SOUNDINGS TAKEN DURING THE DISCOVERY INVESTIGATIONS, 1932-1939 59 



at a depth of 182 m. (100 fm.), gave a single spot of light on the scale equivalent to a depth of 1-5 m. 

 (5 ft.) these multiple spots have been interpreted as echoes from successive layers of the bottom deposit. 

 This may be correct, for the greatly increased frequency of the short-wave quartz transmitter 

 (slightly more than twice that of the Admiralty system) may allow of greater penetration, but without 

 the evidence provided by a permanent record there must remain an element of doubt in this assump- 

 tion.^ With the British Admiralty system of permanent recording there does not appear to be an 

 instance published of the soundwaves penetrating more than what is apparently a single overlay on 

 a harder bottom. Stocks (1935) and Rust (1935) have pointed out that the correct width of each layer 

 in the deposit could not be accurately assessed, since the velocity of sound in the respective media 

 was certain to show a difference from that determined for sea water. Theoretically this is true, but it 

 is doubtful whether the accuracy in reading off depths from a visual light scale would be in itself 

 sufficient to warrant an exact measurement being given for the extent of each layer. In practice it is 

 doubtful whether a change in velocity would have much effect even on the measurements obtained 

 by permanent recording at the very shallow depths mentioned, and it is probably true to say that the 

 measurements of the extent of an overlay, obtained from a permanent record, would not be far from 

 the actual figures. Confirmation of this could only be obtained from cores of the bottom deposit taken 

 at the time when the soundings are recorded. 



Rust (1935) mentions that layering in the bottom sedimentation has been detected with sonic 

 transmissions but he does not give any particulars. No account of any such occurrence appears to 

 have been published in this country but it is known- that in 1935 H.M. Survey Ship 'Challenger' 

 obtained an excellent record of mud overlying rock near Trinidad, in the West Indies. The echo- 

 sounding set used was the ordinary Admiralty pattern sonic set for shallow water, but with the 

 addition of a recorder. The depth of water over the mud varied between 90 and 100 fm. (165 and 183 m.) 

 and the depth of the mud itself from 180 to 240 ft. (55 to 73 m.). The record, unfortunately, was not 

 retained after the completion of the survey. A careful examination of the many records we possess 

 from the ' Acadia ' recorder fails to show any such definite evidence that the soundwaves penetrated 

 the bottom deposit. Admittedly, the great majority of these records were made in depths of water 

 much beyond the range at which layering has hitherto been detected with supersonic equipment ; but 

 with the sonic transmitter and hydrophone both exposed to the sea it had been thought that if as many 

 as four re-echoes could be recorded from shallow water, then there was a reasonable chance of the 

 sound penetrating beyond the immediate surface of the bottom. The attenuation of the soundwaves from 

 the deep-sea transmitter, however, was very great, and as can be seen from some of the records shown m 

 Pis. XXVI-XXX the echo marking which thus results from a normal echo may easily cover any trace 

 of an echo from a harder layer below the surface of the bottom. Among the records which were closely 

 examined are certain peculiar recordings of fairly flat bottoms which might be considered as evidence 

 of layering. The spread of sonic transmissions, however, is such that these secondary traces may well 

 have their origin in other features of the bottom, at depths of more than 100 fm. (183 m.), and although 

 there remains an element of doubt in some instances, these records have generally thus been mterpreted. 

 We have been equally disappointed in such few records as remain from the M/S XII D recorder. The 

 great majority of these have faded (see Appendix I, p. 97) and although the contour of the bottom was 

 pencilled or inked-in shortly after the record was dry (see Plate XXXI, fig. 2), little else remains, except 

 1 Conversations with the Marconi International Marine Co., Ltd., confirm our doubts on this point. The Marconi Co. 

 have much experience of visual recording by light in conjunction with a quartz transmitter (or projector) and are not by any 

 means prepared to assume that multiple flashes on the scale are echoes from successive layers of the bottom. In fact, they 

 consider that the presence of such multiple effects (which to them is a well-known occurrence) is more probably caused by 

 attenuation of the reflected soundwaves, owing to a rough or confused bottom. 

 ■^ Private communication to the author. 



