Viscosity-based theory of phase velocity and attenuation of sound in mud consisting of water and flocculated clay particles

Abstract

Recent theory of authors ascribes attenuation in typical marine mud sediments to be caused by viscous interaction of sea water with embedded silt particles. Influence of underlying clay matrix is regarded as passive and of minor importance. Present paper considers silt-less mud where clay particles are flocculated to a card-house structure, with the flocculation hypothesis yielding a porosity of 90%. During the passage of a sound wave, Van-der-Waals forces between platelets cause the matrix to move to-and-fro as a unit; viscous forces are insufficient to cause the matrix to move perfectly with the water. The silt-less theory assumes that the local force on the matrix is the sum of the viscous forces on the platelets in the matrix. Forces on clay particles, which are thin platelets, are given by a low-Reynolds flow theory initiated by Stokes, and further developed by Oberbeck, Lamb, and Brenner. For each particle there is a characteristic frequency inversely proportional to the platelet thickness, which turns out to be extremely high for clay. Consequently, at acoustic frequencies, the clay matrix moves nearly in lock-step with the fluid motion associated with the sound wave. The inevitable very-small slip leads to an attenuation that is proportional to the square of the frequency, but which is very small compared with that of mud with embedded silt-particles. Current idealized theory results in prediction of attenuation inversely proportional to viscosity and of nearly constant phase velocity. [Work supported by ONR.]

Measurement of Sound Speed in Fine-Grained Sediments During the Seabed Characterization Experiment

“…It has been shown that reliable attenuation records are challenging to obtain in mud due to primitive techniques and complication from ambient noise [33]. With limited understanding and measurement of the geoacoustic properties in mud, sediment acoustic modeling is currently an ongoing research topic [34].…”