Consideration of fine-scale coastal oceanography and 3-D acoustics effects for the ESME sound exposure model

Results and recommendations for evaluating the effects of fine-scale oceanographic scattering and three-dimensional (3-D) acoustic propagation variability on the Effects of Sound on the Marine Environment (ESME) acoustic exposure model are presented. Pertinent acoustic scattering theory is briefly reviewed and ocean sound-speed fluctuation models are discussed. Particular attention is given to the nonlinear and linear components of the ocean internal wave field as a source of sound-speed inhomogeneities. Sound scattering through the mainly isotropic linear internal wave field is presented and new results relating to acoustic scattering by the nonlinear internal wave field in both along and across internal wave wavefront orientations are examined. In many cases, there are noteworthy fine-scale induced intensity biases and fluctuations of order 5-20 dB.     

Characteristics of acoustic propagation to the eastern vertical line array receiver during the summer 1996 New England shelfbreak PRIMER experiment

During July and August of 1996, the summer component of the New England shelfbreak front PRIMER experiment was fielded in the Mid-Atlantic Bight, at a site due south of Martha's Vineyard, MA. This study produced acoustic transmission data from a network of moored sources and receivers in conjunction with very-high-resolution oceanography measurements. This paper analyzes receptions at the northeast array receiver from two 400 Hz acoustic tomography sources, with the transmission paths going from the continental slope onto the continental shelf. These data, along with forward acoustic-propagation modeling based on moored oceanographic data, SeaSoar hydrography measurements, and bottom measurements, reveal many new and interesting aspects of acoustic propagation in a complicated slope-shelf environment. For example, one sees that both the shelfbreak front and tidally generated soliton internal wave packets produce stronger mode coupling than previously expected, leading to an interesting time-and-range-variable population of the acoustic normal modes. Additionally, the arrival time wander and the signal spread of acoustic pulses show variability that can be attributed to the presence of a frontal meander and variability in the soliton field. These and other effects are discussed in this paper, with an emphasis on creating a strong connection between the environmental measurements and the acoustic field characteristics.     

A software workbench for estimating the effects of cumulative sound exposure in marine mammals

The U.S. Navy has initiated a multidisciplinary research and development program to study the Effects of Sound on the Marine Environment (ESME). As part of this effort, the Naval Research Laboratory (NRL) is developing a software workbench for estimating the effects of cumulative sound exposure on marine mammals. This paper describes the workbench, and presents some preliminary simulation results. The workbench integrates data sets and computer models contributed by researchers in the areas of oceanography, underwater acoustic propagation, and marine mammal physiology and behavior. Models are incorporated for simulating animal movement, for estimating the received acoustic time series along an animal's track, and for predicting temporary threshold shifts (TTSs) induced by acoustic exposure. TTS predictions are shown to be highly dependent upon both the environmental variables and the movement of the marine mammals.     

Acoustic Ducting, Reflection, Refraction, and Dispersion by Curved Nonlinear Internal Waves in Shallow Water

Nonlinear internal waves in shallow water have been shown to be effective ducts of acoustic energy, through theory, numerical modeling, and experiment. To date, most work on such ducting has concentrated on rectilinear internal wave ducts or those with very slight curvature. In this paper, we examine the acoustic effects of significant curvature of these internal waves. (By significant curvature, we mean lateral deviation of the internal wave duct by more than half the spacing between internal waves over an acoustic path, giving a transition from ducting to antiducting.) We develop basic analytical models of these effects, employ fully 3-D numerical models of sound propagation and scattering, and examine simultaneous acoustical and oceanographic data from the 2006 Shallow Water Experiment (SW06). It will be seen that the effects of curvature should be evident in the mode amplitudes and arrival angles, and that observations are consistent with curvature, though with some possible ambiguity with other scattering mechanisms.      

Internal wave effects on high-frequency acoustic propagation tohorizontal arrays-experiment and implications to imaging

An experiment was carried out over a nine day period from August 18 to 27, 1996 to examine acoustic wave propagation in random media at frequencies applicable to synthetic aperture sonar. The objective was to test experimentally the hypothesized imaging effects of variations in the sound speed along two different acoustic paths as put forth by F.S. Henyey et al. (1997). The focus of this paper is on describing the experiment and carrying out an initial analysis of the data in the context of the effect of ocean internal waves on imaging resolution. The oceanography is summarized to the extent needed to discuss important aspects relative to the acoustics experiment. In the acoustics experiment transmissions at 6, 20, 75, and 129 kHz between sources and receiver arrays were carried out. Source to receiver separation was about 815 m. All sources and receivers were mounted on bottom-deployed towers and were at least 9 m off the seafloor. The analysis concentrates on the 75-kHz data acquired during one day of the experiment. The time span examined Is sufficient to examine a diurnal tidal cycle of the oceanographic conditions. The results indicate the IW phase perturbations would have a significant effect on imaging for even the most benign conditions of the experiment if no autofocusing scheme is used. Also, though autofocusing should be useful in recovering the focus for these conditions, there are conditions (e.g., for the path that has a turning point at the thermocline and during times when solibores are present), where more sophisticated compensation schemes would be needed     

