• There is a good relationship between mean grain size and sound velocity 

 and the velocity versus mean grain size regression equations for continental-terrace sands- 

 silts-clays adequately describes these relations for these calcareous sediments. This is 

 probably because the hollow shells interact as large, solid particles. 



The continued measurements and studies in calcareous materials should furnish 

 fairly definitive information on interrelationships between common properties. 



COMPRESSIONAL-WAVE VELOCITY PROFILES AND GRADIENTS IN THE 

 SEA FLOOR 



Introduction 



Continuous reflection profiling (as with a sparker or air gun) and wide-angle reflec- 

 tion measurements of sediment and rock layer interval velocities have become important 

 sources of critical data in underwater acoustics. Continuous reflection profiling measures 

 sound travel time between impedance mismatches within the sediment and rock layers of 

 the sea floor. To derive the true thicknesses of these layers it is necessary to measure or 

 predict the mean or interval layer velocity or use a sediment surface velocity and velocity 

 gradient. At the present time, the simplest method of measuring layer interval velocities 

 involves the use of expendable Navy sonobuoys. 



Sonobuoy measurements of interval velocity also provide basic data for determining 

 velocity profiles and gradients in the sea floor. When velocity measurements in sediment 

 cores are available, these can be corrected to in situ values at the water-sediment interface 

 and used with layer mean velocities to establish velocity profiles and gradients. 



It has been shown by Morris (1970) and others that the presence of a positive veloc- 

 ity gradient in the sea floor is of critical importance in acoustic studies when sound interacts 

 with the sea floor. In general, when a sediment layer has a positive velocity gradient, sound 

 energy is refracted back into the water column at certain grazing angles and energy is lost 

 over long refraction paths. 



In summary, reflection profiling records yield data critical to underwater acoustics 

 in two categories: the form and true thicknesses of sediment and rock layers and the pres- 

 ence and values of velocity gradients. These data, when examined statistically, yield re- 

 gional velocity profiles and general, averaged velocity gradients that can be used to predict 

 velocity gradients in similar sediments elsewhere. 



Studies From 1974 to 1977 



During the three-year period of this project, three sets of sonobuoy data were ana- 

 lyzed. Two sets from the Northeast Indian Ocean have been analyzed, added to previous 

 data in the area and reported (Hamilton et al, 1977). A suite of 17 sonobuoy measurements 

 of interval velocity were made in the thick calcareous sediments and rocks in the Ontong- 

 Java Plateau in the West Central Pacific. These records have been analyzed and reported 

 (Johnson etal, 1978). 



13 



