Acoustic propagation under tidally driven, stratified flow

Abstract

Amplitude and phase variability in acoustic fields are simulated within a canonical shelf-break ocean environment using sound speed distributions computed from hydrodynamics. The submesoscale description of the space and time varying environment is physically consistent with tidal forcing of stratified flows over variable bathymetry and includes the generation, evolution and propagation of internal tides and solibores. For selected time periods, two-dimensional acoustic transmission examples are presented for which signal gain degradation is computed between 200 and 500 Hz on vertical arrays positioned both on the shelf and beyond the shelf break. Decorrelation of the field is dominated by the phase contribution and occurs over 2-3 min, with significant recorrelation often noted for selected frequency subbands. Detection range is also determined in this frequency band. Azimuth-time variations in the acoustic field are illustrated for 100 Hz sources by extending the acoustic simulations to three spatial dimensions. The azimuthal and temporal structure of both the depth-averaged transmission loss and temporal correlation of the acoustic fields under different environmental conditions are considered. Depth-averaged transmission loss varies up to 4 dB, depending on a combination of source depth, location relative to the slope and tidally induced volumetric changes in the sound speed distribution.

Coupled Ocean–Acoustic Prediction of Transmission Loss in a Continental Shelfbreak Region: Predictive Skill, Uncertainty Quantification, and Dynamical Sensitivities

“…12(h). Our simulation results are overall in accord with observations of [26] for the South China Sea and canonical simulations of [61], even though amplitudes are different. Finally, the TL over the shelfbreak (event B) is much higher between 120 and 260 than the TL over the shelf (event A), regardless of the internal tide properties (phase, etc.).…”

Scales of time and space variability of sound fields reflecting obliquely from underwater slopes

“…In the subsequent decades the complexity of ocean environmental models used in sound propagation predictions has grown in complexity, allowing greater insight into the dominant drivers of the patterns encountered in nature. Recently created levels of complexity include so-called N×2D independent radial simulations of sound through environmental conditions drawn from data-driven regional ocean flow models (Lermusiaux et al, 2010), and time-stepped fully 3D simulations involving representative environments (Oba and Finette, 2002;Finette and Oba, 2003;Finette et al, 2007) or realistically modeled ocean environments (Duda et al, 2010;Duda et al, 2011). This study uses the latter methodology.…”

Modal Theory of Transverse Acoustic Coherence in Shallow Oceans

“…One candidate is a modification of the Garrett-Munk spectrum [50] for shallow water. While the mechanism for the generation of nonlinear internal waves by tidal flow across the continental shelf break is well established [22,51,52], the generation of diffuse shallow-water internal waves is less well understood. One source of such waves is the arrival from afar of diffuse deep-water linear internal waves that were generated in the open ocean.…”