Low-Frequency Shallow Water Acoustics (20 to 500 Hz)

摘要

The parameters affecting shallow water acoustic behavior at low frequencies (20 to 500 Hz) have been reviewed. The depth dependence of the sediment parameters, and stratigraphic layering in depth, form the basis of geoacoustic models for which the wave equation may be solved. There is no unique approach to geoacoustic modeling. Typically, the additional information required includes in situ refraction, dispersion, and reflection requirements. Application of the Biuot theory of sediment acoustics, which uses poroviscous parameters leads to different conclusions in the frequency range of interest from those calculated using standard viscoelastic parameters alone. The Biot acoustic theory also explains successful data fits to semiempirical compressional and shear wave results. The most important sediment property is the flow permeability, which is equal to the choice of acoustic frequency in its effect. Its range of variability is so large that it is necessary to make a specific in situ determination of its magnitude for use in modeling and prediction. The permeability controls the relaxation frequency of the sediment and thus the rate of attenuation of both compressional and shear waves, their frequency dependence, and their velocity dispersion. Next in importance are the shear properties of the sediments, their related interface waves, and the skeletal frame loss. Determination of these parameters in situ and further study in the laboratory are most important for progress.