Acoustic Sources In Shallow Water Research Articles

On the Dispersive Characteristics of Shallow Water Seismic Waves Excited by Air Gun Sound Source in Shallow Water

In order to study the dispersion characteristics of seabed seismic waves excited by acoustic sources in shallow water, the sound field at liquid-solid interface is derived based on the wave equation firstly, which results demonstrate that the longitudinal wave and transversal wave are not dispersive, while the Scholte wave is normal dispersive. Then, the numerical simulation is carried out on the sound field at the liquid-solid interface excited by the air gun sound source based on high-order staggered grid finite difference method, which results demonstrate that the dispersion phenomenon of Scholte waves is observed and the theoretical analysis results were verified. Finally, the data of experiment in the sea are analyzed based on the transformation, and the dispersion curve of Scholte wave is finely extracted.

Shallow water source localization using a mobile short horizontal array

This paper presents an approach to the challenging issue of passive source localization in shallow water using a mobile short horizontal linear array with length less than ten meters. The short array can be conveniently placed on autonomous underwater vehicles and deployed for adaptive spatial sampling. However, the use of such small aperture passive sonar systems makes it difficult to acquire sufficient spatial gain for localizing long-range sources. To meet the requirement, a localization approach that employs matched-field based techniques that enable the short horizontal linear array is used to passively localize acoustic sources in shallow water. Furthermore, the broadband processing and inter-position processing provide robustness against ocean environmental mismatch and enhance the stability of the estimation process. The proposed approach's ability to localize acoustic sources in shallow water at different signal-to-noise ratios is examined through the synthetic test cases where the sources are located at the endfire and some other bearing of the mobile short horizontal linear array. The presented results demonstrate that the positional parameters of the estimated source build up over time as the array moves at a low speed along a straight line at a constant depth.

Passive localization using flexible hydrophone array

Localizing a radiated acoustic source in shallow water precisely is a difficult problem. To achieve this goal, a large aperture between hydrophones should be considered. But a large and rigid array is not convenient when used by ship. Here a flexible hydrophone array is designed to solve this problem. The position of each array hydrophone is randomly placed. Spherical interpolation method is used to estimate the position of acoustic source. This method need precise 3-D coordinates of each hydrophone to estimate 3-D coordinates of the source. In this paper a new method is derived to estimate the coordinates of each hydrophone. Computer simulation is conducted to analysis the performance of passive localization algorithm using the flexible array. Factors such as time-delay, hydrophone coordinates, and array shape, which affect the localization performance are discussed. Lake trial results are presented and show that the experiment results agree highly with the simulation results.

Bayesian geoacoustic inversion and source tracking for horizontal line array data

This thesis develops non-linear Bayesian methods for three-dimensional tracking of a moving acoustic source in shallow water despite environmental uncertainty, with application to data from a horizontal line array (HLA) of hydrophones. Focalization-tracking maximizes the posterior probability density (PPD) over track and environmental parameters. Synthetic test cases show that the algorithm substantially outperforms tracking with poor environmental estimates and generally obtains results close to those achieved with exact environmental knowledge. Marginalization-tracking integrates the PPD over environmental parameters to obtain joint marginal distributions over source coordinates, from which track uncertainty estimates and the most probable track are extracted. Both approaches are applied to HLA data from a shallow-water experiment conducted in the Barents Sea. Focalization-tracking successfully estimates the tracks of a towed source and a surface ship in cases where simpler tracking algorithms fail. Marginalization-tracking generally outperforms focalization-tracking and gives uncertainty estimates that encompass the true tracks. As a precursor, Bayesian geoacoustic inversion is applied (to both controlled-source and ship-noise data) to estimate seabed model parameters and their uncertainties. It is demonstrated that combining data from multiple, independent time-series segments in the inversion can significantly reduce geoacoustic parameter uncertainties. Geoacoustic uncertainties are also shown to depend on ship range and orientation.

An electrical marine vibrator with a flextensional shell

A completely new electro-mechanical marine Vibroseis concept is introduced that uses a flextensional shell with a uniquely developed form. Two controllable output resonances achieve a very high efficiency and repeatability, with a typical frequency bandwidth of 6-100 Hz. The marine vibrator can be used as a standard towed acoustic source in shallow water, or as a stationary source in transition zone environments. It is particularly significant that the vibrator requires only an electrical power supply, cannot leak hydraulic fluids, is easily transported, and is suitable for applications requiring very low environmental impact. A marine vibrator will provide several environmental advantages. Vibrator technology spreads the net source energy over a long period, reducing the acoustic power in comparison to impulsive sources. This is attractive for applications where high peak power may be problematic. This fact was established by the original hydraulic vibrators developed previously. An electrical marine vibrator offers even more advantages than hydraulic vibrators. There is no need for heavy equipment and hydraulic systems that can cause hydraulic oil spills. As the electrical vibrator requires only an electrical power supply it can be easily transported to different vessels and locations without any costly installations and potential environmental hazards. The marine vibrators discussed here have been tested as a standard towed acoustic source in shallow water, and as a stationary source in transition zone environments (4-6 ft of water). These applications are particularly well suited for this type of source.

