Approximations for acoustic reflection from solid seabeds
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Much earlier work in underwater acoustics ignored the presence of shear waves in the seabed and treated the seabed as fluid. However, many seabeds can support low speed shear waves. The presence of low speed shear waves increases both the bottom reflection phase shift and the bottom reflection loss. These two shear wave effects on the waterborne sound propagation can be accounted for by several simple approximations. An "equivalent fluid" with real parameters can be used to approximate the solid if the shear wave speed is less than about 600 m/s. The real equivalent fluid has a reduced density and an increased attenuation. The reduction in density accounts for the increase in reflection phase shift; the increase in attenuation accounts for the increase in reflection loss. The shear wave effects can also be modelled by an equivalent fluid of complex density. The complex density is obtained by multiplying the bottom density by a pre-calculated complex number. The real part of the complex number accounts for the increase in reflection phase shift and the imaginary part accounts for the increase in reflection loss. The approximation by an equivalent fluid of complex density is more accurate than the approximation by the real equivalent fluid and is applicable for shear wave speeds up to about 1000 m/s. Bottom reflections can also be simulated by a perfect reflector placed at a "complex effective depth". The real part of the complex effective depth simulates the reflection phase shift and the imaginary part simulates the reflection loss. The acoustic field at long ranges is dominated by reflections at low grazing angles and is well reproduced by all three approximations. In all three approximations algebraic expressions are given for the various parameters. In a waveguide, matching the complex effective depth for each mode by a few iteration provides a fast and easy to use method of finding the exact eigenvalues for both trapped and leaky modes. Because of the reflection loss associated with shear waves, all the modes, except the interface wave, are leaky for a solid seabed with shear wave speeds less than the sound speed in the water. Leaky modes have complex eigenvalues and thay attenuate with range. Interface waves are not formed by the reinforcement of multiple reflections at real angles and therefore are not accounted for by our approximations.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1993