2005黄海声学实验-声传播起伏

2005年夏末、秋初在黄海南部进行一次多家联合海洋声学实验研究,主要目的是研究浅海内波(线形、非线性)及其海洋温度峰面对远距离声传播的起伏效应及其声学方法监测海洋新方法研究。文章综述这几年实验数据处理的一些结果。     

Effects of internal waves and turbulence on a horizontal aperturesonar

Random variability in the water column will affect the operation of a horizontal aperture sonar. Two sources of variability in shallow water are turbulence and internal waves. In a modeling study, the effects of turbulence and internal waves on a shallow-water imaging system are compared. The operational principles of a large aperture imaging system are first reviewed. A shallow-water internal wave model is developed by modifying the Garrett-Munk model. The internal waves are assumed to dissipate and drive the small-scale turbulence. The two phenomena are predicted to have markedly different effects on a system. Turbulence has short spatial correlation scales whose primary effects will be manifested in the variance of the acoustic phase. By contrast, internal waves will have much larger scattering but also a longer correlation scale. The primary acoustic quantity of interest for internal waves is shown to be the curvature of the phase as observed along the aperture. Properties of shallow-water internal waves are shown to preclude the use of standard acoustic calculations based on the Markov approximation. Using archival environmental data, sample calculations are presented for the site of a planned August 1996 experiment     

Acoustic oceanography by remote sensing

Ocean dynamical processes exist over a wide range of temporal and spatial scales. Remotely stationed acoustic devices are being used to "sense" the interior of the oceans at previously unattainable scales. The method is similar to computer-aided tomography (CAT) scans of the brain, but the measurement difficulties are far more severe. The results of a demonstration experiment are reviewed and some directions for acoustic oceanography are discussed.     

Effects of internal waves on sound pulse propagation in the Straitsof Florida

An unexplained result of broad-band transmission experiments made more than ten years ago by DeFerrari in the Straits of Florida (center frequency ~500 Hz, bandwidth ~100 Hz, water depth ~200-m, range ~20 km) is that the measured pulse response functions failed to show the expected multipath replicas of the transmitted pulse and instead were smeared into a single broad cluster (duration ~50-~350 ms) in which the unresolved multipaths fluctuated rapidly in geophysical time (coherence time ≪12 min) leaving only a relatively stable envelope that is useful for oceanographic inversion. It is demonstrated here that the effects of internal waves on sound pulse propagation in the Straits of Florida can explain these observed results, and it is suggested that similar instabilities of acoustic multipaths due to internal waves are to be expected in other shallow-water propagation conditions. The demonstration is based on numerical simulations with the broad-band UMPE acoustic model that includes multiple forward scattering from volume inhomogeneities induced by internal wave fluctuations that are described by a broad spectrum of excitation. The simulated temporal variability, stability, and coherence of acoustic pulse arrivals are displayed on geophysical time scales from seconds to many hours and are qualitatively in agreement with the measured data in the Straits of Florida     

Acoustic Measurement of Multisubtidal Internal Modes Generated in Hiroshima Bay, Japan

A coastal acoustic tomography (CAT) experiment with seven acoustic stations was successfully carried out to measure multisubtidal internal modes, generated in Hiroshima Bay, Japan, in spite of severe data gaps due to many oyster aquaculture rafts, accompanied by an array of vertical wire strings for growing oysters and distributed widely over the bay. The travel-time data acquired along five transmission lines with better data acquisition rates in one-way or reciprocal directions are analyzed, focusing on subtidal variations of sound speed (proportional to temperature). Significant multisubtidal internal modes are observed in the period range of 1.0–4.0 h. To specify the source region of internal modes and to estimate their propagation speed and direction, the power-spectral and cross-spectral analyses are applied to the time-series data of range-averaged sound speed along the successful transmission lines. Waves of period ranges 3.8–4.2, 2.0–2.3, and 1.4–1.5 h are identified as the second, third, and fourth modes of long internal waves, respectively, and the first internal mode is attributed to the semidiurnal tide. It is suggested that the source region of internal modes is located in the narrow channel at the northeastern corner of the tomography site and the modes radiate out from the source region, satisfying the propagation parameters (period, phase speed, and propagation direction) determined by this study. The propagation parameters of the subtidal internal modes can also be measured by point temperature sensors, attached to aquaculture rafts with an appropriate arrangement. However, it should be noted that the tomographic technique can confirm temperature variations through a spatial average along each ray path.      

