Analysis and modeling of broadband airgun data influenced by nonlinear internal waves

Abstract

To investigate acoustic effects of nonlinear internal waves, the two southwest tracks of the SWARM 95 experiment are considered. An airgun source produced broadband acoustic signals while a packet of large nonlinear internal waves passed between the source and two vertical linear arrays. The broadband data and its frequency range ͑10-180 Hz͒ distinguish this study from previous work. Models are developed for the internal wave environment, the geoacoustic parameters, and the airgun source signature. Parabolic equation simulations demonstrate that observed variations in intensity and wavelet time-frequency plots can be attributed to nonlinear internal waves. Empirical tests are provided of the internal wave-acoustic resonance condition that is the apparent theoretical mechanism responsible for the variations. Peaks of the effective internal wave spectrum are shown to coincide with differences in dominant acoustic wavenumbers comprising the airgun signal. The robustness of these relationships is investigated by simulations for a variety of geoacoustic and nonlinear internal wave model parameters.

Temporal sound field fluctuations in the presence of internal solitary waves in shallow water

“…3 However, the same mechanism is not adequately depicting the temporal variations observed in the NRL-VLA data, for example, Ref. 3. In this paper, the authors present theoretical estimates, supported by experimental data, stating that specific features of mode coupling are responsible for the intensity fluctuations.…”

“…This is in contrary to the case of small angle ͑ϳ5°͒, which was shown to be governed by horizontal refraction. 3 In shallow water the sound speed perturbations (or IS) are usually concentrated in the thermocline region. Therefore, most significant mode coupling takes place between adjacent modes, at some optimal frequencies opt m (Fig.…”

“…Other numerical calculations involved mode coupling. 3 However, the same mechanism is not adequately depicting the temporal variations observed in the NRL-VLA data, for example, Ref. 3.…”

“…3 This mechanism can uniquely describe the behavior of all the pulses that have passed through the IS and were received by the NRL-VLA. A pulse radiated from the airgun is a sum of separate normal modes propagating with separate group velocities.…”

Temporal and cross-range coherence of sound traveling through shallow-water nonlinear internal wave packets

“…Instead, resonance and phasing of dominant modes propagating in the wave center ͑the trough for a nonlinear wave of depression͒ were found to be of central importance. This work shows that resonance between the internal wave scale and the mode interference scale, as discussed in Frank et al ͑2004͒ and papers cited therein, must play a role, but that the mode pair or pairs responsible for the largest effects may be difficult to ascertain.…”