 

 velocity correction curves on Form e/-100-5. 



There are three velocity correction graphs, numbered ^-1229-1, y1-1228-1, and 

 ^-1230-1, for instruments calibrated for a velocity of sound of 800, 810, and 820 

 fathoms per second, respectively. Copies of these graphs are obtainable from the 

 Washington Office. The v^elocity correction graph ^-1230-1 is reproduced at a re- 

 duced scale in figure 115. 



The graph utilizes temperatures in degrees centigrade as an ordinate, and a system of curves to 

 represent sahnities, with three abscissa scales giving the velocity corrections to be applied for depth 

 layers of three thicknesses. The principle of utilization of depth layers is exactly the same as in the 

 numerical method described in 5613, except that in the graphic method no corrections for pressure are 

 applied for depths less than 200 fathoms. Values taken from the graphs will, therefore, differ slightly 

 from corresponding values taken from the tables, but the differences will not be enough to affect the 

 ultimate accuracy of the corrections. 



A. SCALES OF THE GRAPHS 



The velocity correction graphs are designed for use directly with Form J- 100-5 on which the 

 velocity correction curves are drawn. They are constructed so that horizontal scales for the 20- 

 fathom and 200-fathom layers correspond to the scales used on the above form (see 5614). Where 

 5-fathom layer-intervals are used, the values taken graphically from the velocity correction graph 

 must be divided by four to reduce them to the 20-fathom layer scale for use on Form J-100-5. This 

 can be done by using proportional dividers instead of regular dividers when scaling the intercepts. 



B. METHOD OF USE 



Each velocity correction graph contains a vertical index line at which the correction is zero. It 

 should be noted that the position of this index line and of the abscissa scales differs, with reference to the 

 system of salinity curves, for the different standard velocities. In using the graphs one must be 

 certain to use the graph corresponding to the calibration velocity of his echo-sounding instrument. 

 In the rare case of an instrument operated for a velocity of sound other than one of the three standard 

 velocities, the difference may be accounted for by using the graph for a standard velocity nearest that 

 at which the instrument is operated, and moving the index line and the vertical grid lines in the following 

 manner: 



For each fathom variation in velocity from the standard, move the index line 0.00602 



fathom (on the 5-fathom layer scale) — to the right if the instrumental velocity is less than the 



standard, and to the left if it is greater than the standard. Any error due to this procedure is 



considerably less than one-tenth percent. 



Before using the velocity correction graph, the approximate range of the corrections should be 

 determined so that the zero of the correction curve may be properly placed on Form J-100-5. This is 

 particularly necessary for the deeper depths where the curve is frequently a reverse one. It may be 

 necessary, therefore, to determine graphically the positions of a few points on the velocity correction 

 curve as a preliminary step. 



a. For depths of 200 fathoms or less. — Using the mid-depth of each layer and corresponding 

 temperature and salinity values, as in columns (A), (B), and (O in table 20 of 5613, enter the velocity 

 correction graph with the temperature as an ordinate and find where it intersects the salinity curve for 

 the corresponding salinity. On the edge of a strip of paper laid horizontally, mark off the intercept 