AN EXPERIMENTAL STUDY OF DEEP SCATTERING LAYER IN THE SOUTH CHINA SEA

Discussed in this paper is the deep scattering layer (DSL) observed during the experiment in the South China Sea in October 1983. The authors have investigated the acoustic characteristics of the DSL, such as the volume scattering strength of the water column, the thickness of the layer and the coefficient of the backscattering. Resonant scattering, presumably caused by the swimbladder of fishes, is responsible for the bulk of observed reverberation. In addition, the size of the swimbladder forming the acoustic scatter of the DSL is also estimated.     

Performance Analysis for Matched-Field Source Localization: Simulations and Experimental Results

Matched-field methods concern estimation of source locations and/or ocean environmental parameters by exploiting full wave modeling of acoustic waveguide propagation. Typical estimation performance demonstrates two fundamental limitations. First, sidelobe ambiguities dominate the estimation at low signal-to-noise ratio (SNR), leading to a threshold performance behavior. Second, most matched-field algorithms show a strong sensitivity to environmental/system mismatch, introducing biased estimates at high SNR. In this paper, some theoretical developments on matched-field performance analysis are summarized, including Bayesian performance bounds and probabilistic ambiguity analysis, both incorporating environmental/system uncertainty/mismatch. Performance analysis is then implemented for source localization in a typical shallow water environment chosen from the Shallow Water Evaluation Cell Experiments (SWellEX). The performance predictions describe the simulations of the maximum-likelihood estimator (MLE) well, including the mean-square error (MSE) in all SNR regions as well as the bias at high SNR. The threshold SNR and bias predictions are also validated through SWellEX experimental data processing. The results suggest the current environmental, acoustic, and statistical modeling has developed to such a level that the optimum theoretical matched-field performance can be achieved in a well-controlled experiment.     

Experimental matched-field localization results using a short vertical array and mid-frequency signals in shallow water

Traditionally, matched-field processing (MFP) has been used to localize low-frequency sources (e.g., <300 Hz) from their acoustic signals received on long vertical arrays. However, some sources emit acoustic signals of much higher frequency. Applying MFP to signals in the mid-frequency range (e.g., 1-4 kHz) is a very challenging problem because MFP's sensitivity to environmental parameter mismatch becomes more severe with increasing frequency. Robust MFP techniques are required to process signals in the mid-frequency range. As a practical issue, short vertical arrays are more convenient to work with than are long vertical arrays; they are easier to deploy and are less prone to large amounts of deformation. However, short vertical arrays undersample the water column, which can result in severely degraded MFP performance. In this paper, we present experimental data results for this nonconventional paradigm. Using the environmentally robust broad-band L/sub /spl infin//-norm estimator, MFP results are given using shallow-water experimental data. This data consisted of broad-band signals in the 3-4-kHz band collected on an eight-element 2.13-m vertical array. These results serve to demonstrate that good localization performance can be attained for this difficult problem. Guidelines on the appropriate use of ray and normal-mode propagation models are also presented.     

Deterministic three-dimensional analysis of long-range soundpropagation through internal-wave fields

A Munk profile and a set of propagating internal-wave modes are used to construct a three-dimensional time-varying ocean sound-speed model. Three-dimensional ray tracing is employed to simulate long-range sound propagation of a broadband acoustic signal. Methods are developed to convert three-dimensional ray-tracing results to acoustic time-domain amplitude and phase measurements. The ocean sound-speed model is defined deterministically, and the model acoustic receptions are analyzed deterministically. A single internal-wave mode that is “spatially synchronizes” to an arrival can coherently focus and defocus the acoustic energy. These internal waves can cause an arrival's amplitude fluctuation to mimic Rayleigh fading; however, the time-domain phase is stable, in contradiction to the classical Rayleigh fading environment where the received phase is uniformly distributed. For example, the received power attributed to an early arrival propagated over a 750-km range can fluctuate over 40 dB, while the time-domain phase remains within a quarter of a 75 Hz cycle. The characteristics of the time-domain phase are important for establishing coherent integration times at the receiver     

海洋声速起伏对声传播及定位精度影响分析

海洋中声速起伏导致水声信道发生变化,进而引起声线到达结构的变化,对水声传播及定位精度产生一定影响。为讨论这一效应,基于TDOA体制建立了考虑声线弯曲的水下目标无源定位模型,分析了声速起伏对水下声传播路径及传播时间的影响,进而研究了声速起伏对水下无源定位测量精度影响程度。结果表明:当水平传播距离较大时,声速剖面起伏对声传播路径及传播时间的影响更为显著;以典型四元阵为例,若基线长度为20 km,接收阵位于水下5 km处,在不考虑其它随机误差影响下,海洋声速起伏造成的声源定位误差量级在0.5 m以内。分析结果有助于更好地利用环境特征优化无源定位测量方案,可为高精度水下无源定位系统设计及精度评估提供依据。     

