OCEANOGRAPHIC INFORMATION FOR SUBMARINE CABLES 1053 



be added to the depth read from the echo trace. This is done graphically 

 by swinging arcs representing the echo distances, using the cUstance 

 between soundings as the arc center spacings. The envelope of a suc- 

 cession of such arcs is the best approximation to the actual bottom con- 

 figuration that can be made. This method, of course, reciuires that the 

 sounding track l^e run approximately at right angles to the slope, and 

 thus, that the trend of the topography be determined. In addition, when 

 the relief becomes more complicated, constructing and interpreting the 

 en^■elope becomes much more difficult. Although all hills are shown on 

 echograms, small \'alleys are often completely missed because their width 

 is much less than the breadth of the cone of sound. This problem is partly 

 eliminated by the use of a PDR, where second echoes indicate the exist- 

 ence of valleys not recognized on standard echo sounders. 



Fig. 2 (a) shows the trace made by a PDR in passing over a rugged 

 slope in mid- Atlantic. The multiple echo on the left hand side of the 

 figure illustrates how echoes from the wide sound cone are returned 

 from different parts of a steep slope. Similarl}^, the multiple echoes in 

 the center show a deep valley with energy from the same pulse reflected 

 from the steep slopes as well as the bottom. Fig. 2 (b) shows a corrected 

 profile constructed from the echogram illustrated in Fig. 2 (a), and Fig. 

 2 (c) is a profile of the same track with 40: 1 vertical exaggeration, which 

 is typically used in describing bottom topography. 



To construct a profile and interpret each sounding taken would re- 

 quire a prohibitive amount of work. Thus, in preparing contours and 

 uncorrected profiles for ordinary mapping purposes, a sufficient number 

 of the initial echoes are taken to allow a reasonably accurate profile to 

 be constructed. Where 1 : 1 corrected profiles are required, all the echoes 

 are used and the best possible envelope representing the bottom is 

 constructed. 



2.3 CHARACTERISTICS OF AVAILABLE INFORMATION 



2.3.1 Sources of Data 



The \-ast majority of data for a study of submarine topograph}^ are 

 obtained as a series of "soundings," or depths below sea level.* Prior 

 to the development of echo sounding apparatus, soundings were made 

 by measuring the length of either a weighted piano wire or hemp line 

 which was lowered to the bottom. This method of line or wire sounding 



* Features to be studied by detailed soundings can often be located by measure- 

 ments of the earth's magnetic and gravitational fields and by acoustic methods. 



