• Session: 24. Inversion methods in underwater acoustics
    Organiser(s): N/A
  • Lecture: RELAXATION MODELS FOR MUD [invited]
    Paper ID: 999
    Author(s): Chotiros Nicholas
    Presenter: Chotiros Nicholas
    Presentation type: oral
    Abstract: Mud has a few distinctive characteristics: Its sound speed is relatively constant and the attenuation increases linearly with frequency. It supports shear waves, and it exhibits creep. These facts suggest a relaxation process associated with the shear stiffness. The creep suggests that the shear stiffness vanishes at very low frequencies. This resembles a relaxation model, controlled by a time constant. At frequencies above the inverse of the time constant, the shear modulus is approximately constant, but below, it decreases with frequency. The Grain Shearing (GS) model of Buckingham has these properties. The constant T in the GS model is in fact the relaxation time constant of the creep process. However, the GS model is not adaptable to other types of sediments, such as sands and silts, although other variants of the GS model, with additional relaxation times have been put forward. The Extended Biot (EB) model [2], which can model a range of sands and silts, may also be adjusted to fit the properties of mud. While finding mathematical models that happen to fit the observations is useful, it gives little insight to the physical processes involved. In the case of clean sand, the division between the solid frame and the pore fluid is very clear. The solid grains are mechanically held together to form a skeletal frame through which the pore fluid flows. In the case of mud, the skeletal frame needs to be redefined, because a significant fraction of the pore fluid is attached to the solid platelets by electrostatic forces. Multiple platelets, along with the attached water, are connected together to form relatively rigid domains. They are connected together in a “floc”, which may be considered as the skeletal frame in the context of a porous medium [Supported by the Office of Naval Research, Ocean Acoustics Program].
  • Corresponding author: Dr Chotiros Nicholas
    Affiliation: ONRG/ARL:UT
    Country: United States