Using the Biot theory to establish a baseline geoacoustic model for seafloor sediments

By modeling the soft, fine-grained sediments of Eckernförde Bay, Germany, as porous viscoelastic materials filled with a viscous fluid, the Biot theory is used to establish a baseline geoacoustic model that is useful as a starting point for studies of scattering and other phenomena caused by the introduction of free gas into the sediment. The basic parameters of the model are derived from a wide variety of field measurements made during the Coastal Benthic Boundary Layer research program. The low permeability revealed by these measurements indicated that the overall fluid mobility would be small and therefore it would be necessary to include the effects of local fluid motion in “hidden” coordinate systems in order to model the dispersion and attenuation that is observed over a wide fre-quency range. This was accomplished by using a very general viscoelastic model (Cole–Cole model) to describe the response of the skeletal frame in a water environment. By adjusting the three free parameters of this model it was possible to match both overall attenuation and velocity in both high- and low-frequency ranges, however, because of the fact that compressional attenuation and velocity were only measured at two relatively high frequencies and shear wave attenuation determined only at low frequency, it is not clear whether the best rheological model was chosen to model the skeletal frame. This suggests that more specially designed experimental studies are desirable if the objective is to define the most general baseline model, applicable over a wide frequency range.