Page 533 echo sounding 5614 



(3) Derive the velocity for each temperature and salinity and depth, and enter the velocities 

 in column (D). The velocities may be found from the tables in 9611, or from the diagrams in 9612. 

 The method of deriving the velocities from the tables is described in 6343. It is to be noted that 

 each velocity of sound entered in column (D) is strictly applicable to the one depth entered in column 

 (A) on the same horizontal line, but for the purpose of deriving velocity corrections it is considered 

 to apply throughout the layer of which the value in column (A) is the mid-depth. 



(4) Column (E) is ordinarily not needed for echo sounding. If mean velocities from surface to 

 bottom are needed in R.A.R., the values in column (D) should be progressively meaned and entered 

 in column (E), each entry being the mean of all the velocities down to and including that depth. In 

 computing these means one should be careful of the transition from layers of one thickness to layers 

 of another thickness. 



(5) Derive the factors (see 561) corresponding to each layer velocity and enter them in column 

 (F). Table 35 in 9613 gives these correction factors for velocities of sound to the nearest meter 

 and for instruments operating at three calibration velocities. Most instruments used by the Coast 

 and Geodetic Survey are operated for a velocity of 820 fathoms per second and the correction factors 

 can be taken from the table by interpolation,. For the rare case of an instrument operated for any 

 other velocity of sound the correction factors may be derived from the formula in 561. 



(6) Multiply each correction factor in column (F) by the layer thickness in fathoms, to derive the 

 correction applicable to this depth interval. Enter these products in column (G). Each of these 

 entries is the correction in fathoms for that layer, whose mid-depth is in column (A) . Be sure to allow 

 properly for the layer whose mid-depth is 98.5 fathoms — this layer, in the example, is only 3 fathoms 

 thick, and the proper entry in column (G) is the factor multiplied by three — not by five, as for pre- 

 ceding values. 



(7) Add the values in column (G) algebraically, entering the progressive sums with their correct 

 signs in column (H) ; thus each entry in column (H) is the sum of all the values in column (G) on a line 

 with and above it. 



(8) For convenience enter in column (/) the depth of the bottom of each layer. This is the depth 

 for which the entry on the same horizontal line in column (H) is the correction; any entry in column 

 {H) is the total correction to be applied to a sounding whose depth is equal to the corresponding entry 

 in column (7), 



(9) The values in column (H) are then plotted with reference to the depths in (7) to which they 

 apply and a smooth curve is drawn through them, from which curve the correction to apply to any 

 sounding may be scaled (see 5614). 



5614. Velocity Correction Curves 



After the values in columns {H) and (J) in table 20 have been derived and 

 entered, they shall be utilized to plot a graph of the velocity corrections. A commercial 

 graph paper with overprint (Form J-100-5) shall be used for this purpose. This paper 

 has 20 divisions to the inch. The overprint provides a vertical depth scale in fathoms 

 for depths to 200 fathoms. When used for such depths the horizontal scale shall be 1 

 inch equals 0.4 fathom. For deep-water correction curves, both scales shall be multi- 

 plied by 10, adding a zero to the printed vertical scale, and making the horizontal scale 

 1 inch equals 4 fathoms. Where a deep-water correction curve is included on the same 

 sheet with a shoal-water correction curve, the deep-water curve shall be properly 

 identified. The form, with velocity correction curves drawn and a tabulation of the 

 corrections is reproduced at reduced scale in figure 114. 



Where velocity corrections in feet are required, the units on Form e/- 100-5 should 

 be changed to feet and the scales used as "for depths to 200 fathoms." 



After the values in column (ET) have been plotted, smooth curves should be drawn 

 through them and a tabulation of velocity corrections should be made for use in making 

 entries in the Sounding Records (see 8223). The tabulation should provide a depth 

 range through which each correction is applicable. The units of the corrections depend 

 on the depths and character of the area (see 821 and table 29) . For example, if corrections 



