Elasticity of partially saturated frozen sand (short note)

Seasonal thawing and freezing of near-surface sediments significantly affect the interpretation of seismic reflection surveys and vertical seismic profiling, as well as activity on the surface. Permafrost covers much of the Earth’s colder regions and is also subject to periodic thawing and freezing. The significance of the regions with large seasonal temperature variations for exploration and engineering activities makes understanding the elasticity of frozen ground an important practical and scientific goal. Previous theoretical and experimental studies of seismic velocities in frozen rocks (e.g., Timur, 1968) showed that P-wave velocity in fully water-saturated rocks increases with decreasing temperature, whereas it is almost independent of temperature in dry rocks. As temperature decreases, saturated rocks become (given enough time) completely frozen, and velocity reaches its constant (terminal) value. A theoretical scheme for estimating both Pand S-wave velocities in saturated freezing rocks has been proposed by Zimmerman and King (1986); the method accurately predicted the experimental data. In this study, we are concerned with velocities in partially saturated unconsolidated sands. Our experiments show that in hydrophilic granular materials, velocities may be close to their full-saturation terminal values at saturations as small as 10– 15%. The reason for this strong velocity increase from the dry state is the cementing action of ice that forms pendular rings at the grain contacts. This freezing effect is a special case of a more general phenomenon: even small amounts of relatively soft cement, if placed at the grain contacts, act to dramatically increase the stiffness of the aggregate. By applying the Dvorkin et al. (1994) intergranular cementation theory, we accurately predict the measured experimental values. These accurate predictions cannot be achieved by effective medium theories that do not take into consideration the specific location of the cement—at the grain contacts.