Shallow Water Acoustic Waveguide Research Articles

Random Matrix Theory for Sound Propagation in a Shallow-Water Acoustic Waveguide with Sea Bottom Roughness

The problem of sound propagation in a shallow sea with a rough sea bottom is considered. A random matrix approach for studying sound scattering by the water–bottom interface inhomogeneities is developed. This approach is based on the construction of a statistical ensemble of the propagator matrices that describe the evolution of the wavefield in the basis of normal modes. A formula for the coupling term corresponding to inter-mode transitions due to scattering by the sea bottom is derived. The Weisskopf–Wigner approximation is utilized for the coupling between waterborne and sediment modes. A model of a waveguide with the bottom roughness described by the stochastic Ornstein–Uhlenbeck process is considered as an example. Range dependencies of mode energies, modal cross coherences and scintillation indices are computed using Monte Carlo simulations. It is shown that decreasing the roughness correlation length enhances mode coupling and facilitates sound scattering.

Mode Coupling Due to a Local Inhomogeneity in a Shallow-Water Acoustic Waveguide in a Broad Frequency Band

Mode Coupling Due to a Local Inhomogeneity in a Shallow-Water Acoustic Waveguide in a Broad Frequency Band

Variability of the modal response of a shallow water acoustic waveguide in the presence of uncertain sediment properties

Variability of the modal response of a shallow water acoustic waveguide in the presence of uncertain sediment properties

An Arctic-Type Shallow-Water Acoustic Waveguide as an Information Transmission Channel for Underwater Communications

Model experiments are used to obtain the transfer functions of an Arctic-type shallow-water acoustic waveguide in the spring–summer period, including in the presence of surface waves. The calculations were performed for frequencies from 150 to 1000 Hz using the theory of coupling modes. From the numerical simulation results, the usable frequency range is determined for stable underwater communications suitable for waveguides with both acoustically soft and acoustically hard bottoms. It is demonstrated that for certain distances between the sound source and receiver and their location depths, errorless information transmission can be achieved even without special processing methods. Channel compensation with passive time reversal leads to increased transmission reliability predominantly in a waveguide with an acoustically hard bottom.

On the possibility of representing an acoustic field in shallow water as the sum of normal modes and quasimodes

Using the example of a shallow-water acoustic waveguide with a homogeneous water layer of constant thickness H lying on a homogeneous fluid absorbing half-space (bottom), we obtain estimates of distance r from a source, for which it is possible to ignore the continuous spectrum for the mode description of the depth dependence of the intensity of a low-frequency sound field in the bottom layer. We have compared two discrete representations of the field using (1) the total set of normal modes and (2) the total set of normal modes and quasimodes. It is shown that in the case when there is at least one normal mode in the channel, additional allowance for quasimodes makes it possible by an order of magnitude to approximate the boundary of applicability of mode theory and on average establish it at a level of r ~ H or less. We explain the functional dependences of the contribution of the continuous spectrum to the total field on the waveguide parameters and find the conditions of its minimization. We present examples of description of the field in the bottom, where the advantage of using quasimodes at short distances is also demonstrated.

Determining the depth of a sound source in shallow water against intense background noise

We consider a method for estimating the depth of a sound source in a shallow water acoustic waveguide for a weak signal, based on information on the ratio of the amplitude of neighboring modes of the wave field. Results of a numerical experiment using a single receiver and a horizontal linear array in the lowfrequency region are given. We demonstrate the stability of the method to errors in measuring the amplitudes of filtered modes and variations of the waveguide model, as well as high noise immunity. It is established that the error in reconstructing the depth of a source with increasing noise tends to the established value. We give a qualitative and quantitative explanation of the simulation results.

MODELING OF GREEN'S FUNCTION WITH BOTTOM REFLECTIVE PARAMETERS (P, Q) INSTEAD OF GA PARAMETERS

In shallow-water acoustic waveguide, the bottom of the sea (as the boundary condition that determines the Green's function of the sound propagation) plays an extremely important role in the sound field in water medium, especially at low frequency. Commonly the bottom is characterized by geoacoustic (GA) parameters and nearly all the existing numerical codes for sound field prediction use the GA parameters as the input of the bottom. However, the GA parameters are model based (needs a specified layered bottom model to mount the parameters in) which in practical applications may cause a series of problems such as model mismatching, extra uncertainty for field prediction and time consuming in MFP inverting. When the two bottom reflective parameters (P, Q) are used instead of GA parameters, which are model-free and directly determine the effect of the bottom on the sound field respectively in phase and strength, those problems will be solved naturally. In this paper, for range-independent waveguide using (P, Q) directly as input to construct the Green's function in terms of normal mode field is fully developed. For range-dependent waveguide, we propose an effective-equivalence based mapping between the bottom reflective parameters (P, Q) and the effective half-space (EHS) GA parameters to make (P, Q) as input parameters indirectly. Then all the existed numerical codes can still be used for the calculation of sound field without any modification except using P and Q as the input parameters of the bottom. The accuracy of this mapping is verified for both range-independent and range-dependent waveguide. At last the uncertainty analysis in GA space as well as in PQ space is briefly discussed to reveal the "extra uncertainty" due to the inter-coupling among the GA parameters.

The coherence of low-frequency sound in shallow water in the presence of internal waves

The coherence time and transverse coherence length of a low-frequency (100–300 Hz) sound field that is formed by an omnidirectional point source at a distance of 10–30 km in a shallow-water acoustic waveguide, which is characteristic of an open ocean shelf, were estimated analytically and in a numerical experiment. An anisotropic field of background internal waves is considered as a source of spatiotemporal fluctuations. It is shown that the coherence time decreases as the frequency increases, and strongly depends on the perturbation-movement direction. The transverse coherence length is primarily determined by phase incursions that are related to the cylindrical shape of the acoustic-wave front. In the case of transverse propagation, background internal waves may lead to significant variations in this length. The introduction of compensating phase corrections during processing provides a considerable increase in the average transverse coherence length.

