Speculations on the cause of finite density and sound speed gradients near the interface of muddy sediments

Abstract

Holland et al. [JASA (2013)] at the previous ASA meeting reported positive density gradients and negative sound speed gradients at the water-mud sediment interface and asked “what processes drive them.” A derivation using the Mallock-Wood relations yields a simple high-porosity approximate relation giving a negative proportionality between sound speed gradient and density gradient independent of depth and porosity. It is also argued that the solid portion of the mud consists of tiny mineral platelets, each of which carries a net negative charge. The presence of dissolved salts causes each platelet to behave as an electrical quadrupole, so that there is, on the average, a small electrical repulsive force between the platelets. With gravity taken into account, the equilibrium separation distance between two parallel vertically aligned platelets is one to two orders of magnitude greater than a typical length scale of the face of a platelet. However, when platelets touch, edge to face, there is an attractive force between platelets, and the net effect is that the platelets tend to be separated at a much shorter distance than the stand-off distance deep within the sediment. Paper reports ongoing efforts to estimate the depth over which the transition occurs.