5. HYDROGRAPHY 



181 



comparisons shall be made with the vessel 

 stopped or drifting slowly in calm water 

 over an even or gently sloping bottom. A 

 minimum of 10 simultaneous comparisons 

 shall be made between soundings obtained in 

 the overlap zones of the various scales. 



Phase errors are not always constant, and 

 when the results of phase comparisons made 

 during the season differ significantly they 

 should not be averaged. Abnormal phase cor- 

 rections should be noted by the echo sounder 

 operator and reported to the recorder dur- 

 ing sounding operations. 



5-113 Corrections to EDO-255 sound- 

 ings. — The EDO-255 depth recorder (see 

 3-84) is designed to eliminate most of the 

 operational and mechanical errors inherent 

 in 808 depth recorders. However, it is not 

 fool-proof and certain features must be moni- 

 tored or corrections to soundings will be 

 required. 



With the initial or index correctly set and 

 maintained there are two possible sources 

 of error in EDO-255 depth recordings: in- 

 correct frequency and improper phase ad- 

 justment. The instrument can be operated 

 at a variable frequency and the converter 

 or inverter must be adjusted whenever a 

 variation from the adopted frequency is 

 noted on the meter. A drift of 0.3 cycle 

 will cause an error of one-half percent in 

 the soundings. A change in frequency of 

 0.2 cycle or more shall be noted in the record 

 book, the frequency shall be adjusted and 

 the times noted. Corrections to soundings 

 shall be applied as a percentage with respect 

 to variation from the correct frequency. 



Phase changes are accomplished electri- 

 cally. The contacts for the various phases 

 are adjusted on the bench (see 3-86) and 

 will ordinarily maintain their positions so 

 that there will be no phase correction. It 

 should not be assumed that this is always 

 true, and checks should be made on the bar 

 when sounding in feet, and during hydrog- 

 raphy in either mode. If it is found that a 

 phase correction is required, the amount of 

 the error shall be determined by the method 

 described in 5-112, and the instrument ad- 

 justed to eliminate the error. All pertinent 



information shall be entered in the sounding 

 record. 



5-114 Velocity corrections. — In echo 

 sounding the sound wave passes vertically 

 downward and back through a column of 

 water in which the velocity of sound differs 

 at different depths, and since the true depth 

 is a product of velocity and time, the average 

 velocity from surface to bottom must be 

 known at each sounding. The velocity of 

 sound in sea water depends on the tempera- 

 ture, salinity, and pressure, and the velocity 

 used in echo sounding is usually calculated 

 from these characteristics (see 3-116) . 



The hydrostatic pressure increases in di- 

 rect proportion to depth, the temperature 

 decreases with depth but not uniformly, and 

 salinity usually increases with depth. The 

 result is that the velocity of sound is seldom 

 uniform from top to bottom, and seasonal 

 changes within any region will change the 

 average velocity. 



Echo sounders are calibrated for sounding 

 at assumed velocities of 800 or 820 fathoms 

 per second. The actual velocity of sound in 

 sea water must be determined in all hydro- 

 graphic surveys except in shoal areas where 

 bar checks are used to determine corrections 

 through the full depth range of soundings. 

 Velocity corrections shall be ignored when 

 they are less than one-half percent of the 

 depth, but shall be applied when they equal 

 or exceed one-half percent of the depth. 



For use in correcting echo soundings the 

 velocity of sound must be known with suffi- 

 cient accuracy to ensure that no sounding 

 will be in error from this cause alone by as 

 much as one-half percent of the depth. There- 

 fore, the mean velocity must be known with- 

 in 7.5 meters per second. Of the character- 

 istics of sea water affecting the velocity of 

 sound, temperature is the most variable, and 

 to satisfy the above requirements the mean 

 temperature of the water must be known 

 approximately within 2° C. A sufficient 

 number of salinity and temperature observa- 

 tions must be made so that the velocity may 

 be known with the specified accuracy. The 

 number required depends on the physical 

 characteristics of the water and the physiog- 



