50 DISCOVERY REPORTS 



DIFFICULTIES OF OBTAINING SOUNDINGS IN BAD WEATHER 

 AND IN CERTAIN PARTS OF THE OCEANS 



As a general rule the strength of the echo is inversely proportional to the depth of the water, but 

 the echo is very much affected also by the physical condition of the water and the nature of the bottom, 

 and it is specially important that the various causes of weak or inaudible echoes should be distinguished 

 so far as they are known. Weak echoes can be caused by defects in the sounding set, and in the earlier 

 years of our work it was assumed that weak echoes were generally attributable to such mechanical 

 defects. During the later voyages of the 'Discovery II', however, it was found that difficulties in 

 obtaining soundings could usually be traced to one or more of the following external causes: water 

 noises, aeration, layering of the water, a soft or badly reflecting bottom, or an irregular bottom. 



In bad weather the echo is liable to be obscured by a background of water noise associated with the 

 ship's progress, and this may necessitate slowing down or heaving-to while sounding. In deep water, 

 however, the echo strength was usually impaired during a gale, and this weakening often persisted 

 for more than 24 hours after a severe gale had ceased and the water-noise background had subsided. 

 This efli^ect is probably due to the persistence of aeration in the surface water with consequent 

 blanketing of transmission and echo. Such conditions rendered it difficult to obtain soundings in 

 deep water even with the ship stopped. 



Conclusive evidence that aeration can impede, or indeed completely cut off", the passage of super- 

 sonic sound in water was obtained on occasions when the ship's engine was put astern after letting-go 

 an anchor. This sets up considerable local aeration, especially in shallow water, and when the ship 

 moved over this patch of aerated water the echo trace on the M/S XII D recorder completely 

 disappeared, returning only when the turbulence and aeration had subsided. With the deep-sea sonic 

 set the strength of the signal appears to have been sufficient to overcome this local shallow area of 

 aeration in depths of less than 50 fm. (91 m.). 



Difficulty in obtaining an echo was also noticed on many occasions in certain latitudes even when 

 weather conditions were such that perfect or almost perfect reception was to be expected. This applies 

 especially to the neighbourhood of the Antarctic Convergence. The Convergence (see Deacon, 1937, 

 and Mackintosh, 1946) is continuous round the Southern Ocean, and may be described simply as 

 the line at which the Antarctic surface water sinks below the less dense sub-Antarctic water. In lines 

 of soundings running for instance southwards it was usually found that the echo failed just north of 

 the Convergence, especially where the latter was well defined, and was commonly very weak for 

 a hundred or more miles to the south of it, even when it was ill defined. Deacon draws attention to 

 the area of mixed water which lies just north of the Convergence and it is in this area that echoes 

 become weak or have, on a few occasions, disappeared entirely. Further reference to Deacon (figs. 12, 

 13) shows that north of the Antarctic Convergence the discontinuity layers are well marked and have a 

 pronounced horizontal or oblique trend. It is thus extremely probable that not only is the greater pro- 

 portion of the outgoing soundwaves deffected by these layers of diff'ering densities, but that the echoes 

 from any soundwaves which may have penetrated in a direct line to the bottom are themselves 

 deflected on their upward journey. Text-fig. 6, which is a section showing the vertical distribution of 

 density in terms of (T,, is a good example of these conditions which prevailed on the Greenwich 

 meridian in February 1939, in positions where we noted a distinctly weaker echo. This diagram has 

 been prepared for me by Mr A. J. Clowes and will eventually form part of a further report on the 

 hydrology of the Southern Ocean. 



It would appear that the weak echoes obtained to the south of the Convergence are attributable to 

 the composition of the bottom deposit. A bottom of rock or hard clay will return a strong, clear echo, 



