Acoustic Propagation in Continental Shelf Break and Slope Environments (original) (raw)
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IEEE Journal of Oceanic Engineering, 2010
In this paper, we quantify the dynamical causes and uncertainties of striking differences in acoustic transmission data collected on the shelf and shelfbreak in the northeastern Taiwan region within the context of the 2008 Quantifying, Predicting, and Exploiting Uncertainty (QPE 2008) pilot experiment. To do so, we employ our coupled oceanographic (4-D) and acoustic (Nx2-D) modeling systems with ocean data assimilation and a best-fit depth-dependent geoacoustic model. Predictions are compared to the measured acoustic data, showing skill. Using an ensemble approach, we study the sensitivity of our results to uncertainties in several factors, including geoacoustic parameters, bottom layer thickness, bathymetry, and ocean conditions. We find that the lack of signal received on the shelfbreak is due to a 20-dB increase in transmission loss (TL) caused by bottom trapping of sound energy during up-slope transmissions over the complex and deeper bathymetry. Sensitivity studies on sediment properties show larger but isotropic TL variations on the shelf and smaller but more anisotropic TL variations over the shelfbreak. Sediment sound-speed uncertainties affect the shape of the probability density functions of the TLs more than uncertainties in sediment densities and attenuations. Diverse thicknesses of sediments lead to only limited effects on the TL. The small bathymetric data uncertainty is modeled and also leads to small TL variations. We discover that the initial transport conditions in the Taiwan Strait can affect acoustic transmissions downstream more than 100 km away, especially above the shelfbreak. Simulations also reveal internal tides and we quantify their spatial and temporal effects on the ocean and acoustic fields. One type of predicted waves are semidiurnal shelfbreak internal tides propagating up-slope with wavelengths around 40-80 km, horizontal phase speeds of 0.5-1 m/s, and vertical peak-to-peak displacements of isotherms of 20-60 m. These waves lead to variations of broadband TL estimates over 5-6-km range that are more isotropic and on bearing average larger (up to 5-8-dB amplitudes) on the shelf than on the complex shelfbreak where the TL varies rapidly with bearing angles.
Global Oceans 2020: Singapore – U.S. Gulf Coast, 2020
The reliability of sonar systems in the littoral environment is greatly affected by the variability of the surrounding nonlinear ocean dynamics. This variability occurs on multiple scales in space and time, and involves multiple interacting processes, from internal tides and waves to meandering fronts, eddies, boundary layers, and strong air-sea interactions. We utilize our high-resolution MSEAS-PE ocean modeling system to hindcast the ocean physical environment off the New Jersey continental shelf for the end of June 2009, and then utilize our new MSEAS acoustic NAPE and WAPE solvers in a coupled ocean physics-acoustic modeling fashion to predict the transmission and integrated transmission losses, respectively. The coupled models are described, and their predictions are then verified against independent ocean physics observations and sound propagation measurements from several acoustic sources and receivers in the region.
Bottom Interaction in Ocean Acoustic Propagation
The long term objective here is to understand the dominant physical mechanisms responsible for propagation and scattering over distances from tens to thousands of kilometers in the deep ocean where the sound channel is not bottom limited. The specific goal is to study the role of bottom interaction and bathymetry on the stability, statistics, spatial distribution and predictability of broadband acoustic signals observed just above and on the deep seafloor (greater than the critical depth). What is the relationship between the seismic (ground motion) noise on the seafloor and the acoustic noise in the water column? What governs the trade-offs in contributions from local and distant storms and in contributions from local and distant shipping? How effective is seafloor bathymetry at stripping distant shipping noise from the ambient noise field? This project addresses "the effects of environmental variability induced by ocean internal waves, internal tides and mesoscale processes, and by bathymetric features including seamounts and ridges, on the stability, statistics, spatial distribution and predictability of broadband acoustic signals..." (quote from the Ocean Acoustics web page). Understanding long range acoustic propagation in the ocean is essential for a broad range of Navy applications such as the acoustic detection of ships and submarines at long ranges, avoiding detection of ships and submarines, long range command and communications to submerged assets, and improving understanding of the environment through which the Navy operates. The long-term objective here is to understand the dominant physical mechanisms responsible for propagation and scattering in the deep ocean where the sound channel is not bottom limited.
