. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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 impo
. Discovery reports. Discovery (Ship); Scientific expeditions; Ocean; Antarctica; Falkland Islands. 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
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