The physics of bubbles in surficial, soft, cohesive sediments

This paper reviews current understanding of the physics of bubbles in soft (unlithified), cohesive sediments, certainly in the top 100 m from the sediment-water interface, but likely much deeper. Experimental evidence, primarily CT scans and internal pressure records, indicate that near-surface bubbles grow by elastically compressing and fracturing these sediments, which results in thin, irregular disks of gas. These data do not suggest either plastic or fluid behavior on the part of the bulk sediment. In addition there is no hint of capillary invasion of gas into the pores of fine-grained sediments. The growth rate of an elastic-fracturing, disk-shaped, bubble can vastly exceed that of a spherical bubble, depending on the eccentricity of the bubble. This effect results from the far more favorable surface-to-volume ratio of flattened bubbles. The initial rate of rise of bubbles in cohesive sediments also appears to be governed by a fracture process, which is driven by pseudo-buoyancy. The rate of bubble rise could be controlled by various possible mechanisms, but only the predictions from a visco-elastic-fracture propagation model seem to produce rise velocities that are not ballistic (<m s(-1)) and unobserved for bubbles leaving sediments. Bubbles can repeatedly utilize the same fracture path to rise to the surface and incomplete annealing of the path between subsequent bubbles greatly facilitates this process. Experimental evidence demonstrates that a rising bubble can deviate from its path to the surface if it encounters a "zone" of sediment with lower lateral fracture toughness; such a bubble will begin to move in a lateral direction, but stall because pseudo-buoyancy is no longer operative. Subsequent bubbles will do the same and accumulate in the low fracture-toughness zone. This explains the occurrence of shallow sub-seafloor domes of gas, and modeling of this process predicts only modest over-pressures in such structures. (C) 2012 Elsevier Ltd. All rights reserved.