Velocity unconsolidated sediments

Nonionizable chemicals (e.g., hydrocarbons, ethers, alcohol) that sorb onto organic materials in an aquifer (i.e., organic carbon) are retarded in their movement in groundwater. The sorbing solute travels at linear velocity that is lower than the groundwater flow velocity by a factor of Ra, the retardation factor. If the Koc of a compound is known, the retardation factor may be calculated using the following equation from Freeze and Cherry (1974) for unconsolidated sediments ... 

The differences in the elastic behaviour of these two groups are based on different physical conditions at the contacts of the rock particles. For the first group, conditions are controlled by friction effects, whereas for the second group, physiochemical phenomena are dominant. Similar to consolidated sedimentary rocks, there exists a significant correlation between velocity and porosity for unconsolidated sediments. Velocity in unconsolidated sediments is distinctly lower than in consolidated sediments. The compressional wave velocity shows a clear difference for the dry sediment (about 200-500 m s ) and water-saturated sediment (about 1600-2000 m s ). 

With increasing clay content, the velocity generally decreases in unconsolidated sediments. This is the result of the low stiffness of the clay-water aggregates in the sediments. 

FIGURE 6.12 Compressional wave velocity versus porosity for unconsolidated sediments (a) dry unconsolidated sediments (Schim, 1964, 1969, 1983) and (b) water-saturated marine sediments (Hamilton, 1971). 

Frozen unconsolidated sediments (permafrost) show behaviour more comparable to consolidated sediments. Ice between grains acts like cement and the velocity of longitudinal and transverse waves is distinctly higher than in the non-frozen state. Some data from Canadian Arctic permafrost samples are shown in Table 6.8. Velocity changes in freezing temperatures are discussed also in Pandit and King (1979) and Scott et al. (1990). 

A practical explanation for the velocity-porosity variations is provided by a simple elastic theory (Wood, 1941) where seawater-saturated, unconsolidated marine sediments are considered to be nonrigid systems, consisting of discrete, noninteracting mineral grains suspended in seawater (Hamilton, 1971,1972). Sound velocity would only depend on the relative proportion of solid and fluid, and their respective compressibilities and densities, expressed through Hookean elastic equations (except for attenuation which must be treated viscoelastically) (Hamilton, 1972,1980). 

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