Page 525 echo sounding 556 



the ratio of the depth of water to the distance between acoustic units is greater than 5 to 

 1, this effect may be neglected — -but where the ratio is less it must be taken into account 

 by compensation or correction. Soundings may be corrected by use of the formula: 



where £'=echo sounding corrected for separation effect; c?=mean draft of the acoustic 

 units; Z)= depth registered on the echo-sounding instrument; and <S'= horizontal dis- 

 tance between the centers of the acoustic imits. All values must be expressed in the 

 same units. 



Some echo-sounding instruments, for example the 312 Fathometer, compensate 

 for this separation eiTor by having the shoal end of the scale on which the soundings 

 are registered, graduated to compensate for an assumed distance between the acoustic 

 units. For the 312 Fathometer, this distance 

 is 24 feet, and the instructions for installation 

 specify that the units shall be installed this 

 distance apart. If acoustic units separated 

 by any other distance are used with the 312 

 Fathometer, the registered soundings must be 

 corrected for the error introduced by the 

 difference between the actual distance of \ / 



separation and 24 feet. \ / 



This separation effect does not exist for \ / 



instruments using a transceiver, such as the \ / 



Dorsey Fathometers No. 1 and No. 3. It may vyy///y///////////////////7 7^;^ /y//y/yyyy/yy/y/y/yy^ 

 be neglected entirely for instruments with only ^ „ . . 



'^ "^ , . Figure 113.— Separate acoustic units. 



a small separation between the acoustic units, 



such as the 808 Fathometer, and the Hughes MS 12 D. And it may be disregarded in 

 all cases where the soundings are greater than five times the distance between the 

 acoustic units. 



The second effect of separation is the lag in registration of the transmitted signal. 

 This is caused by the time required for the sound to travel from the transmitting 

 unit to the receiving unit. The effect of this may be better understood by comparing 

 an instrument using a transceiver with an instrument which has a separation between 

 its transmitting and receiving units. In the transceiver the same unit is used for trans- 

 mitting and receiving, and it is obvious that there can be no lag between the time of 

 emission and the time of reception of the transmitted signal. By comparison, where 

 there is a separation, the soimd of the transmitted signal must travel tlirough the water, 

 or steel hull of the vessel, to the receiving unit, registering late by an amount approxi- 

 mately equal to one-half of the distance *§ (fig. 113) between the units, in tjrm.s of the 

 depth scale. In the 312 Fathometer, for example, if the units are mstalled 24 feet 

 apart, disregarding any other adjustments, the transmitted signal should register on 

 the depth scale 2 fathoms beyond the point where the index was when the signal was 

 transmitted. 



This lag in the registration of the transmitted signal does not, of itself, introduce 

 any error in the registered depths read independently. However, it must be considered 

 when making index adjustments to certain instruments and also when scaling fatho- 

 grams. Where there is a separation between the transmitting and the receiving unit. 



