Wettability dependent P-wave scattering and velocity saturation relation in granular medium

Abstract

We investigate elastic wave scattering and the velocitysaturation- relation (VSR) in glass bead packings resembling unconsolidated granular sediments in the shallow subsurface. While it is known that wave scattering in dry granular media is dictated by the presence of force chains, the effect on the VSR in partially saturated granular media under different wettability conditions remains unexplored. To make progress in this direction, we design and conduct laboratory experiments by combining core flooding and ultrasonic measurement in glass bead packings with different wettability. The experimental results show that there is a transition from a stable P-wave pulse propagating in an effective medium at low and moderate saturations to a set of incoherently scattered waves at high saturation. This observation holds true for either the water- or the gaswetting glass bead packings. However, the incoherent scattering in the gas-wetting case is negligibly small. We interpret these observations through alteration of the force chains. Only if water wets the grains, can the liquid enter the grain contacts. Then, its viscoelastic properties contribute to the reinforcement of the force chain and also increases its characteristic length scale. This leads to a P-wave velocity enhancement and to scattering. On the contrast, in the gas wetting case, the wetting phase gas prevents the water from the direct contact with the glass beads therefore stops the extension of the force chains, which inhibits the wave scattering and viscous coupling between the water and the glass beads. Show more