SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 57 



It should perhaps be pointed out that these ' crossovers ', or simuhaneous echoes from a valley, can 

 only be recorded when the two echo distances are within the range covered by the width of the record 

 paper which, in our recorder, was equivalent to a depth of 250 fni. (457 m.). If the difference is outside 

 these limits, ' phasing ' to bring the second echo on the record will only result in the loss of the one 

 originally being recorded. Normally the differences in echo distances at a ' crossover ' are small and 

 it is seldom that they exceed 150 fm. (274 m.). The average, for uncomplicated records, appears to be 

 from 50 to 100 fm. (91-183 m.) but many of them fade out within smaller limits. It seems probable, 

 therefore, that the simple ' crossover ' effect is obtained only when crossing a fairly shallow depression 

 or valley, which at the same time is comparatively narrow. It would seem also that the absence of 

 a secondary echo from a wider valley at moderate depths is further evidence of a possible limit to the 

 spread of soundwaves from a sonic transmitter. 



In oceanic sounding with the recorder, multiple traces do not appear on the record from depths 

 much greater than 2000 fm. (3658 m.) and are seldom recorded from shallow water of less than 100 fm. 

 (183 m.), or from the continental shelf. On the other hand, records from various depths between 

 2000 and 3000 fm. (3658-5486 m.) mostly show the bottom as a flat plain; it is in the depths between 

 1000 and 2000 fm. (1829-3658 m.) that the greatest variety of bottom in the Southern Ocean is met. 

 These really interesting regions are mostly in mid-ocean and on known but ill-defined ridges, but 

 owing to the great irregularity of the bottom echoes are faint and difficult to record except in very fine 

 weather — a condition which is not normally satisfied in these parts — and thus we have not been able 

 to obtain more than a few good records. It is on these records, however, that the greatest number of 

 multiple traces is to be found (see PI. XXIX, fig. i), and it is probable that the failure to obtain good 

 results is for the most part due to the very steep slopes and to a diversity of features which causes 

 scattering of transmissions and echoes. Limitation in the angle of spread of the transmissions may be 

 a contributory cause when a slope is very steep and comparatively simple. 



Soundings are not always difficult at intermediate depths, and some good records were obtained by 

 the 'Discovery II' on the Antarctic continental slope, especially on the Greenwich meridian in 

 March 1939 (PI. XXVI, fig. 3). Here the slope is peculiarly free from complexities, and it is further 

 discussed in a later section (pp. 86 to 88). 



In the waters adjacent to the Scotia Arc an exception is found to the general rule that multiple 

 traces are not usually recorded from depths of less than 100 fm. (183 m.). Areas such as the continental 

 shelf around South America and the Falkland Islands, the Burdwood Bank, the approaches to South 

 Africa, Australia and New Zealand, and other shallow areas show, on the whole, a remarkably level 

 bottom sloping up gently towards the land. In the shallow water off the Scotia Arc conditions are 

 quite diflFerent, and the bottom is extremely varied, though perhaps not quite so irregular as in the 

 slopes at intermediate depths. ' Crossovers ' on sonic records are common but are of small extent, and 

 there are very many steep slopes. Instances, however, of more than two traces from these shallower 

 depths are rare.^ An example of this very varied bottom from the Palmer Archipelago has already been 

 mentioned (see p. 54 and PI. XXVII, fig. i), and more will be given when the running surveys of the 

 South Shetland Islands are dealt with (pp. 79 to 81). 



1 A possible explanation of the absence of multiple traces on records from waters of less than 200 fm. (366 m.) in depth 

 here may be that the spread of the transmission at this depth is not sufficient for echoes to be recorded from other than the 

 nearest point of the bottom. It is probable that the bottom is no less varied than that of the intermediate depths between 

 200 and 2000 fm. (366 and 3658 m.) but at these greater ranges the spread in the transmission will allow features which are 

 not directly below the ship to be recorded. 



