52 DISCOVERY REPORTS 



producing an excessive angle between the ocean floor and the face of the hydrophone tank, made the 

 incoming echo less audible. The former explanation seems the more probable, for heavy rolling may 

 take place anyw^here in the Southern Ocean, and it was certainly not the cause of the faint echoes found 

 by the 'Discovery II' near the Convergence, for, as mentioned above, the rolling could always be 

 remedied by manoeuvring the ship to suit the weather. 



Another local area in which only weak echoes were received was found by the 'Discovery II' on 

 the Greenwich meridian, south of the region of diatom ooze. It lay roughly between 54° S and 60° S 

 and from the number of soundings which we eventually obtained during a series of repeated cruises 

 in that area it seems likely that an extremely irregular bottom was the cause of our weak echoes. 

 Rapid changes in depth were experienced and on one occasion the depth shoaled by nearly 2000 m. 

 (1094 fm.) in 30 min., with the ship proceeding at normal speed. Changes of depth nearly of this 

 magnitude were by no means uncommon, and attempts to take a record of some of the slopes with the 

 ' Acadia ' recorder were not very successful. On some occasions, indeed, the depth altered so rapidly 

 that it was impossible to get an accurate reading even on the old ' listening ' receiver. This problem of 

 echoes from steep slopes will be considered in more detail in the next section of this report ; but it 

 may be stated here that owing to the extreme irregularity of the bottom now known to exist in many 

 areas of the Southern Ocean, it seems probable that the weakness of echo strength reported often from 

 south of the Antarctic Convergence-ooze area is more than likely to be due to this cause. 



SLOPE CORRECTION AND THE CONDITION OF THE OCEAN FLOOR 



In general it appears to have been accepted that the soundwaves transmitted by sonic echo-sounding 

 apparatus are reflected from the bottom at the point nearest the ship. If this is established, then with 

 a level bottom, the depth recorded is the true depth at that point, but if the sea bed is sloping or 

 comprises irregular features then the echo depth obtained is not the true depth and will require 

 a correction for the angle of the slope. This correction is, of course, in addition to the normal 

 corrections for draught and the speed of sound in sea water in difl^erent areas. In discussing the 

 question of slope correction for sonic soundings, therefore, it is assumed that the expression ' echo 

 distance' is the recorded depth which has been corrected for the speed of sound, etc., and that, unless 

 otherwise mentioned, the soundwaves are reflected from the nearest point of the bottom. 



The various methods of correction for slope have already been described at length in the Hydro- 

 graphic Reviezv, and Shalowitz (1930), Vanssay de Blavous 

 (1930, 1933) and Hayes (1933) are some of the most recent 

 contributors to this subject. Briefly it may be stated here that 

 either the echo distance must be corrected for the slope of the 

 bottom, or the position of the sounding must be moved up the 

 slope until the actual depth at the new position equals the echo 

 distance actually recorded. In a third method corrections 

 embodying both the above systems can be applied. Text-fig. 7 

 is a simple diagram to illustrate the first two methods, and it 

 will easily be seen that HA is the position at which the sounding 

 is taken then either the echo distance AB must be corrected for 

 slope by a plus correction to give the actual depth AC; or the 

 position of the sounding must be moved from ^ to ^' so that 

 the echo distance AB equals the actual depth A'C. In this figure it is assumed that the slope of the 

 bottom is regular ; if it is not then the angle of slope must be determined for the point of observation 

 and the correct echo distance or displacement calculated accordingly. 



Fig. 7. Diagram to illustrate the theory of 

 slope correction. 



