197 



to place complete reliance on it. 



ACOUSTIC METHODS OF EXPLORING THE SEA FLOOR 



The classical method of obtaining information on the nature of the sea 

 floor by direct sampling is now being supplemented by information obtained by 

 means of various types of radiation. Fortunately, water is an excellent medium 

 of transmission of sound. In fact, of all the geophysical methods, those using 

 underwater sound are the most important so that they are the only ones which 

 will be discussed here. Military necessity has promoted the development of the 

 science of underwater sound for detecting and locating underwater targets. The 

 study of the sea floor promises to be greatly aided by the developments in this 

 flourishing science. The recent use of low frequency sound from an explosive 

 source to determine the nature of the crustal layers beneath the sea is a good ex- 

 ample but inasmuch as we are concerned here only with method of exploring the 

 water-sea floor interface, we need not discuss this technique here. 



Of all devices for exploring the sea floor, the most useful is the echo 

 sounder. This is in spite of the fact that the echo sounder was developed as an 

 aid to navigation and all existing models are still poorly designed as far as being 

 a tool for deep sea exploration and research. An innportant advance in the echo 

 sounder was made when a recorder was added as in the NMC and the NMC-1 and 

 2, models which permitted a graphic profile of the bottom to be obtained. Un- 

 fortunately, a maximum scale depth of 2,000 fathoms was chosen. Inasmuch as 

 there is little sea floor which lies between 2,000 fathoms and the continental 

 shelf maximum depth of about 70 fathoms, this scale has been of little value. 

 Few graphic records of the deep sea floor have ever been obtained with these 

 echo sounders. In the Pacific Ocean, other than those taken by Scripps and 

 NEL vessels with especially modified equipments, the writer is only aware of 

 the existence of one fathogram in the central Pacific by USS KERSTIN and sever- 

 al in the Gulf of Alaska by vessels of the U,S. Coast and Geodetic Survey. 



The recent development of the Edo echo sounder is most promising. The 

 larger paper size, the rectilinear scale, the greater power (apparently sufficient 

 to record the bottom at any depth in the ocean), and the general mechanical and 

 electronic excellence of design are great improvements over the older equip- 

 ment. However, the paper speed and the choice of scale (on deep scale) are, it 

 seems to the writer, poorly chosen so that the great advantages of this new in- 

 strument for exploring the deep sea floor are largely lost. Fortunately, this 

 can be corrected by the addition of second stylus to the stylus belt so that the 

 shoaler scales can be made to record at multiples of the basic scale, e.g. , the 

 0-600 fathom scale then will record at 600-1200 fathoms, etc. Such an altera- 

 tion will undoubtedly be made on survey and research vessels but it appears that 

 the great mass of detailed bottom data that should come in from the fleet and 

 revolutionize the science of the sea floor, unfortunately will not materialize with 

 the present gear. 



Another application of underwater sound to the studying of the sea floor is 

 the determination of bottom type directly from a sound pulse and determination 

 of the structure of the sedimentary layers in the immediate vicinity of the bot- 

 tom. If such methods can be developed they will be a great boon to the sea floor 

 science. This matter deserves some discussion in this paper. Many persons 

 who have worked with echo sounders feel that they can tell bottom type from fath- 

 ogram traces and from the audio quality of echo-- but this has more the status 

 of an art than a science. Some of the criteria which seem to be useful for iden- 

 tifying the bottom are (1) topography, (2) echo extension, (3) structure in the 

 bottom echo, (4) the strength of the echo, and (5) audio quality. 



