Conditions for the excitation of interface waves in a thin unconsolidated sediment layer

The reflection of a plane acoustic wave from the seabed is considered. Conditions for the generation of an interface wave in a thin unconsolidated sediment layer overlying a solid substrate are examined, with particular attention to a resonance excited at low frequency and near grazing incidence. Assuming that the sediment layer can be treated as a fluid, the main requirement is for the sediment–substrate impedance ratio (evaluated at grazing incidence) to be less than −1, and very high reflection loss is shown to arise when this condition is only just met. The bottom reflection loss is evaluated at resonance for realistic combinations of seabed parameters, and lower limits are placed on the substrate compressional and shear speeds required for the high losses to occur. Finally, the influence of a small non-zero sediment rigidity is considered.