188 



HYDROGRAPHIC MANUAL 



5-120 Mechanical correction of velocities 



— The application of arithmetical corrections 

 for differences between calibrated and actual 

 velocities can be avoided when using the 

 EDO-255 depth recorder under certain con- 

 ditions. The frequency of the power supply 

 can be varied to accommodate changes in 

 velocity of sound in sea water over a range 

 of about 4,600 to 5,000 feet per second (see 

 3-85) . The procedure is as follows : 



(a) Determine the velocity of sound to 

 the maximum depth in the area to be sur- 

 veyed, by veloclmeter or from serial tem- 

 perature and salinity observations, and 

 draw a velocity curve. Determine the mean 

 velocity for this depth range. 



(b) Compute the frequency corresponding 

 to the mean velocity and adjust the power 

 supply to provide this reading on the fre- 

 quency meter. 



If the velocity at any point on«the curve 

 differs from the mean velocity by one-half 

 percent or more, this system cannot be used 

 without applying velocity corrections. The 

 velocity curve must be determined with suffi- 

 cient frequency to assure correct setting of 

 the frequency meter. The frequency must 

 be held constant and no attempt shall be 

 made to vary the frequency during the day's 

 work. 



The paper speed (see 3-84 (1) ) will change 

 with change of frequency and by the same 

 percent. 



The adopted frequency shall be noted in 

 the sounding record and the fact that such 

 frequency is maintained shall also be noted. 

 The descriptive report shall contain a full 

 report on the procedure used. 



Bar check requirements are not altered 

 when this system is used, however the sys- 

 tem will probably affect the results. The 

 initial shall be set to compensate for draft 

 of the transducer and instrumental error. 

 The curve of actual velocity of sound will 

 cross the vertical line representing the mean 

 velocity adopted for sounding. A bar check 

 at this depth of crossing should show a 

 zero correction, and at other depths may 

 show slight differences which may be dis- 



regarded provided that they are less than 

 one-half percent of the depth. 



5-121 Scanning fathograms. — Before 

 soundings are plotted on the boat sheet (see 

 5-63) the hydrographer shall inspect the 

 fathogram to assure himself that all impor- 

 tant soundings have been scaled and recorded 

 in the sounding record. As the soundings are 

 plotted, he should verify any recorded 

 sounding which appears doubtful when com- 

 pared with other soundings on adjacent lines, 

 or shows abrupt changes in slope which are 

 contrary to the general characteristics of 

 the area. 



Some hydrographers make a complete re- 

 view of the fathogram, check the recorded 

 soundings, and record additional soundings 

 as necessary, prior to plotting the soundings 

 on the boat sheet. In many instances this 

 is considered a final check of the scanning 

 prior to the smooth plot of the survey. The 

 advisability of this practice is questionable, 

 since the checking is usually done rather 

 hurriedly. 



When the procedure described in the first 

 paragraph of this section is followed, it is 

 necessary to rescan the fathograms in detail 

 at a later date and prior to the reduction of 

 the soundings. This is a relatively simple 

 operation for the majority of surveys, how- 

 ever the work must be done carefully and 

 accurately by competent personnel. When 

 the bottom is irregular or when the bottom 

 trace is partially obscured by kelp, grass, 

 strays, or side echoes a considerable amount 

 of experience is required for proper inter- 

 pretation of the record (see 5-122). Regard- 

 less of the character of the bottom, the 

 fathogram may contain marks attributable 

 to echoes from fish, debris floating at various 

 depths, turbulence in the water, abrupt 

 changes in temperature or density of the 

 water, noises generated in the sounding ves- 

 sel, or they may be caused by instrumental 

 faults. Some skill is required in order that 

 each trace may be correctly identified and 

 the greatest value and the most accurate 

 results obtained from the records. 



The fathogram should first be examined 

 to determine whether the initial trace has 



