future work, where possible, should emphasize in situ 

 measurements to parallel, or corroborate, experimental 

 and theoretical work in the laboratory. Where in situ 

 measurements are not feasible or possible, a great deal of 

 consideration must go into the collection of "undisturbed" 

 samples. 



In reviewing the mass physical properties of sedi- 

 ments which are necessary for some theoretical studies of 

 underwater sound, it is apparent that the elastic properties 

 and the frequency-attenuation relationships are the least 

 known. The speeds of compressional elastic waves and 

 densities are fairly well known, or can be easily measured 

 or estimated (although there is much work to be done to 

 reconcile various theoretical and experimental studies). 

 For the shear modulus (or rigidity) and compressibility 

 values, little can be done at present except to estimate, or 

 extrapolate. The single greatest need now is for valid 

 measurements of the speed of shear waves within natural 

 sea-floor sediments. Given the speeds of the compressional 

 and shear waves, plus the density, all of the other pertinent 

 elastic properties may be computed. Another problem, not 

 yet resolved, is which theoretical elastic body (e.g., Voigt, 

 Maxwell, ideal) best represents a natural sediment under 

 the light pressures of a passing acoustic wave at the fre- 

 quencies of interest in underwater acoustic studies. There 

 is no such thing as an ideal elastic medium in nature, since 

 the energy of any elastic wave is dissipated through con- 

 version to heat and by other mechanisms. This is especially 

 true of sediments, which vary widely in mass properties 

 and constituents, both horizontally and vertically. 



Miscellaneous 



A complete model study requires the vertical sound- 

 velocity profile in the sea water above the sea floor. This 

 profile is generally obtained by using Nansen cast data, 

 either in a computer program or in conjunction with tables 

 for the speed of sound in sea water. (These tables are 

 conveniently interpolated and published by the U. S. Navy 

 Oceanographic Office. 6-8 ) A less common way, but one to 



12 



