Modeling nonlinear internal wave effects on broadband shallow water acoustics

The 1995 SWARM experiment collected high quality environmental and acoustic data to investigate how nonlinear internal waves affect acoustic signals. In the southwest portion of SWARM, an airgun source produced broadband acoustic pulses which were received by two vertical linear arrays. A packet of large nonlinear internal waves passed between the source and arrays during these transmissions. The frequency range (10–180 Hz) of the data distinguishes this study from others involving effects of nonlinear internal waves.; A notable feature of the experiment is how the nonlinear internal waves crossed the acoustic tracks at two different incidence angles of approximately &thetas; ₁ = 52° and &thetas;₂ = 85°. Models are developed for the internal wave environment, geoacoustic parameters, and airgun source signature for the 91 acoustic track. Parabolic equation (PE) simulations demonstrate observed time-frequency and intensity variations can be attributed to nonlinear internal waves. Moreover, an empirical test is provided of the internal wave-acoustic resonance condition using internal wave model parameters and acoustic wavenumber estimates. The incidence angle &thetas;₂ is below the 11° total internal reflection angle for an interface. This suggests acoustic horizontal refraction occurs as nonlinear internal waves pass the source. Three-dimensional adiabatic mode PE simulations are used to reproduce acoustic variations observed in the data. The match between data and simulation provides strong evidence of acoustic horizontal refraction due to internal waves. Current literature contains little, if any, such data.; The sensitivity of acoustic variations to geoacoustic and nonlinear internal wave model parameters is investigated. Dependence of nonlinear internal wave packet spectra on packet width and spacing is examined. Variations in dominant geoacoustic features of SWARM change acoustic horizontal wavenumbers and these effects are investigated. PE simulations are used to demonstrate and predict the occurrence of resonant interactions. Results support the use of the internal waveacoustic resonance condition in a deterministic setting.