12 The Ocean 



appearance of the echo in the receiver. The structure and form of the returning wave is 

 dependent on the nature of the reflecting surface. If the oscillatory form of the re- 

 flected wave can be ascertained in the receiver it is possible to decide whether the 

 bottom is rock, sand, mud, or other material. It is very frequently found that the echo 

 is split into broader or narrower bands which are clearly connected with the different 

 layers in the bottom sediment (mud or rock). The echo sounder thus gives a pre- 

 liminary idea of the nature of the bottom and often the thickness of the soft upper 

 sediment. This was first mentioned by Stocks (1935). For further details reference 

 may be made to Evving, Crary and Rutherford (1917), Bullard (1938) and 

 EwiNG and Vine (1938). Another method of studying the structure and thickness of 

 the deep sea sediments has recently been developed by Weibull (1947). Very good 

 results were obtained with this by the Swedish "Albatross" Expedition (Pettersson 

 1946). 



Indirect depth determination with an unprotected reversing thermometer. Ruppin 

 (1906. 1912) first suggested the use of the difference between protected and unpro- 

 tected reversing thermometers for the measurement of the depth at which the reversing 

 frame or the water sampler on which the thermometers are mounted is reversed. The 

 usefulness of the method has been shown by the investigations which he carried out at 

 depths up to 100 m and by those of von Perlewitz at up to 1000 m. Brennecke (1921) 

 on the "Deutschland" Expedition of 191 1-12 made valuable use of it, and it was used 

 systematically for the first time on the "Meteor" Expedition of 1925-7 (WiJST, 1932). 

 In both wire sounding and in oceanographic serial observations there is always a wire 

 angle of greater or lesser magnitude and it is therefore extremely valuable to have a 

 method available which allows a reduction of the temperature and salinity values to 

 true depth or which ascertains a determination of depth independent of the wire angle. 



For the construction and function of the reversing thermometer, the corrections 

 applied and the accuracy of the depths obtained [see particularly Oceanographic 

 Instrumentation (Report of conference, Rancho Santa Fe, Cahfornia, 21-23 June 

 1952, p. 55)]. 



2. The General Morphology of the Sea Bottom 



The topography of the bottom of an ocean or part of an ocean can be conveniently 

 shown on a depth chart on which all available soundings are recorded after critical 

 interpretation. The reliefs of the sea bottom can be shown by drawing lines of equal 

 depth (isobaths) at fixed intervals. Constructing the isobaths between separate soundings 

 isessentially a question of interpolation which is considerably facilitated if the soundings 

 are distributed as evenly as possible over the whole area. This condition is unfortu- 

 nately very rarely satisfied, even less so after the introduction of sonic sounding. 

 Apart from the more sporadic distribution of earlier wire soundings there is now a 

 greater concentration of soundings along isolated fines of echo soundings resulting 

 in an extremely uneven distribution of depths and, while some parts are extremely well 

 surveyed, there are very large areas with only single soundings. The task of preparing 

 isobaths for an entire ocean has thus become more difl[icult than before the introduction 

 of echo sounding. 



The construction of the isobaths for an ocean area depends on subjective considera- 

 tions; the lines must of course be fitted to the soundings, but the available points 



I 



