Estimation of geoacoustic parameters of ocean mud

Abstract

Current geoacoustic models fail to estimate accurately the shear speed and attenuation of high-porosity marine mud, because they do not account for the physics of its colloidal and aggregation properties. Marine mud is comprised of seawater and thin clay mineral platelets that possess a net negative charge. Positive ions in the seawater congregate near the platelet surface, suggesting an electrical model of a platelet as a sheet of uniformly distributed longitudinal quadrupoles aligned perpendicular to the surface. This electrical structure causes interactions that lead platelets to aggregate into card-house structures wherein platelets have nearly perpendicular contact. The shear speed of the card-house is calculated from an effective shear modulus using electric and elastic forces between platelets when the card-house is perturbed. Several platelet interaction models are considered, and one with rigid rotation (neglecting elasticity) of platelets separated by a small channel produces shear speed estimates consistent with experimental observations. Estimates of card-house porosity are calculated from two-dimensional idealized computational models based on two different aggregation processes. Particle-cluster and cluster-cluster models lead to porosities consistent with observations. Additionally, for the cluster-cluster model, solid content is dependent on the aggregate length scale in a fractal manner. Bubbles in mud are observed to have thin shapes, which may arise from the electrical properties of the platelets. Electrostatic effects on bubbles are investigated by considering the experimental observation of decreased coalescence of bubbles in salt water.