Low Frequency Acoustic Propagation in Shallow Water Over an Elastic Bottom

Abstract eng:
The paper presents an overview of the methods applied at FORTH to calculate the acoustic field in shallow water over an elastic bottom. The acoustic wave equation in the water column for a monochromatic source and range-independent environment is solved with either an impedance boundary condition at the water bottom interface, or by assuming an effective depth in the bottom at which perfect reflection is assumed. The acoustic field is expressed as a series expansion of modes, which are associated with the eigenvalues corresponding to a local problem defined on the basis of the appropriate boundary conditions. The eigenvalues are in general complex, even when no absorption is assumed, with real eigenvalues corresponding to very hard bottoms. When a general source excitation function is assumed, the actual acoustic field is composed by inverse Fourier transform of the system transfer function (acoustic field for a monochromatic source). Numerical examples are presented to simulate propagation of sound due to low frequency sources and assess the propagation characteristics for various types of elastic bottoms from soft to hard, through a parametric analysis. The applications envisaged include studies of the propagation of sound related to earthquakes. This task is very important for the acoustic monitoring of the seismic activity in the oceanic environment by listening to the low frequency acoustic noise field. This is a novel concept for ocean observatories which include a passive acoustic modulus.

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