A coupled local mode approach to laterally heterogeneous anisotropic media, volume scattering, and T-wave excitation.

摘要

This dissertation presents theoretical and numerical results for the coupled local mode formalism applied to the seismo-acoustic wavefield in generally anisotropic range-dependent media. General anisotropy affects the form of the elastic stiffness tensor, which directly affects the polarization of the local modes, the frequency and angular dispersion curves, the coupling and scattering of the local modes in range-dependent media, and also introduces the effects of nearly degenerate modes. The effects of anisotropy and the combination of anisotropy and lateral heterogeneity are examined for 1-D and 2-D models, respectively. Horizontally polarized shear motion is determined to play an important role in seismo-acoustic wave propagation, where modes have significant bottom interaction with anisotropic sediments at low frequencies. The discrete modes for tilted anisotropy are best described as quasi-P-SV, quasi-SH, and generalized P-SV-SH modes with particle motions in all three Cartesian coordinate directions.; Lateral heterogeneity is introduced through interface and volume terms. The relative significance of deterministic and stochastic effects from the interface and volume scattering terms are considered. New stochastic volume scattering terms are derived by applying perturbation theory to the elastic equations of motion and boundary conditions. Anisotropy enhances modal scattering, which leads to the loss of signal coherence, and apparent energy loss if not properly accounted for.; Coupled-mode scattering theory is applied to the T-wave excitation problem. Modal scattering from lateral heterogeneity along the seabottom is shown to convert energy from the directly excited hybrid crustal-acoustic modes to propagating acoustic modes. Both refraction from a sloping seafloor and seafloor scattering act as T-wave generation mechanisms, with each mechanism entering the modal scattering theory as separate terms. In addition, fault type is strongly correlated with T-wave excitation efficiency, while low shear speed sediment cover enhances T-wave generation. The discrete modes contribute to the majority of the seismic source field for shallow sources, and the continuum spectrum becomes increasingly important with greater source depth.