798 NAFE AND DRAKE [CHAP. 29 



3. Methods of Measurement 



Among the methods described below some are direct measurements and 

 some yield information on physical proj^erties only through a comparison of 

 experiments and theory involving observables other than the ones sought. Thus, 

 velocity may be measured by direct timing or it may be determined indirectly 

 by comparison of measured with computed curves of phase velocity as a func- 

 tion of period. It is not intended to provide a comprehensive summary of 

 available methods but rather to indicate some that are typical and that are 

 currently in use. Methods of static determination of elastic constants are not 

 included on the grounds that they are adequately treated elsewhere. Moreover, 

 results are frequently incompatible with observed velocities, which depend on 

 djmamic values. Particular notice is given to methods having special advantages 

 for use at sea. 



A. Density, Porosity and Average Grain Density 



These three quantities have been measured (Sutton ef al.. 1957; Hamilton 

 et al., 1956) directly for sediment samples by weighing a sample of known 

 volume and reweighing after drying at 100° to 115°C. 



Then 



P = wiv 



Rg 



Dp,r 



W{pulp)-W + D 

 ■W-D\ _p_ 



Piv 



* - (^) 



where W and D represent wet and dry weights respectively, p„- the density of 

 water, pg the average grain density and V the original volume. Alternative 

 expressions may be written by replacing W by Vp. 



Sutton et al. (1957) have described a method for measuring bulk density that 

 is particularly useful aboard ship where accurate weighing is not possible. The 

 density of a known volume of sediment from a freshly obtained core was 

 determined by suspending the sample beneath a modified spirit hydrometer in 

 distilled water. The sample was obtained by inserting a small cyhnder into the 

 core, at the same time holding a piston fixed at the core surface to prevent 

 compaction of the sample. After density measurement, samples were preserved 

 in sealed containers so that dry and wet weights could also be determined in the 

 laboratory for determination of pg and an independent measurement of p. 



Hamilton et al. (1956) have described underwater density sample collection 

 by a diver. A metal tube, after insertion into the sediment, was sealed at the 

 top by a plastic disk. Another disk was placed at the bottom of the tube by 

 burrowing down along the side. As the methods of Hamilton et al. and Sutton 

 et al. suggest, it is important to avoid any compaction in the process of samjDle 

 collection. An excellent example of the distortion in length that can be produced 



