Numerical Simulation of Shallow Water Propagation using Normal Mode Theory

This paper presents a computer model of acoustic field using normal mode theory in range-independent shallow water environment. Simulation is performed in various acoustic environments while changing physical parameters such as sound speed profiles, geometrical dimensions (water-depth) and sea surface & bottom roughness. For computational speed, the model uses the Helmholtz equation solutions for phase-speed (eigenvalues) and modes (eigenfunctions). The normal modes and eigenvalues for different sound speed profiles are obtained through visual Fortran. Transmission loss is calculated by using Matlab for analyzing results. Simulation results show that many factors can affect the acoustic propagation in shallow water, including the source and receiver depths, sea surface, sound speed profile (SSP) in water, sound speed in bottom, propagation range and sound frequency. To estimate optimum frequency simulation is done to calculate incoherent transmission loss for frequency range from 15 Hz to 500 Hz at different receiver's depth. Our simulation results depict that when the source frequency is less than the cut-off frequency, the acoustic wave does not travel to long distance due to degradation of propagating modes with respect to horizontal range. The optimum frequency determination at specific observation depth will improve the performance of sonar systems.