Development and evaluation of equivalent-fluid approximations for sea-bottom sound reflection

Almost all marine sediments possess enough rigidity to transmit shear waves. Shear waves are important in underwater sound propagation because compressional waves can be partially converted to shear waves or Stoneley waves at reflection boundaries. An equivalent-seabed model is an approximate method to simplify the mathematical analysis and reduce the calculational expense in modelling water-borne shallow-water sound propagation, taking seabed shear-wave effects into account. According to this method, the seabed with rigidity is treated as an equivalent fluid. However, seabed shear-wave effects are included in the adjustable parameters of the equivalent fluid. The objective of this work was to develop and evaluate equivalent-fluid seabed models. Existing equivalent-fluid seabeds have been evaluated by calculating the reflection coefficient of the bottom. Meanwhile, shear-wave effects on reflection and on the total impedance of seabeds have been studied. A new effective-seabed model is proposed from the calculation of the effective impedance of the seabed. Comparison of the new model with the existing model shows that the new model agrees better with the solid seabed at low grazing angles. Furthermore, grazing-angle-dependent parameters of the equivalent-fluid seabed are proposed.