Marine sediment acoustics

Abstract

New field and laboratory data indicate that the assumption of a constant ‘‘Q,’’ or equivalently, an attenuation coefficient, α, that depends on the first power of frequency, is unacceptable in the case of nearly all marine sediments. The fluid component of marine sediments causes various forms of viscous damping that are always frequency dependent. In coarser sediments, overall motion of the fluid field relative to the skeletal frame, causes a strong frequency dependence that depends on permeability and the tortuosity of the intergranular pores. In the finer sediments, local viscous losses occur near the intergranular contacts producing a viscoelastic response similar to the ‘‘squeeze film’’ phenomenon that is well known in lubrication theory. Both kinds of viscous damping, as well as frictional losses at the grain contacts, can be included in the generalized Biot model which has been used to describe the acoustical response of marine sediments. In this paper we summarize recent field data which illustrate the wide spread of ‘‘Q’’ values that are to be expected and present some new experimental data that demonstrate both kinds of viscous damping mentioned above. Finally we use the model to estimate the response of a p wave based on laboratory measurements of torsional vibrations. A simple mathematical model is used to establish the connection between these two modes of motion.