Grant B. Deane
Marine Phys. Lab.-0238, Scripps Inst. of Oceanogr., 9500 Gilman Dr., La Jolla, CA 92093-0238
During the last decade the traditional view that acoustic attenuation in marine sediments is linear in frequency from seismic to ultrasonic frequencies has been re-examined [A. C. Kibblewhite, J. Acoust. Soc. Am. 86, 716--738 (1989)]. In particular, there is a growing body of evidence from the ocean acoustic and marine seismology communities that the characteristics of attenuation in marine sediments are compatible with the Biot--Stoll theory for porous media. The Biot--Stoll formulation predicts a range of frequency dependencies, according to which of a number of attenuation mechanisms, including viscous dissipation, is dominant. The question of how different attenuation mechanisms can be accounted for in a fluid description of sediments will be examined. The convention of complex sound speed does not account for all the effects of absorption due to viscous dissipation when quantities involving both the pressure and velocity field are considered. An example of such a problem is the Pekeris waveguide with a lossy basement. The effect of viscous dissipation can be accounted for by adopting a convention that amounts to introducing a complex quiescent density.