An electrical marine vibrator with a flextensional shell: DECEMBER 2005 DRAFT

A completely new electro-mechanical marine vibroseis concept is introduced that uses a flextensional shell with a uniquely developed form. Two controllable output resonances achieve a very high efficiency and repeatability, with a typical frequency bandwidth of 6 - 100 Hz. The marine vibrator can be used as a standard towed acoustic source in shallow water or as a stationary source in transition zone environments. Of particular significance, the vibrator requires only an electrical power supply, cannot leak hydraulic fluids, is easily transported, and is suitable for applications requiring very low environmental impact.

Acoustic tomography for monitoring the Sea of Japan: a pilot experiment

A pilot experiment was conducted in the Sea of Japan (also called the East Sea) in September-October 1999, to assess the possibility of using acoustic tomographic techniques for monitoring water mass structure and dynamics. Acoustic m-sequence signals at various frequencies between 250 and 634 Hz were transmitted from bottom-mounted acoustic sources in shallow water off the coast of Vladivostok to vertical-array receiving systems deployed off the north coast of Ulleung-Do island (S. Korea), 558 km to the south. The data are analyzed for temporal correlation, time spread, and transmission loss and are interpreted in terms of a tomographic system for monitoring the East Sea.

Identification of acoustic sources in shallow water: Errors in the measured parameters of the source model

Expressions for the variance of the estimated power of an acoustic source are presented for the case of the source identification in a natural shallow-water environment. The effect of errors in the estimated parameters of a shallow-water waveguide and in the estimated coordinates of the phase center of the source on the confidence interval of the estimated directional characteristics of the source is investigated using a model signal as an example.

Matched-field tracking of acoustic sources in shallow water

A simple matched-field tracking algorithm is proposed for transit detection of acoustic sources in shallow water. Let A and B represent the end points of a track. A track correlation function, CAB, is calculated using the formula CAB= ∑f∑t{(Re ∑lγlγ̂*l) /[(∑lγlγ̂*l) (∑γ̂lγ̂*l)]}. In the equation above, the indices f, t, and l are for different frequencies, time periods, and sensor pairs, respectively, and the * denotes complex conjugate. The term γ is the short period time average of the j,k element of the covariance matrix. That is, γl=〈FjF*k〉, where Fj is the signal received at sensor j. In a similar manner, γ̂l=〈F̂jF̂*k〉, where Fj is the expected field if the acoustic source followed the track AB. Realistic simulations will be shown for detection of sound sources by a vertical line of sensors in a shallow water area with a sloping bottom.

Vertical array matched‐field/mode processing results from marginal ice zone data

Matched‐field/mode processing (MFP) has been used to successfully detect and localize submerged acoustic sources in shallow water, arctic waters, and open ocean environments. A very challenging environment for the application of these techniques is the marginal ice zone. The spatial and temporal oceanographic variability of this region makes precise acoustic modeling impossible with the modeling techniques commonly available today. Hence, successful MFP in the marginal ice zone requires the use of robust processing schemes that can perform satisfactorily in an inherently mismatched environment. In this paper, successful MFP results are reported for marginal ice zone data from a vertical, near‐surface array deployed as part of the MIZEX‐88 experiment. A robust, dimensional reduction processing scheme, the reduced maximum likelihood method [Byrne et al., J. Acoust. Soc. Am. 87, 2493–2502 (1990)] applied to a few of the low‐order modes, was used for successful localization of a 190‐dB source at a range of 9 km, with some depth error, using measured environmental parameters. The depth error was eliminated by refinements made to the environmental and array element location parameters. [This work is supported by the AEAS program.]

Range and depth estimation by line arrays in shallow water

In shallow water sound propagates in terms of normal modes. The interference between the modes can be utilized to estimate range and depth of an acoustic source in shallow water by applying high resolution power estimators to a hydrophone array. The paper presents results obtained from a theoretical study based on a normal mode model. Various aspects are covered such as depth dependence of resolution, influence of surface fluctuations, comparison of horizontal and vertical line arrays and mismatch between processing and the acoustic field. Conclusions are drawn which give some more insight in the problems.

Use of generalised resolution methods to locate sources in random dispersive media

Random dispersive media such as the shallow-water sound channel cause the far field of a point source to be random in time and space. Consequently, resolution methods such as d.f.t., `maximum-likelihood estimator? or`maximum-entropy method? are useful only to a limited extent for estimation of source parameters such as range, bearing or depth. This is because those methods are based on the assumption that the signal wavefront is coherent. The resolution methods can easily be generalised to the situation in which the desired signal is random. A shallow water sound propagation model is used to demonstrate the application of generalised power estimators to the problem of locating point acoustic sources in shallow water.