Page 447 echo sounding 5151 



But instruments for use in navigation are constructed with a view to getting soundings 

 rapidly, with a fair degree of accuracy, and with a minimum of adjustment and main- 

 tenance; and they must be simple in construction, which prohibits the refinements 

 necessary in a survey instrument. 



5151. Depth Range Classification 



As to depth, echo-sounding instruments may be broadly classified as shoal, moder- 

 ately deep, and deep-water instruments. The depths are those in which the instrument 

 was designed to sound most efficiently. For purposes of classification, shoal depths 

 may be considered to be those from to 20 fathoms, moderate depths those from 20 

 to 100 fathoms, and deep water all depths of more than 100 fathoms. Some instru- 

 ments of special design may be equally effective throughout the entire depth range and 

 therefore cannot be conveniently classified in this manner. 



The echo-sounding instruments used by the Coast and Geodetic Survey may be 

 classified as to depth range as follows: for shoal water — Dorsey Fathometer No. 1; for 

 shoal and moderately deep water — 808 Fathometer, Simplex-Bludworth Fathometer, 

 and the Hughes MS 12 D; and for moderately deep and deep water — 312 Fathometer, 

 Veslekari Graphic Recorder, and Dorsey Fathometer No. 2. The Dorsey Fathometer 

 No. 3 may be considered as included in all these classifications, since it embodies the 

 features of both the Dorsey Fathometer No. 1 and No. 2 as well as the 312 Fathometer. 



5152. Frequency Classification 



When echo-sounding equipment is classified according to the frequency of the 

 transmitted sound, it is said to be either sonic or supersonic. Devices that utilize 

 acoustic waves of a frequency that are audible to the human ear are generally classified 

 as sonic instruments; and those that utilize frequencies above auditory perception are 

 called supersonic, or ultrasonic, instruments. There is a close relation between the 

 classification of echo-sounding instruments by depth and by frequency. This is 

 because each range of frequencies has distmct advantages for use in a certain depth 

 range. 



A. SONIC FREQUENCIES 



Echo-sounding instruments used in navigation, especially in the United States, 

 employ sonic frequencies to a large extent. These frequencies can be generated in the 

 water at a high-energy level with relatively simple and inexpensive equipment. Because 

 of the low absorption at sonic frequency, their high penetrating power also makes them 

 useful for deep soundings. Sound-producing and receiving units can be made to 

 convert energy very efficiently at sonic frequencies, which adds to the total efficiency 

 of the system. 



Sonic frequencies have, on the other hand, certain restrictive limitations. These 

 frequencies cannot be used to measure extremely shoal depths with a high degree ot 

 accuracy, because the period of these frequencies approaches the time interval to be 

 measured. Most of the energy of water and ship noises is in the sonic-frequency range 

 and, therefore, sonic soundings are more susceptible to interference by strays. Because 

 of their long wave lengths these low frequencies cannot be directed, or beamed, to an ad- 

 vantageous degree, without using transmitting and receiving units of a prohibitive size. 



