detail to show the prominent reflectors in the upper few tens 

 of feet, and (3) enough measurements (preferably in situ) 

 to establish the lateral and vertical gradients of those prop- 

 erties having effects on sound propagation. All of these 

 items are now within technological feasibility. 



Sub-bottom layering in the detail required is not 

 feasible using the spark-source type of sub-bottom acoustic 

 reflection equipment; that part of principal interest is a 

 mere black line on the record. The direction of research 

 and development to obtain the required information should 

 lie with deep-towed transducers and/or equipment mounted 

 on deep submersibles. 



The information on mass physical properties of sedi- 

 ments, in the detail required, can best be obtained from 

 in situ measurements either from probes, boreholes, or 

 from submersibles; these measurements can also validate 

 those taken from surface ships, or in the laboratory. 

 Those properties in most need of study at present are the 

 velocity of the elastic shear wave, and the attenuation of 

 sound at frequencies of interest in underwater acoustics. 

 Studies of the compressional wave, and density, are well 

 advanced, and the values of these properties can usually be 

 predicted with some accuracy. More study, however, is 

 needed of the vertical gradients of these properties. 



The two geoacoustic models which were developed 

 contrast strongly. 



The Bering Sea area reported here is extremely flat 

 and is covered with a fine, hard sand, in which the sound 

 speed is from 4 to 12 percent greater than in the sea water 

 just above the bottom; the density varies from 1.82 to 1.93 

 g/cc. In general, there is about 5 to 50 feet of sediment 

 overlying a prominent reflector ("rock"). 



In situ sound speeds at the Mohole Site vary from 

 1484 m/sec at the sediment surface to 1584 m/sec at a 

 depth in the sediment of 136 meters. The resulting sound- 

 velocity gradient is approximately 1.0 sec" ; the interval 



35 



