The Ocean 11 



Soundings at depths greater than 200 m cannot easily be made with a hemp line 

 by hand since the weight of the line and lead is too great and it is difficult to feel the 

 contact with the bottom. The measurement of greater depths is extremely difficult and 

 it took several decades of experimental work before deep-sea soundings could be made 

 reliably at any point in the ocean. 



The measurement of the depth of the sea (Stahlberg, 1920) is the determination of 

 the perpendicular distance between the surface and the sea bottom. At great depths 

 this is difficult because: (1) the bottom contact is not easy to detect, and (2) hauling 

 in the increased sounding weight is very laborious unless it is done by machine. Two 

 conditions are necessary for a reliable deep-sea sounding: (1) the use of a thin steel 

 wire in place of the hemp line used previously, and (2) the release of the sounding 

 weight on contact with the sea bottom. 



The wire sounding method used at great depths will not be described in detail here 

 since it is essentially a technical question. Further details can be found in technical 

 handbooks [see especially Pratje (1952), and Oceanographic Instrumentation (Re- 

 port of conference, Rancho Santa Fe, Cahfornia, 21-23 June 1952)]. 



The development of echo sounding has revolutionized the investigation of sea- 

 bottom topography; wire soundings could never have been made in such large 

 numbers nor have given such good results for the rapid and precise elucidation of 

 conditions at the bottom of the ocean, and centuries would have been needed to get 

 the results that can be obtained without difficulty in a few years by echo sounding. 

 The basic principle of echo sounding is very simple; it measures the time required for 

 a sound wave to travel from the bottom of a vessel (the sea surface) to the sea bed and 

 back. The returning wave can be detected as an echo and amplified. To calculate the 

 depth, knowing the speed of sound in sea water, it is only necessary to determine the 

 time from emission of the sound until the echo is detected- — the echo time. If the time 

 is /, the speed of sound in water v and the depth of the sea h, then 



Echo sounding makes it possible to sense the bottom of the sea accurately and to 

 ascertain its actual topography. A vessel equipped with echo sounding can fix the 

 depth of the sea without loss of time while moving at full speed. Scientifically, sonic 

 sounding is of value only when: (1) it is combined with an accurate determination of 

 the position of the vessel which in general should not be determined less accurately 

 than ± 1 nautical mile and (2) when the mean velocity of the sound emitted by the 

 echo sounding apparatus is known in addition to the echo distance. Only then is it 

 possible to convert the value obtained to the true depth. The enormously increasing 

 number of echo soundings requires the establishment of an international office to 

 correct and unify the mass of data and to chart it after critical interpretation. This 

 would give results of great utility both scientifically and for the improvement of the 

 sea charts of all nations (Defant, 1938). 



Echo sounding has only one disadvantage compared with wire sounding; it cannot 

 be combined with the collection of bottom samples which are necessary to ascertain 

 the nature of the bottom sediments. If these are needed wire sounding is indispensable. 

 However, it is possible with more modern types of echo sounding equipment to draw 

 some conclusions about the nature and thickness of the bottom sediments from the 



