Additionally, the records are useful in the planning of excavation, 

 dredging, and other engineering tasks which require knowledge of the 

 "rippability" and traf f icability of an area. 



To properly interpret a seismic record to gain knowledge of the 

 physical and load-bearing properties of marine sediments, the acoustic 

 properties of the material must be known. Sediment reflectivity (or 

 bottom loss) is strongly correlated with sediment porosity; thus, with 

 due caution exercised, reflectivity may indicate the sediment type pres- 

 ent: high porosity implies silts and clays, low porosity implies sands 

 and gravel. 



The acoustic compressional velocitv for a sediment type is required 

 to calculate the overburden thickness accurately from a reflection record. 

 Additionally, acoustic velocities also correlate with some of the physical 

 properties of marine sediments, in particular sediment porosity and bulk 

 density: generally, sound velocity increases as porosity decreases. Cor- 

 relations have also been established between acoustic velocity and moisture 

 content, void ratio, and saturated bulk density. Although acoustic proper- 

 ties are not considered to be a reliable index of the shear strength of 

 marine sediments, there is, generally, an increase in sound velocitv as 

 shear strengths increase. 



Acoustic velocities also provide an estimate of the facility with 

 which material mav be excavated ("rippability"). Granites, for example, 

 have velocities larger than limestone, and limestone has velocities 

 larger than those in unconsolidated sediments. 



An increase in compressional velocity generally implies an increase 

 in the firmness or the relative bearing capacity of a material. 



Shear wave velocities of sediments, much more difficult to measure 

 than compressional velocities, are related to the shear strength para- 

 meters of the sediment and, therefore, of immense value to the marine 

 construction engineer. Furthermore, if the compressional and shear wave 

 velocities are known, the dynamic elastic properties of the material, such 

 as Poisson's ratio, can be calculated. 



Although the in-situ determination of shear x^ave velocities requires 

 bottom-sitting instruments as seismometers or probes, the determination 

 of in-situ compressional velocities is relatively simple. Standard seismic 

 refraction measurements or the more feasible techniques of wide-angle 

 reflection mav readily be used. 



In addition to a knowledge of the acoustic reflectivity and velocities, 

 information about the acoustic attenuation properties of marine sediments 

 is of value. The attenuation coefficient, in general, increases with an 

 increase in the mean diameter of the sediment grains; therefore, the 



23 



