Three-dimensional propagation and scattering around a conical seamount

Abstract

In this thesis, a numerically effcient three-dimensional propagation and scattering model is developed based on the three-dimensional coupled mode theory for axisymmetric bathymetry. The three-dimensional coupled mode approach applied in this thesis is fundamentally identical to the one applied in earlier models, such as the one presented by Taroudakis [20]. Thus, it is based on a Fourier expansion of the acoustic field around a seamount, with each azimuthal expansion coefficient being represented by a two-way coupled mode formulation. However, earlier formulations were severely limited in terms of frequency, size and geometry of the seamount, the seabed composition, and the distance between the source and the seamount, and are totally inadequate for modeling high-frequency, large-scale seamount problems. By introducing a number of changes in the numerical formulation and using a standard normal mode model (C-SNAP) for determining the fundamental modal solutions and coupling coefficients, orders of magnitude improvement in efficiency and fidelity has been achieved, allowing for realistic propagation and scattering scenarios to be modeled, including effects of seamount roughness and realistic sedimentary structure.

(cont.) Also, by the simple superposition principle, the computational requirements are made independent of the distance between the seamount and the source and receivers, and dependent only on the geometry of the seamount and the frequency of the source. First, this thesis investigates the scattering from a cylindrical island, which is the simplest case of a conical seamount problem. Our model, using the superposition method, can solve the cylindrical problem in Athanassoulis and Prospathopoulos's paper [3] with the same accuracy while saving about 4/5 computational effort. Second, this thesis demonstrates the spectral coupled mode approach, which includes a two-way coupled mode model and a superposition representation of the field. Third, this thesis applies the three-dimensional model to investigate some physics issues of three-dimensional seamount scattering. As a result of the investigation, we learn that the Nx2D model is a poor approximation of the true three-dimensional model when the three-dimensional effects are significant, though it is a good approximation of the three-dimensional model otherwise. The convergence of the model in terms of the seamount discretization is also discussed and demonstrated.

(cont.) Finally, our three-dimensional spectral coupled mode model is tested by the application of the Kermit Seamount problem with realistic ocean environmental data from the 2004 BASSEX experiment.