Modeling Propagation and Reverberation Sensitivity to Oceanographic and Seabed Variability
IEEE Journal of Oceanic Engineering, 2006
The propagation of bottom and oceanographic variability through to the variability of acoustic transmissions and reverberation is evaluated with a simple adiabatic model interacting with Gaussian distributed uncertainty in a narrow frequency band. Results show that there is significant sensitivity of time series and reverberation uncertainty to different types of environmental uncertainty. For propagation over uncertain bottoms, it is shown that it is that later part of the time series, corresponding to the highest angle energy reflecting most often off the surface and bottom, that is most sensitive to bottom uncertainty. This implies that the larger reverberation contributions from the highest grazing angles with the largest scattering strength is also the most uncertain. Conversely, it is the lowest angle arrivals which are most sensitive to uncertainty in the sound-speed profile. These behaviors are predicted analytically by the theory [K.D. LePage, in "Impact of Littoral Environmental Variability on Acoustic Predictions and Sonar Performance," Kluwer, 2002, pp. 353-360].
IEEE Journal of Oceanic Engineering, 2001
Channel temporal variability, resulting from fluctuations in oceanographic parameters, is an important issue for reliable communications in shallow-water-long-range acoustic propagation. As part of an acoustic model validation exercise, audio-band acoustic data and oceanographic data were collected from shallow waters off the West Coast of Scotland. These data have been analyzed for temporal effects. The average impulse response for this channel has been compared with simulations using a fast broad-band normal-mode propagation model. In this paper, we also introduce a novel technique for estimating and removing the bistatic reverberation contribution from the data. As propagation models do not necessarily account for reverberation, it has to be extracted from the signals when comparing measured and modeled transmission loss.
A review of recent results on ocean acoustic wave propagation in random media: basin scales
IEEE Journal of Oceanic Engineering, 1999
for an assortment of parameters of the GM model can be formulated in terms of a mixed linear/nonlinear inverse. This is a significant improvement upon a Monte Carlo approach presented in this paper which is used to infer average internal wave energies as a function of depth for the SLICE89 experiment. However, this Monte Carlo approach demonstrated, for the SLICE89 experiment, that the GM model failed to render a consistent inverse for acoustic energy which sampled the upper 100 m of the ocean. Until a new theory for the forward problem is advanced, internal wave tomography utilizing the signal from strong mode coupling can only be carried out using time-consuming Monte Carlo methods.
IEEE Journal of Oceanic Engineering, 2000
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Inferences on Seabed Acoustics in the East China Sea From Distributed Acoustic Measurements
IEEE Journal of Oceanic Engineering, 2006
Low-frequency acoustic data acquired in the central East China Sea basin at two locations are analyzed for the purpose of making inferences on seabed acoustics. Previous geophysical studies indicate that the first sediment layer is composed of a fine to medium sand. The current analysis employs octave-averaged transmission loss (TL) versus range data and pressure time series generated from explosive sources. The TL and time series data were collected in locations separated by about 65 km during the same month of the year. Both locations are near the same longitude, with water depths of 100-120 m. A linear frequency dependence of the attenuation in the 25-800 Hz band, with or without sound speed dispersion, leads to a geoacoustic solution using the TL data consistent with a soft clay, and thus inconsistent with the existing geophysical data. However, seabed representations that allow for a nonlinear frequency dependence of the attenuation, such as a Kramers-Kronig dispersion relationship, a simplified six-parameter Biot description, and an empirical frequency power law of the attenuation, all give similar values of the attenuation as a function of frequency and sediment sound speeds that are consistent with the previous geophysical studies in the area. Geoacoustic solutions obtained with the TL inversions produce reasonably good fits to the measured time series data. Inversions of the time series indicate that the sound speed at the top of the sediment is lower as compared to the values estimated from the location where the TL data were acquired. While the data have significant limitations as to the information they contain on the properties of the seabed, the analysis aids in quantifying the sensitivity of geoacoustic inversion of acoustic data in shallow water littoral regions to assumptions about the frequency dependence of attenuation and sound speed.