Ill 



ness of the sedimentary carpet. Greater resolution of bottom shapes could be 

 achieved by an increase of effective directionality. This in turn would have to 

 be accompanied by a high degree of stability in the orientation of the transducer. 

 This could be achieved quite practically by going to a larger transducer below 

 the ship, not only for increased stability, but also to get it closer to the bottom. 

 Penetration of the bottom can be achieved by ennploying lower frequencies. It is 

 not impossible to incorporate all these features in a single device. However, it 

 would certainly not be of any present commercial or naval interest, except for 

 the data about the oceans it would produce. It appears to me that its develop- 

 ment will have to come from scientific interest and out of research funds, and 

 is a possibility in oceanographic instrumentation which merits serious consider- 

 ation. 



The general theory of scattering from small objects has long demanded 

 that marine scattering be studied with an acoustical system in which the scatter- 

 ing as a function of frequency can be studied in detail. A beginning has been 

 made using an explosive sound source and a broad band directional receiver. 

 Past instrumentation has been exceedingly crude acoustically, and is being de- 

 veloped as research in this field progresses. For analysis, similar techniques 

 have been employed as in passive listening, and here the spectrograph type ana- 

 lyzers have proven very useful. This whole field requires extensive exploration 

 and concomitant development of technique before its full potentialities will be ap- 

 proached. Its use as a research tool in marine biology or a commercial tool 

 for fishing is just beginning. It is an exciting and promising field. 



CONCLUSIONS 



In the foregoing I have barely scratched the surface of problems which 

 may be attacked in oceanography with acoustical tools, and I feel that the subject 

 of available acoustical instrumentation has been covered only very sketchily. 



Only beginnings have been made in the development of acoustics in 

 oceanographic science. We have relied rather heavily on makeshift acoustical 

 and electronic instrumentation. This picture is gradually changing, but it re- 

 quires continued emphasis and the interest of competent acousticians and elec- 

 tronics engineers. At present very few thoroughly trained and experienced 

 people are at work in this field -- I should guess less than two dozen acousti- 

 cians and engineers who are relatively free to contribute to purely scientific 

 research. With this sort of a roster progress can be expected to take place 

 very slowly. 



DISCUSSION: R.J. Christensen 



Dr. Hersey has treated comprehensively the philosophy and general 

 technical aspects of acoustic instrumentation. It is my purpose to comment 

 briefly on several specific acoustic tools of limited application to the study of 

 oceanography. 



The first is the system devised for air-sea rescue, SOFAR (Sound Fix- 

 ing And Ranging). This system employs a network of three or more hydrophones 

 installed~at an ocean depth near that of minimum acoustic velocity. Ashore 

 monitoring stations, cable-connected to the hydrophones, record the signals re- 

 ceived from explosive sources at distances up to several thousand miles. 



Dr. Press has already pointed out the usefulness of these SOFAR sta- 

 tions in recording the "T" phase of signals of seismic origin, and Dr. Worzel 

 has discussed the use of reflections of the SOFAR type of signal, in locating sea 

 mounts and other types of lithosphere intrusions into the SOFAR sound channel. 



