Abstract

This is one of several interrelated reports concerning sound velocity, elasticity, and related properties of marine sediments from several major environments in the North Pacific and adjacent areas. Previous reports were concerned with methods, measured properties (e.g., compressional-wave velocity, density, porosity), environmental control, empirical relationships, elastic models for saturated, porous marine sediments, and the computation of elastic moduli. These measured and computed properties were referred to 1 atmosphere pressure and 23 degrees C; all samples were from the sediment surface (0 to 30 cm). This report is concerned with the correction of laboratory values to in-situ values, and the prediction of in-situ values in the absence of any sediment data. Although the samples and values are related to the North Pacific and adjacent areas, the methods and techniques of prediction are applicable to other areas and sediments. The sediments were assigned to three general sedimentary environments associated with three great physiographic provinces: continental terrace (shelf and slope), deep-water abyssal plains, and abyssal hills. The sediment types and their properties in the latter two environments can be predicted with some confidence; detailed sediment charts are required for continental shelf areas. The keys to prediction of in-situ properties of marine sediments are knowledge of (1) general physiographic provinces and their associated sediments, (2) sedimentary processes within these environments, (3) laboratory or in-situ values of properties of the principal sediment types within the environments, and (4) methods of correcting laboratory values to in-situ values. Sound velocity in deep-sea areas can be predicted within about 1 to 2 percent for the sediment surface; velocity should be predicted directly rather than predicting porosity or another property, from which velocity is predicted. Laboratory values of velocity can be corrected to in-situ conditions by multiplying the bottom-water velocity by the velocity ratio (velocity in sediment/velocity in water); this ratio is the same in the laboratory as in situ. Sediment porosity can be considered to be the same in the laboratory as in situ, provided a correction is made to the apparent laboratory porosity to allow for salts evaporated from pore water and weighed with the mineral grains during laboratory analyses. This correction amounts to an increment to porosity of 0.5 to 1.0 percent. Density in situ is slightly greater than in the laboratory because of more dense water in pore spaces; the correction, at a depth of 6000 m, amounts to about 0.025 gm/cm 3 and can usually be disregarded. Using measured values of density and compressional-wave velocity (corrected to in-situ values), plus a computed value of the bulk modulus, we can compute the other elastic constants; these include compressibility, rigidity (shear) modulus, Poisson's ratio, and velocity of shear waves. Numerical examples are given of all computations.

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