-. DISCOVERY REPORTS 



S4 



object, a faint echo from the anchor. This echo, however, disappeared when the angle of the trans- 

 mitter to the anchor was 48°, and a hmit of spread of the soundwaves of significant strength might thus 

 be indicated. 



Difficulties in obtaining echoes from steep slopes may be due to the limited spread of the beam or 

 to scattering, dispersal or absorption of the echo owing to the irregular form of the bottom which 

 often prevails at moderate depths. 



Before the advent of echo sounding deep-sea soundings were normally spaced some considerable 

 distance apart and profiles drawn from these observations differ considerably from those constructed 

 from lines of echo soundings at fairly close intervals. A good example of this is given by Sverdrup, 

 Johnson & Fleming (1942, p. 18) where a profile including South Georgia and the South Sandwich 

 Trench is shown, first as based on 13 existing wire soundings and then as constructed from 1300 

 echo soundings made by the German research ship 'Meteor' in 1926.1 Our results are comparable 

 to these, and in 1938-9, when a series of cruises was made between the latitude of 40° S and the 

 ice-edge, on the Greenwich meridian, the profiles drawn for each cruise varied considerably, although 

 the tracks were often within a few miles of each other. Two of these profiles are shown in Text-fig. 8 

 and for the latitudes between which they are comparable (i.e. 45-55° S) the tracks were within 3 miles 

 of each other. From these it will be seen that although the main characteristics are similar, there is 

 a considerable divergence in detail. These profiles, however, are again relatively simple when compared 

 with the continuous records representing soundings every 2I sec, and although the latter do not 

 cover a large area, they show a degree of irregularity in the sea bottom, especially to seaward of the 

 continental shelf around Antarctica, which makes correction for slope quite impracticable. A section 

 of such a record, which is typical of the sea bed in the neighbourhood of the Scotia Arc, is shown in 

 PI. XXVII, fig. I. It was taken in waters of approximately 125-340 fm. (229-622 m.) in the Palmer 

 Archipelago, off Graham Land, but in the interpretation of this it must be remembered that the 

 vertical scale is much exaggerated, and that, since the paper feeds through the recorder at a fixed 

 rate, the degree of exaggeration is dependent on the speed of the ship. To obtain the exact amount 

 of exaggeration the speed of the ship (in knots) should be multiplied by i -08. In this instance the speed 

 was 9 knots and hence the vertical scale is magnified 972 times. Records taken on the Kerguelen- 

 Gaussberg ridge and other such localities in the Southern Ocean show similar though less well-marked 

 irregular features. 



Apart from these irregularities in the bottom there are, of course, the difficulties previously mentioned 

 in fixing the ship's position, and thus it has not been considered generally possible to correct our 

 soundings for slope. In certain isolated instances, however, this correction was possible, as for 

 instance when the ship's position could be accurately fixed from bearings on land which she was 

 approaching almost directly up a slope. Such an instance was our approach to Tristan da Cunha in 

 1933 (PL XXVI, fig. 2), and in Text-fig. 9 is shown the profile of the slope drawn to true scale, 

 together with the profile after correction for slope error. It is thought that on this occasion the ship's 

 course towards the land was nearly enough at 90° to the bottom contours to justify slope correction, 

 though this cannot usually be assumed without an adequate survey of the bottom.^ As can be seen 

 from the figure the correction is of little value for navigational purposes. 



So far it has been assumed that echoes are only being received from one point on the bottom, but 



1 We are not fully in agreement with this method of presenting a section of the ocean bed with such an exaggerated 

 vertical scale, since it tends to give a false impression of the relief of the ocean floor. 



2 In 1937 the International Hydrographic Bureau at Monaco circulated a questionnaire which, among other queries, 

 asked the Hydrographic Departments of their member states whether their echo soundings were corrected for slope before 

 insertion on the charts. From the replies received it appeared that very few countries applied slope correction and that many 

 considered the practical error far to outweigh the advantages of such a correction. 



