Influence of surface waves on plane wave bottom reflection loss for realistic ocean sediments

Abstract

Plane-wave reflection coefficients have been computed for several cases of fluid sediments, containing a constant sound-speed gradient, overlying a homogeneous solid substrate. It is shown that for realistic sediment descriptions, a pronounced maximum in bottom loss can occur at low grazing angles (well below the shear-wave critical angle). This maxima is well defined in angle and can occur only for special combinations of sound-speed gradient, frequency, and sediment thickness. It is shown that this phenomena is due to Stoneley waves propagating in the neighborhood of the sediment-substrate interface. These surface waves can become excited only when their phase velocity equals the parallel component of the phase velocity equals the parallel component of the phase velocity of the incident plane waves. Calculations are presented showing excellent agreement between the location of the bottom loss maxima and the predictions of a simplied theory based upon the excitation of Stoneley waves. The frequency, layer thickness, and sound speed gradient dependence are all adequately accounted for.