A study of rough surface scattering in electromagnetics, acoustics, and elastodynamics applications using finite-difference time-domain simulations

The shallow water waveguide contains a number of scattering mechanisms. The air-sea surface and the sediment bottom are two important contributors to the general reverberation level. In this dissertation, we examine surface scattering using the finite-difference time-domain method via Monte Carlo simulations. The FDTD method is a full wave technique that has been widely used to model wave scattering in electromagnetics, acoustics, and elastodynamics. The first problem considered is the air-sea interface. Scattering strengths are computed for rough Dirichlet surfaces which satisfy Gaussian and Pierson-Moskowitz (PM) spectra. The Dirichlet boundary condition can be used to model either a PEC surface for electromagnetics or a pressure release surface for acoustics. A tapered plane wave illuminates the surface for incident angles of 0{dollar}spcirc{dollar}, 45{dollar}spcirc{dollar}, 70{dollar}spcirc{dollar}, and 80{dollar}spcirc{dollar} (measured from vertical). A second case is considered for the air-sea interface. The forward scattered quasi-near-field pressure is found for a single realization of a PM pressure-release surface. Small errors are more apparent for a single surface realization, and the fields near the surface have a more complex structure. Hence, this is a more numerically challenging problem. The third problem considered is the ocean bottom, which for the frequencies and material parameters considered is well approximated by a lossless fluid interface. Scattering strengths are determined for surfaces with Gaussian and modified power law spectra. A tapered plane wave is used to insonify the surface at incident angles, {dollar}thetasb{lcub}i{rcub}{dollar} = 70{dollar}spcirc{dollar} and {dollar}thetasb{lcub}i{rcub}{dollar} = 80{dollar}spcirc.{dollar} Integral equation results are used for comparison in all of the surface scattering cases, and generally agreement is excellent. The final problem considered is one component of the elastic FDTD implementation which is useful in modeling sediment bottoms for lower frequencies where shear wave effects cannot be ignored. The perfectly matched layer absorbing boundary condition (ABC) taken from the electromagnetics literature is adapted for elastic waves. Broadband and CW simulation tests show that the new ABC has excellent performance.