5153 HYDROGRAPHIC MANUAL PaGE 448 



Directivity is desirable because it adds to the discrimination against spurious noises, in 

 addition to concentrating the sound energy in the desired direction. Sonic frequencies 

 do not permit as minutely detailed a representation of the bottom as might be desired 

 in some cases. This is especially true where the bottom irregularities are small 

 compared to the wave length of the sound employed. 



B. SUPERSONIC FREQUENCIES 



Supersonic frequencies overcome, to a large extent, most of the disadvantages of sonic 

 frequencies and still possess many of their advantages. The advantages of supersonic 

 frequencies are: 



(1) High directivity with small transmitting and receiving units. 



(2) Concentration of sound energy, because of directivity, in the desired direction, i. e., normal to 

 the bottom. 



(3) Discrimination against spurious noises by reason of directivity and because of the generally 

 lower frequency of the ship and water noises. 



(4) Measurement of shoal depths because a shorter signal can be made. 



(5) Detailed profile of irregular bottom due to the short wave lengths. 



(6) Reduction of side echoes because of the narrow beam of sound. 



Supersonic frequencies are used in both navigation and survey echo-sounding 

 instruments, and are employed in all depths from the shoalest to the deepest. Greater 

 attenuation of sound occurs at these higher frequencies and the sound waves are more 

 likely to be reflected within the medium. Wliile these are hindering limitations they 

 are serious only in extreme cases. 



Only one type of echo-sounding instrument used by the Coast and Geodetic 

 Survey employs sonic frequencies exclusively; this is the 312 Fathometer. Those 

 which are classified as supersonic instruments are the Dorsey Fathometer No, 1, the 

 808 Fathometer, the Simplex-Bludworth Fathometer, the Veslekari, and the Hughes 

 MS 12 D. The Dorsey Fathometer Nos. 2 and 3 are combined sonic and supersonic 

 sounding instruments. 



5153. Registering Device Classification 



A common method of classifying echo-sounding equipment is by reference to the 

 method of depth registration. There are two broad classifications; the visual and the 

 graphic-recording types. This classification is irrespective of the kind of acoustic 

 signals employed or whether the instrument is intended for navigation or surveying. 

 On a visual indicator the depth can be read quickly and accurately but it must be 

 observed constantly if a profile is desired. It is of most value when only an occasional 

 sounding is desired. The visual indicator types used by the Coast and Geodetic 

 Survey, because of their design, do not register silt deposits less than 2 fathoms thick. 

 The principal advantage of the graphic-recording type is that it makes a permanent 

 record of a profile of the bottom, whose details may be examined at leisure at any later 

 date. Unusual characteristics of the bottom, such as silt layers and the underlying 

 substrata, are often registered by a graphic-recording instrument. All these features 

 are of special value to the hydrographer. 



Instruments used by the Coast and Geodetic Survey classified as visual types are 

 the Dorsey Fathometer Nos. 1, 2, and 3, and the 312 Fathometer. Those classified 

 as graphic recording are the 808 Fathometer, the Simplex-Bludworth Fathometer, 

 the Veslekari, and the Hughes MS 12 D. 



