58 



DISCOVERY REPORTS 



COMPOSITION OF THE BOTTOM 



It is an established fact that the composition of the sea bed has an appreciable effect on the strength 

 of the echo. We have already mentioned the difficulty of obtaining soundings in deep water in certain 

 regions where the bottom is very soft, but with a hard bottom re-echoes can often be obtained down to 

 moderately great depths. An instance of this has also been quoted (p. 51) and there were many other 

 occasions when re-echoes were obtained from lesser depths. In deep water the automatic recorder 

 cannot record both echo and re-echo unless, by chance, the re-echo falls in the same phase as the 

 first echo, i.e. an echo from, say, 1050 fm. (1920 m.) and a re-echo at 2100 fm. (3840 m.) will fall in the 

 phases 1000- 1200 fm. (0-200 fm. on the phasing dial) and so be recorded simultaneously on the paper. 

 An example of this from soundings of 890 to 1000 fm. (1628 to 1829 m.) is shown in PI. XXX, fig. 2. 

 On the other hand an echo from, say, 1275 fm. would be picked up in the 200-400 fm. phase, and its 

 re-echo at 2550 fm. in the 400-600 fm. phase. In shallow water of 50 fm. (91 m.) or less, with a bottom 

 of rock or hard sand, as many as four re-echoes have been recorded with sonic transmissions. Similar 

 records have been obtained in shallow water with the M/S XII D recorder, though with the supersonic 

 transmissions the number of re-echoes does not appear to be so great. The reason for this is almost 

 certainly that the faces of both the deep-sea transmitter and hydrophone were exposed to the sea, 

 whereas the supersonic transmitting and receiving tanks were secured inside the hull plating, which 

 at this point was i in. in thickness. 



For some years it has been known that supersonic transmissions in very shallow water could detect 



an overlay of mud, silt or sand on rock. Records from many parts of the world have been obtained 



with Messrs Henry Hughes and Son's machines, with the British Admiralty system of transmission, 



but references to these in the meagre literature on echo sounding in this country appear to be scarce. 



Among the few illustrations traced are portions of two very good records taken respectively in Canada 



(Lake St Peter, Quebec) and Denmark (Aarhus). Portions of these records were published in Messrs 



Hughes' private journal. The Hiisun Review (1936), but the reproduction is not sufficiently clear to 



show the depths at which they were taken. It appears fairly obvious, however, that they were in very 



shallow water. Another series, but with the depth scale clearly marked, shows overlays at 5-18 m. 



(3 to 10 fm.), and forms the illustrations to a report on the survey of Lake Windermere in 1937 by 



Mortimer & Worthington (1940). Chapman (1944) has shown echo-sounding records taken in water 



of depths up to 10 fm. (18 m.). These were taken during a survey of the British Coasts, primarily to 



determine the extent of the beds of the seaweeds of the genus Laminar io, but it might be expected 



that some indication would be seen of the presence of more than one layer of bottom deposit. 



Unfortunately from the echo-sounding point of view no depth scale is given, and this, together with 



the difficulty of reproducing such records clearly, makes it doubtful whether these illustrations are of 



much value for the determination of the composition of the bottom. Re-echoes are common but in 



one instance only does there appear any possibility that the soundwaves have penetrated beyond the 



surface of the bottom, and even this example (Chapman, pi. i, fig. i) is open to several interpretations. 



As to work in other countries Th. Stocks (1935) has given an account of experiments in the Baltic, at 



depths of 11-22 m. (6-12 fm.). The echo-sounding set used here, the Debeg 'Radiolot', differed from 



the British Admiralty supersonic system in that the oscillation of a system of quartz plates was used as 



the source of the transmissions, and the returning echoes were observed directly as spots of light on 



a scale. It is claimed for this sounding set that the angular spread of the beam did not exceed 10°. 



From the illustration in Stocks' paper (fig. 3) it would appear that multiple spots of light were 



received, from a single transmission, over a portion of the scale covering a total depth of approximately 



4 m. (13 ft.) and commencing at a reading of 15-5 m. (51 ft.). As the normal echo from one transmission. 



