The connection between grain size, porosity, and Biot model parameters

Abstract

The seabed is a porous acoustic medium, consisting of solid grains permeated by pore water. The Biot model for acoustic propagation in a porous medium is often regarded as a complex model that has an excessive number of input parameters, particularly permeability, pore size and tortuosity. Furthermore, the relationship between these parameters and the sediment classification, which is based on the mean grain size, is unclear. Borrowing from the geophysics and civil engineering communities, a relationship between grainsize and the Biot parameters was developed. This opens the way to predicting the acoustic properties of the seabed from geophysical seabed classifications. The relationship for well-sorted, unconsolidated sands and silts is well established. The transition from sand/silt to clay is where the problem becomes complicated for the following reasons: Because, in clay, a significant fraction of the fluid is attached to the solid platelets by electrostatic forces, and a significant proportion of the clay platelets may be suspended in the pore fluid, the boundary between pore water and skeletal frame needs to be clearly defined. The skeletal frame is sparse and supported by electrostatic forces, and therefore behaves differently to a mechanical packing of grains. [Work supported by ONR, Ocean Acoustics Program.]