Most ocean acoustic propagation was adequately presented using two-dimensional (2D) models. A notable exception is the propagation around seamounts, where the three-dimensional (3D) effects become significant. A variety of 3D models, either analytic or numerical, have been developed during the past decades for addressing this problem. However, all these methods are feasible only at some conditions.

To provide a more general modeling framework, LUO Wenyu of State Key Laboratory of Acoustics, Institute of Acoustics, Chinese Academy of Sciences and SCHMIDT Henrik of Department of Mechanical Engineering, Massachusetts Institute of Technology developed a 3D propagation and scattering model for the field generated by an offset acoustic source in an ocean with axisymmetric bathymetry.

Based on the same theoretical foundation as the formulation presented by Taroudakis ( M I 1996 J. Comput. Acoust. 4 101), the researchers combine a spectral decomposition in azimuth with a coupled-mode theory for two-way, range-dependent propagation. The approach applies a number of modifications to the numerical formulation, leading to orders of magnitude in numerical efficiency for realistic problems. Further, by using a standard normal-mode model for determining the fundamental modal solutions and coupling matrices, and by applying a 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. As a result, realistic propagation and scattering scenarios can then be modeled, including effects of seamount roughness and realistic sedimentary structure.

In the study, it is shown that strong mode coupling occurs at the edge of a conical seamount for the incident normal modes with significant amplitudes below the top of the seamount. Therefore, mode coupling is critical for the investigation of the acoustic field around a seamount. In addition, the researchers suggest the use of random discretization for representing smoothly varying bathymetry. For the use of uniform discretization, when the horizontal step size is greater than half of the wavelength, artificial diffraction lobes appear due to coherent backscatter. However, by using the random discretization scheme instead, such artificial diffraction lobes are diffused, resulting in a faster convergence rate.

This research result was published on the recently issued journal of CHIN. PHYS. LETT. (Vol. 27, No. 11, 2010).