Application of the Acoustic Propagation Model to a deep‐water cross‐shelf curtain (original) (raw)

Time-evolving acoustic propagation modeling in a complex ocean environment

2013 MTS/IEEE OCEANS - Bergen, 2013

During naval operations, sonar performance estimates often need to be computed in-situ with limited environmental information. This calls for the use of fast acoustic propagation models. Many naval operations are carried out in challenging and dynamic environments. This makes acoustic propagation and sonar performance behavior particularly complex and variable, and complicates prediction. Using data from a field experiment, we have investigated the accuracy with which acoustic propagation loss (PL) can be predicted, using only limited modeling capabilities. Environmental input parameters came from various sources that may be available in a typical naval operation.

Acoustic Propagation in Continental Shelf Break and Slope Environments

2012

The long-term goal of the research is to increase the physical understanding of acoustic propagation in continental shelf and slope environments in the 25-5000 Hz band. This includes both the physics of the seabed and the coupling to physical mechanisms in the water column in complex range-and azimuth-dependent littoral waveguides. OBJECTIVES There were two main objectives of the current research. The first objective was to complete a final numerical implementation of a statistical inference approach based on a maximum entropy formalism. The second objective was to combine the waveguide parameter statistical inferences with geophysical data from the Shallow Water 2006 (SW06) experiment on the New Jersey continental shelf to model range-dependent acoustic data for the purpose of determining the bandwidth and range over which the effects of range-dependent inhomogenities in the sub bottom layering can be discerned. APPROACH The approach applied in this work was to continue to use data obtained from the SW06 experiment to test hypotheses made for statistical inference of waveguide parameters and the effects of sub bottom seabed layering on low frequency acoustic propagation. Further, there is ongoing collaboration between Mr. Jason Sagers, my graduate student, and I on (1) identifying mode coupling effects in SW06 data and (2) performing statistical inference in environments with significant horizontal variability. The theoretical approach to statistical inference is based on maximum entropy. The main advancement made by the current work is the discovery of how to specify the average error constraint for maximization of the Shannon entropy. 1 Previously, all statistical inference methods in ocean DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

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

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.

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

IEEE Journal of Oceanic Engineering, 2010

LOAPEX: The Long-Range Ocean Acoustic Propagation EXperiment

IEEE Journal of Oceanic Engineering, 2000

This paper provides an overview of the experimental goals and methods of the Long-range Ocean Acoustic Propagation EXperiment (LOAPEX), which took place in the northeast Pacific Ocean between September 10, 2004 and October 10, 2004. This experiment was designed to address a number of unresolved issues in long-range, deep-water acoustic propagation including the effect of ocean fluctuations such as internal waves on acoustic signal coherence, and the scattering of low-frequency sound, in particular, scattering into the deep acoustic shadow zone. Broadband acoustic transmissions centered near 75 Hz were made from various depths to a pair of vertical hydrophone arrays covering 3500 m of the water column, and to several bottom-mounted horizontal line arrays distributed throughout the northeast Pacific Ocean Basin. Path lengths varied from 50 km to several megameters. Beamformed receptions on the horizontal arrays contained 10-20-ms tidal signals, in agreement with a tidal model. Fifteen consecutive receptions on one of the vertical line arrays with a source range of 3200 km showed the potential for incoherent averaging. Finally, shadow zone receptions were observed on an ocean bottom seismometer at a depth of 5000 m from a source at 3200-250-km range.

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

IEEE Journal of Oceanic Engineering, 2010

Perculiarities of sound propagation over the continental shelf in Bass Strait

Measurements of the transmission loss of airgun signals from an offshore seismic exploration survey were made in 2011 in the western part of Bass Strait, Australia, as part of an 8-month sea noise monitoring and blue whale tracking program supported by Origin Energy. The measurements were made using an array of four autonomous sea noise recorders equipped with single hydrophones and deployed on the seafloor on the continental shelf not far from the continental slope. The distances from the hydrophones to the airgun array varied from 38 km to nearly 75 km. A review of the airgun signals received on the hydrophone array revealed that their waveform and spectral characteristics were atypical of a low-frequency impulsive signal propagated in shallow water over the continental shelf. The acoustic energy in the received signals was noticeable only in three relatively narrow frequency bands below 40 Hz. Moreover, airgun shots from some seismic transects were not detected in sea noise recordings. An explanation of such peculiar sound propagation in this area of the strait is suggested in this paper based on results of numerical modelling and some supplementary data on seafloor properties.

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.

Application of the Acoustic Propagation Model to a deep‐water cross‐shelf curtain (original) (raw)

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