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

By modeling the soft, fine-grained sediments of Eckernforde Bay, Germany, a s porous viscoelastic materials filled with a viscous fluid, the Blot theor y is used to establish a baseline geoacoustic model that is useful as a sta rting point for studies of scattering and other phenomena caused by the int roduction of free gas into the sediment. The basic parameters of the model are derived from a wide variety of field measurements made during the Coast al Benthic Boundary Layer research program. The low permeability revealed b y these measurements indicated that the overall fluid mobility would be sma ll and therefore it would be necessary to include the effects of local flui d motion in "hidden" coordinate systems in order to model the dispersion an d attenuation that is observed over a wide frequency range. This was accomp lished by using a very general viscoelastic model (Cole-Cole model) to desc ribe the response of the skeletal frame in a water environment. By adjustin g the three free parameters of this model it was possible to match both ove rall attenuation and velocity in both high- and low-frequency ranges, howev er, because of the fact that compressional attenuation and velocity were on ly measured at two relatively high frequencies and shear wave attenuation d etermined only at low frequency, it is not clear whether the best rheologic al model was chosen to model the skeletal frame. This suggests that more sp ecially designed experimental studies are desirable if the objective is to define the most general baseline model, applicable over a wide frequency ra nge. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.