56 DISCOVERY REPORTS 



in view of the diverse character of the sea bed, as revealed by the automatic recorder, and the assumed 

 spread of the soundwaves from the transmitter, it appears obvious that echoes may be received 

 simukaneously from more tlian one point. A large number of records from the 'Discovery IT show 

 such multiple traces (see Pis. XXVII-XXIX), and it will be obvious that this 'third dimension' 

 introduces further complications in the interpretation of the records. It would appear impossible, 

 except in certain isolated instances, to determine the relative positions of the points from which the 

 echoes are being received, since with the conical spread of the transmissions these multiple echoes may 

 be from points on the bottom ahead, astern, or on either beam of the ship. The few exceptions are the 

 records which only show two traces and which probably result from crossing a simple but well-defined 

 narrow valley or depression. As shown in PI. XXX, fig. 4, these traces appear on the record in the 

 form of ' crossovers '. 



If it is assumed that the echo returns from the nearest point on the bottom, then a ship crossing 

 a depression of the form shown in Text-fig. 10 a will receive echoes at A from A^ , at C from Cj , and at B 

 from both B^ and Bo . It is probable (and is assumed here) that only one echo would be distinguishable 



Fig. 10. Diagrammatic representations of sounding across a narrow valley, (a) Transmission and echo assumed to be narrow 

 parallel beams, (b) Transmission spreading, and two echoes being recorded as in actual practice. 



as such with the ' listening ' gear, and it follows that echoes from the deepest part of the depression 

 would be missed, and the profile of the bottom could only be assumed to take the form represented by 

 the dotted line. The recorder however is more selective and sensitive than the human ear and it can 

 distinguish echoes from points at diff'erent distances. It seems very probable that the ' simple cross- 

 over ' of the kind shown in PL XXX, fig. 4, is in fact the result of crossing a depression of the type 

 shown in Text-figs. 10a and b. That is to say the 'crossover' is not a precise representation of the 

 actual profile of the bottom but almost certainly represents echoes received simultaneously from both 

 sides of a depression. In Text-fig. 10b positions A, B, C, D and E are presumed to be equally spaced 

 points on the ship's track. It must be remembered that in practice continuous soundings are being 

 taken between these points, but the points shown are sufficient to illustrate our theory. At position A 

 echoes are being recorded from A^ and Ao , the latter being weak. At C the echo strength from C^ 

 and C, is nearly equal and at E the signal being received from E^ is now the secondary trace and 

 will probably be of just sufficient strength to give a recording under good conditions. From this 

 it will be seen that a rounded, or U-shaped, depression may be recorded as a V-shaped 'crossover' 

 as represented by the pecked lines in Text-fig. 10/;, and that analysis of such a 'crossover' may give 

 a more accurate representation of the shape of a depression on the bottom than can be obtained by 

 the ' listening ' gear. 



