DEPTH SURVEY CARRIED OUT BY THE SONIC 



METHOD 



Work was continued during 1928 with that part of the scientific 

 program concerned with determining the bottom contours in the ice 

 patrol regions. The echo or sonic method of depth finding was again 

 used, the Modoc and the Mojai'e being both equipped with commercial 

 instruments. Although one of the ice patrol vessels has been fitted 

 with an experimental echo depth finder for several years, this year 

 was the first one in which both patrol ships were equipped to obtain 

 soundings rapidly and easily. 



The following brief description is given for the benefit of those 

 who may be unfamiliar with modern sonic sounding methods. Ex- 

 periments have dem.onstrated the speed with which sound travels 

 through water under various conditions of temperature, pressure, 

 and salinity. If the time between the outgoing and the return by 

 echo from the bottom of a short sharp note is measured, the depth 

 of water can easily be found out. The principles are simple but in 

 practice many complications arise and the instruments have to be 

 elaborate and ingenious devices. 



As might be expected the echoes are relatively much louder in 

 shoal than in deep water. To get results in great depths the oscil- 

 lators that produce the outgoing notes are strong and powerful. 

 They are placed in the bottom of the ship near the keel. The echoes 

 that com.e back are picked up by hydrophones in small tanks in- 

 sulated from sound from all directions but downward. 



The echoes are carried electrically to the bridge where an instru- 

 ment amplifies the intensity of the signals by vacuum tubes until 

 they are audible in head telephones. There is an indicator by the 

 phones which revolves at a constant speed. When the sound goes 

 out the pointer passes a zero mark on the scale. When the sound 

 comes back it is only necessary to notice the number on the scale 

 to which the marker is pointing to get the depth in fathoms instantly. 



In shoal water, saj^ under about 60 fathoms, the incoming sound 

 comes back so quickly that it merges with and becomes almost indis- 

 tinguishable from the loud outgoing one. A different system is used 

 to get the depth in this case, a svstem much more accurate than that 

 based on the coordination between the ear and the eye of man. The 

 energ}^ coming in from the strong echoes of shoal water is used to 

 cause the flashing of a red neon tube electric light opposite the depth 

 marks on the indicator. The light is carried alo'ig on the rotating 



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