Page 511 echo sounding 5521 



Although the above-mentioned errors are variable to a certain extent, most of the 

 variations are small compared to the total amount, and the important changes are 

 gradual. The various instrumental lags are principally additive, and their sum may 

 be of such a magnitude as to require instrumental compensation or an arithmetical 

 correction to the soundings. For correction purposes these errors are combined with 

 other unknown quantities, the total being called the instrumental error, which is 

 determined periodically and compensated for instrumentally. It is especially important 

 that this be done where the survey includes precise sounding in shoal depths. 



5521. Determination of Instrumental Error 



The amount of the instrumental error is determined by comparing echo soundings 

 with simultaneous direct vertical measurements of the depth. Such simultaneous 

 depth measurements are usually called simultaneous comparisons. The echo-sounding 

 instrument must first be adjusted for the draft of the transmitting and receiving units. 

 A carefully standardized leadline, or wire registering on an accurately calibrated sheave, 

 and a heavy lead must be used for the vertical measurements. The simultaneous 

 comparisons must be corrected — the echo soundings for the velocity of sound, and the 

 leadline or wire soundings for any leadline error or sheave factor. The difference 

 between the simultaneous comparisons after correction, without regard to sign, is the 

 instrumental error. The correction for this error is called the instrumental correction, 

 and may be plus or minus. Wliere the index has been adjusted to compensate for the 

 amount of settlement and squat while underway (see 553), that amount must be applied 

 to the observed echo soundings from the stationary vessel to bring the simultaneous 

 measurements to the same plane (see table in 5522). 



If the draft has been properly compensated for, the instrumental correction is the 

 quantity that must be applied to the echo sounding corrected for velocity, to make it 

 agree with the depth measured by leadline or wire. This is accomplished instrument- 

 ally by adjusting the index for the amount of the instrmnental correction. 



Although some types of echo-sounding instruments may require special consider- 

 ation when the instrumental error is determined, the general procedure is essentially 

 the same for all instruments that have acoustic units permanently installed in the 

 ship's huU. This general procedure is as foUows: 



(1) Operate the echo-sounding instrument for at least one-half hour before use. 



(2) Set the amplifier gain at the value normally used when surveying — not necessarily at the 

 value to give best results in the depth in which the test is made. 



(3) The ship must be stopped, or at anchor, where the bottom is level and comparatively hard, 

 and preferably in a depth not exceeding 25 fathoms. The sea should be moderately smooth so the 

 ship will have little or no vertical motion, and so that the vertical casts wiU be accurate. 



(4) Determine the draft of the acoustic units in accordance with 5512 and adjust the instrument 

 so that the index will compensate for the draft. 



(5) A minimum of five simultaneous comparative soundings should be taken. The echo soundings 

 should be read by the operator without a prior knowledge of the depths obtained by direct measure- 

 ment; each sounding should be read at the instant the lead strikes the bottom. The shoal edge of the 

 neon tube flash or other registration should be read. The leadline or wire soundings should be taken 

 from a position as near the acoustic units as practicable and extra care must be taken to ensure their 

 verticality. 



(6) Correct the echo soundings for the velocity of sound by one of the methods explained in 561, 

 and for settlement and squat if the index has been previously adjusted to compensate for this. Cor- 

 rect the leadline or wire soundings for errors of graduation, or calibration. The difference between 

 the corrected vertical measurements and the corrected echo depths is the instrumental error or the 

 amount for which compensation must be made. 



