120 



Fig. 7. 10-Foot Line Hydrophone 



At the Underwater Sound Laboratory we have encountered certain situa- 

 tions where sound propagation conditions as predicted on the basis of ray theory 

 and the bathythermogram are by no means realized. In these situations a well 

 mixed surface layer was present. Propagation in the surface layer should have 

 been good; it was very poor. In such situations the very great importance of 

 small thermal gradients has been pointed out. It seems entirely possible that 

 under certain conditions current gradients may exist which have an equally im- 

 portant effect. Instrumentation and methods for measurement of current grad- 

 ient with depth will be welcomed by both the acoustician and the oceanographer. 



DISCUSSION: Leonard Liebermann 



I believe it is worth pointing out that Dr. Hersey's excellent and compre- 

 hensive discussion of "Acoustic Instrumentation for Oceanography" has been 

 presented on the framework of existing acoustic instruments and their applica- 

 tions to oceanography. He has preferred to set the emphasis on acoustic equip- 

 ment with possible applications, rather than primarily on the oceanography. 

 Suppose instead of this viewpoint, one begins with the important contemporary 

 problems of oceanography; and inquires what contributions, if any, might be 

 made by applying acoustic techniques to these problems. Two major oceanch- 

 graphic problems immediately come to mind: the study of the dynamics of ocean 

 currents and the study of ocean waves and their prediction. 



Beginning with the first problem, one of the obstacles to the study of 

 ocean currents is the difficulty with which field observations can be made; the 

 ideal current meter has not yet been invented. However, acoustical techniques 

 make it possible to measure doppler frequencies caused by relatively slow vel- 

 ocities. Hence, it now appears feasible to construct echo ranging equipment 

 designed to measure ocean currents. Comparison of the frequency of the bot- 

 tom echo with the reverberation frequency from the body of the ocean water 

 would yield the current velocity. In this manner it may actually be possible to 

 delineate ocean currents at different depths. An important advantage would be 

 the possibility of continuous measurements while the ship was under way. The 

 same equipment may also be used for the accurate determination of relative 

 ship's motion. 



Another closely related problem is the determination of total water mass 

 flow, in various parts of the ocean. Stommel has made observations on the 

 flow of the Gulf Stream through the Straits of Florida. He observed the induced 

 voltage in a pair of electrodes fixed at opposite sides of the stream. However, 

 his technique is limited in the choice of locations because the two electrodes 

 must be electrically connected; in this case by means of a commercial sub- 

 marine cable. 