Information-theoretic quantification of underwater acoustic source localization performance

Metrics that historically have been applied to quantify the performance of signal processing for source localization are algorithm-dependent. For example, performance of conventional beamforming or matched-field processing is usually quantified by main-peak width and secondary-peak levels of the beam response or spatial ambiguity function, while performance of Bayesian localization may be quantified by measures of the statistical dispersion of the a posteriori pdf of source location. While algorithm-dependent performance metrics permit comparisons within a given class of signal processing algorithms, they do not provide comparability across algorithm classes. The present work identifies fundamental information-theoretic quantities that can be used as metrics to quantify the source localization performance of diverse signal processing algorithms and thus provide for performance comparisons across signal-processor classes. These quantities include conditional entropy of source location given processor output, mutual information of source location and processor output, and cross-entropy of actual and posterior source-location probability distributions. Applications of these information-theoretic metrics are illustrated in examples of Bayesian localization, conventional beamforming, and matched-field processing of a time-harmonic source in a range-independent shallow-water acoustic waveguide. The results are interpreted in the light of the data processing inequality of information theory. [Work supported by ONR.]

Acoustic propagation characteristics of the estuarine salt wedge

The estuarine environment often hosts a salt wedge—a layer of denser seawater advected by the rising tide under fresh water discharged by the river. The nature of the stratification is a function of the tide's range and speed of advance, river discharge volumetric flow rate, and river mouth morphology. The competing effects of temperature and salinity on sound speed present the question: Is the salt wedge acoustically observable? Using temperature and salinity profiles collected in situ, numerical results show that the salt wedge can impact acoustic propagation. Acoustically, this environment consists of two isospeed layers separated by a thin gradient. While this three-layer very shallow water acoustic waveguide is typically dominated by high angle multipath propagation, refraction occurring in the gradient layer allows some low-angle energy from near-surface sources to be trapped in the upper layer. Acoustic fluctuations observed at an upstream or downstream receiver depend upon the interaction between the advancing and receding tide and the river discharge, which can include the presence of internal waves propagating along the top of the salt wedge.

Bayesian localization of acoustic sources with information-theoretic analysis of localization performance

Approaches investigated to date for localizing acoustic sources include conventional beamforming, matched field processing, and Bayesian methods [e.g., Pitre and Davis, J. Acoust. Soc. Am., 97, 1995], with recent research revisiting Bayesian methods with focalization and marginalization approaches [Dosso and Wilmut, J. Acoust. Soc. Am., 129, 2011]. Information-theoretic bounds on source localization performance were investigated by Meng and Buck [IEEE Trans. Sig. Proc., 58, 2010] extending earlier work of Buck. The present work investigates direct application of Bayes’ Rule to source localization and information-theoretic quantification and analysis of localization performance, taking as an example the localization of a time-harmonic source in a range-independent shallow-water acoustic waveguide. Signal propagation is represented by normal modes, and additive Gaussian ambient noise is represented by a Kuperman-Ingenito model. The localization performance is quantified by the entropy of the Bayesian posterior pdf of the source location, and an information-theoretic interpretation of this performance measure is presented. Comparisons with matched-field localization performance and extensions of the modeling and localization performance analysis to inhomogeneous media are discussed. [Work supported by ONR.]

Focusing of low-frequency sound fields in shallow water

A numerical experiment is carried out to study the focusing of a low-frequency (100–300 Hz) sound field in a shallow-water acoustic waveguide typical of an oceanic shelf. Focusing with the use of time reversal of broadband acoustic signals, which is called time reversal mirror (TRM) of waves, is considered along with focusing by phase conjugation (PC) of a monochromatic sound field. It is demonstrated that, in the case of focusing by the TRM method in the waveguide of interest, it is sufficient to have a single source-receiving element. The use of a vertical array improves the quality of focusing. The quality achieved in the latter case proves to be approximately the same as that achieved in the case of focusing by phase conjugation of a monochromatic field at a frequency identical to the carrier frequency of the broadband signals. It is also shown that, in a range-independent waveguide, intense surface waves considerably reduce the quality of focusing. This effect is most pronounced in the case of using phase conjugation.

The Pekeris waveguide revisited

The traditional Pekeris treatment of the shallow water acoustic waveguide is marred by a seeming paradox: The so-called nonorthogonality of the propagation modes. This anomaly is due to the use of mistaken orthogonality criteria, and is easily exorcised by the use of normal coordinate theory (which leads to the correct orthonormality conditions for any linear conservative system).

Shallow water single mode excitation using feedback control

This work presents an adaptive algorithm for exciting a single mode in the shallow water acoustic waveguide. Previous efforts have focused on laboratory waveguides using pre-computed weights for the source array based on precise prior knowledge of the channel [Clay and Huang, J. Acoust. Soc. Am. 67, 792–794 (1980), Gazanhes and Garnier, J. Acoust. Soc. Am. 69, 963–969 (1981)]. In contrast to these open-loop systems, the algorithm presented here uses a reference receiver array to implement a closed-loop feedback control system. The channel is characterized through an effective coupling matrix relating the source weights to the coefficients of the excited modes. The algorithm iterates between estimating the coupling matrix from the reference observations, and determining the source array weights from the estimated coupling matrix. Results from detailed simulations using actual sound-speed and bottom parameters indicate this approach allows the algorithm to adapt and compensate for environmental mismatch and some time-dependent fluctuations.