29. PHYSICAL PROPERTIES OF MARINE SEDIMENTSi 



J. E. Nafe and C. L. Drake 



1. Introduction 



The unconsolidated sediments that blanket the ocean floor are of widely 

 varying thickness, but seismic observations indicate that 200 to 400 m in the 

 Pacific (Revelle et at., 1955) and 1 km in the Atlantic (Ewing and Ewing, 1959) 

 are fairly typical values for deep water. At present direct observation of these 

 sediments is limited to such samples as may be recovered by dredging or coring 

 operations, for drilling has been carried out only in the shallow waters of the 

 coastal shelves. Knowledge of the physical properties of the great bulk of the 

 sediments deeper than the few tens of feet reached by coring equipment is thus 

 necessarily derived from geophysical observations. 



The recoverable sediments of the topmost part of the sedimentary column 

 may be studied in detail. Physical properties, such as elastic wave velocities, 

 attenuation, density and thermal conductivity, are directly measurable. The 

 bulk properties depend ultimately on the values of other observables. These 

 include water content, particle density, size and shape of particles, age and 

 chemical composition. Measurements may be static or dynamic. They may be 

 carried out at elevated pressure, they may be made in various frequency ranges, 

 or they may sometimes be made in situ. Measured properties on small samples 

 are related only in some average sense to the results of large-scale measure- 

 ments. 



It will be noted that, whether on the scale of small samples or on the scale 

 of seismic refraction experiments, acoustic measurements play a major role in 

 any study of physical properties of marine sediments. They provide direct 

 information on compressional-wave velocity, shear-wave velocity, impedance, 

 attenuation and Poisson's ratio. If density is known independently, they supply 

 in addition dynamic values of compressibility, shear modulus and other elastic 

 constants. Combined with appropriate travel-time analysis they give the varia- 

 tion with depth of elastic -wave velocities and, consequently, the depth variation 

 of any derived quantities. In certain cases, such as dispersion of elastic waves in 

 shallow water, some information on the depth variation of density is obtained. 

 Approximate depth variation of many physical properties may be derived from 

 velocity measurements by combining them with empirical relationships con- 

 necting velocity with density or velocity with porosity. These relationships are 

 based on experimental observation, and inferences from them are capable of 

 being partially checked by comparison of gravity with seismic interpreta- 

 tions. The existence of such empirical connections is a consequence of the 

 limited range of substances ordinarily occurring in sedimentary deposits and 

 is to be regarded as a fact of geology rather than the operation of physical 

 law. 



1 Lament Geological Observatory (Columbia University) Contribvition No. .598. 



[MS received July, WW] 794 