Refraction of Sound in a Horizontally Inhomogeneous, Time-Dependent Ocean

Measurements of the three-dimensional (3-D) structure of a sound-speed field in the ocean with the spatial and temporal resolution required for prediction of acoustic fields are extremely demanding in terms of experimental assets, and they are rarely available in practice. In this study, a simple analytic technique is developed within the ray approximation to quantify the uncertainty in acoustic travel time and propagation direction that results from an incomplete knowledge or purely statistical characterization of sound-speed variability in the horizontal plane. Variation of frequency of an acoustic wave emitted by a narrowband source due to a temporal variation of environmental parameters is considered for deterministic and random media. In a random medium with locally statistically homogeneous, time-dependent 3-D fluctuations of the sound speed, calculation of the signal frequency and bearing angle variances as well as the travel-time bias due to horizontal refraction is approximately reduced to integration of respective statistical parameters of the environmental fluctuations along a ray in a background, range-dependent, deterministic medium. The technique is applied to acoustic transmissions in a coastal ocean, where tidally generated nonlinear internal waves are the prevailing source of sound-speed fluctuations, and in a deep ocean, where the fluctuations are primarily due to spatially diffuse internal waves with the Garrett–Munk spectrum. The significance of 3-D and four-dimensional (4-D) acoustic effects in deep and shallow water is discussed.     

声学方法监测悬浮物浓度的定标技术研究

海洋中特别是河口海岸中存在着大量的悬浮物，其中，包含有内陆河流夹带的大量泥沙、海洋内浮游生物及残骸，以及大陆倾倒的大量废水废渣，造成对海洋的污染。研究悬浮物随深度剖面的变化关系，即它的时空特性，将给河口和港湾的疏浚、泥沙综合治理、沿岸工程建设、环境污染防止等方面提供可靠的理论依据和直观方便的监测手段。所以，对悬浮物浓度的定标的研究有着十分重要的现实意义。     

Temporal and azimuthal dependence of sound propagation in shallowwater with internal waves

The short time scale (minutes) and azimuthal dependence of sound wave propagation in shallow water regions due to internal waves is examined. Results from the shallow water acoustics in random media (SWARM-95) experiment are presented that reflect these dependencies. Time-dependent internal waves are modeled using the dnoidal solution to the nonlinear internal wave equations, so that the effects of both temporal and spatial variability can be assessed. A full wave parabolic equation model is used to simulate broadband acoustic propagation. It is shown that the short term temporal variability and the azimuthal dependence of the sound field are strongly correlated to the internal wave field     

An overview of SAX99: acoustic measurements

A high-frequency acoustic experiment was performed at a site 2 km from shore on the Florida Panhandle near Fort Walton Beach in water of 18-19 m depth. The goal of the experiment was, for high-frequency acoustic fields (mostly In the 10-300-kHz range), to quantify backscattering from the seafloor sediment, penetration into the sediment, and propagation within the sediment. In addition, spheres and other objects were used to gather data on acoustic detection of buried objects. The high-frequency acoustic interaction with the medium sand sediment was investigated at grazing angles both above and below the critical angle of about 30°. Detailed characterizations of the upper seafloor physical properties were made to aid in quantifying the acoustic interaction with the seafloor. Biological processes within the seabed and the water column were also investigated with the goal of understanding their impact on acoustic properties. This paper summarizes the topics that motivated the experiment, outlines the scope of the measurements done, and presents preliminary acoustics results     

Internal tide and nonlinear internal wave behavior at the continental slope in the northern south China Sea

A field program to measure acoustic propagation characteristics and physical oceanography was undertaken in April and May 2001 in the northern South China Sea. Fluctuating ocean properties were measured with 21 moorings in water of 350- to 71-m depth near the continental slope. The sea floor at the site is gradually sloped at depths less than 90 m, but the deeper area is steppy, having gradual slopes over large areas that are near critical for diurnal internal waves and steep steps between those areas that account for much of the depth change. Large-amplitude nonlinear internal gravity waves incident on the site from the east were observed to change amplitude, horizontal length scale, and energy when shoaling. Beginning as relatively narrow solitary waves of depression, these waves continued onto the shelf much broadened in horizontal scale, where they were trailed by numerous waves of elevation (alternatively described as oscillations) that first appeared in the continental slope region. Internal gravity waves of both diurnal and semidiurnal tidal frequencies (internal tides) were also observed to propagate into shallow water from deeper water, with the diurnal waves dominating. The internal tides were at times sufficiently nonlinear to break down into bores and groups of high-frequency nonlinear internal waves.