excellent reflectors. Such a condition is believed to be caused by these 

 sediments having an acoustic velocity substantiallv less than that of 

 water (Wood, 1941), in turn causing a large acoustic impedance contrast. 



Thus, with the above cautions borne in mind, it is possible to des- 

 cribe the seafloor in terms of either the reflectivitv or porosity. The 

 strong correlation between reflectivitv and porositv can be used to infer 

 the potential load-bearing properties during a survev; or, if a more 

 quantitative description is desired, computer data processing, either in 

 the laboratory or aboard ship, could be used (Breslau, 1965). 



As discussed above, it is feasible to relate porosity to sediment 

 type only in general terms, i.e., high porositv relates to silts and clays, 

 and low porosity to sands and gravels. However, for porosities greater 

 than 55 percent, there is a great overlap in sediment type and precise 

 designations are inconclusive (Smith and Li, 1966; Faas , 1969). 



Acoustic Velocity 



In order to interpret accurately the continuous seismic reflection 

 record, some knowledge of the acoustic compressional velocity in sediments 

 and rock is reauired. The seismic record is a time display; in order to 

 estimate sediment or rock thickness, an interval or average velocity for 

 the particular unit must be assumed or measured. Additionally, numerous 

 correlations between the compressional velocities and sediment and rock 

 physical properties have been established. Thus, if the acoustic veloci- 

 ties in the material can be determined, inferences can be made about the 

 expected load-bearing capabilities of this material. This method of 

 remotely determining sediment and rock properties is of particular value 

 in preliminary construction-site surveying work where a large area is to 

 be investigated and, later, in estimating the physical properties of 

 deeply-buried materials, those not sampled using conventional oceanographic 

 corers . 



Subsequent sections will summarize current information on acoustic 

 compressional and shear wave velocities in sediments and rock, with an 

 emphasis on the engineering applications of these data. 



Compressional Wave Velocity . Marine sediments are porous and loose 

 in structure, having elastic properties which mav or may not be directly 

 measured. When this loose aggregate is placed under compacting pressure, 

 the porosity is reduced, the grain-to-grain contact is increased and, as 

 pressure increases, this aggregate acts more and more like an ideal 

 elastic body (Hamilton et al. , 1956). 



The velocity of acoustic compressional waves in an elastic media is 

 given by Hamilton et al. (1956). 



