involve reflection at the surface at grazing incidence and because the 

 sound might travel long distances near the surface, in an obviously 

 disturbed layer. Measurements of the depth of this layer as a function of 

 sea state would be interesting from the point of view of sound velocity. 

 In this case it seems unlikely that temperature or temperature and salinity 

 measurements would be at all meaningful by themselves, the effects of 

 surface geometry and entrained air being so much stronger. 

 3. Velocity in the Sea-bottom 



A knowledge of the density and the velocity in sea-bottom 

 sediments in specific locations would be of interest from the points of view 

 of sonar reflectivity, seismic reflection and refractive profiling. 

 Reflectivity is currently measured by large pulsed sources or explosives. 



The velocity of sound in sediments is a function of several 

 variables and is strongly dependent on any trapped gas. The dependence 

 of sound speed on pressure in marine sediments is unknown. Measurements 

 at ultrasonic frequencies may be of little value to the Navy if dispersion 

 frequencies occur, since extrapolations to low frequencies might allow 

 gross errors. 



Many of these factors could be determined by laboratory experiments. 

 However, velocity measurements in areas known to have bottoms which are 

 suitable for sonar operation would be of both short and long term value. 



An instrument for making velocity measurements in situ in ocean 

 sediments (Reference 16) has been under development by Hudson Laboratories 

 and the Benthos Company. It is a combination of a sound velocimeter and 

 a free-falling boomerang corer. The transmission frequency is 0.5 MHZ; 

 the path length is 15 cm. The data is recorded on magnetic tape in the 

 buoyant section which is separated from the free-fall vehicle by a timer. 



16 



Arthur ai.HtttMnr. 



