Non-linear modeling of underwater acoustic waves propagation for multi-receiver channels

A non-linear acoustic wave propagation model has been developed to determine the effects of ocean variations in the acoustic field, and to determine the signal measured by a receiver at any distance from an omni-directional source. The model accounts for environmental conditions that include bathymetry, bottom properties, sound velocity profile and sea surface characteristics. First, a stationary estimate of the complex sound attenuation is computed as a function of frequency and location, using the parabolic equation numerical technique. For a given range, the vertical profile of the attenuation frequency spectrum is decomposed in the wave number domain. A specific Doppler shift is associated with each wave number. The space-frequency attenuation filter obtained is applied to the transmitted signal to create time-frequency selective fading. The non-linear acoustic wave propagation model has been specifically applied to the area of Port Everglades, Florida, to simulate the performance of the FAU General Purpose Acoustic Modem (FAU-GPAM). The modem operates in the 15.6 kHz to 31.9 kHz frequency band, with 192 dB of source level, and transmits Multi-Frequency-Shift-Key modulated sequences. The range of operation varied from 1 to 5 km, in 12 meters of water. The sea bottom is mainly composed of medium sand. Experimental data have been collected under sea-state 2 conditions. The performance of the acoustic communication system has been predicted using the non-linear model, the Crepeau model and experimental data. This work was sponsored by the Office of Naval Research, Code #ONR 321OM, Dr. Swean.