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Multipath propagation in shallow water can lead to mismatch losses when single-path replicas are used for horizontal array beamforming. Matched field processing (MFP) seeks to remedy this by using full-field acoustic propagation models to predict the multipath arrival structure. Ideally, MFP can give source localization in range and depth as well as detection gains, but robustly estimating range and depth is difficult in practice. The approach described here seeks to collapse full-field replica outputs to bearing, which is robustly estimated, while retaining any signal gains provided by the full-field model. Cluster analysis is used to group together full-field replicas with similar responses. This yields a less redundant “sampled field” describing a set of representative multipath structures for each bearing. A detection algorithm is introduced that uses clustering to collapse beamformer outputs to bearing such that signal gains are retained while increases in the noise floor are minimized. Horizontal array data from SWELLEX-96 are used to demonstrate the detection benefits of sampled field as compared to single-path beamforming. [ABSTRACT FROM AUTHOR]

In theory, matched field processing offers the significant benefit of higher signal gains and increased localization capability. However, this has not been robustly observed in practice because of inherent uncertainties about details of the shallow water propagation environments which limit the prediction of the channel response. The use of guide sources to directly measure the transfer function between source and receiver arrays has been proposed as a means for reducing mismatch. However, the guide source measurement only provides a measured transfer function at the guide source location. In this paper a method of depth-shifting guide source observations is proposed, making it possible to estimate transfer functions for points in the ocean other than the guide source location. The proposed depth-shifting process does not require knowledge of environmental parameters. The theoretical background for the technique is developed below and its range of applicability is examined [ABSTRACT FROM AUTHOR]

The relationship between array design and ambiguity surface characteristics is not straightforward for matched field processing (MFP). Because MFP makes use of multipath propagation, ambiguities are a function of the environment as well as the array. This paper develops a statistical approach that seeks to provide an analytic link between array design and metrics describing the MFP output. Approximate expressions are derived for the probability distribution of power output across conventional MFP ambiguity surfaces. The validity of the expressions is examined through numerical simulation. This approach can be used as a design tool for comparing the expected performance of different array geometries. [ABSTRACT FROM AUTHOR]

A self-consistent perturbation method for three-dimensional acoustic scattering due to sound speed and density fluctuations is developed below. This method allows calculation of mean-field attenuation due to scattering, as well as second-moment statistics of the scattered field. Scattering from an inhomogeneous sediment bottom in shallow water is considered as an application. The power spectral density of the scattered field is calculated and used to study the effects of fluctuation statistics. Modal attenuation due to scattering is then calculated for several shallow-water scenarios. The scattering loss calculation is straightforward and is suitable for use with standard normal-mode codes. Numerical results show the influence of the statistical model used to represent bottom randomness and demonstrate the importance of scattering into the continuous spectrum. Scattering loss predictions are shown to agree well with a previous wave number-integration approach. © 1999 Acoustical Society of America. [ABSTRACT FROM AUTHOR]

In shallow-water environments scattering from rough surfaces and volume inhomogeneities acts to degrade sonar performance. An efficient normal mode method has been developed to model the resulting reverberation. The self-consistent perturbation theory of Schmidt and Kuperman [J. Acoust. Soc. Am. 97, 2199-2210 (1995)] has been reformulated in terms of normal modes to handle scattering from rough fluid and elastic interfaces. A similar, recently developed volume scattering theory is used to model the effects of sound speed and density fluctuations in the seabed and water column. The two theories are integrated numerically and used to contrast the different scattering mechanisms. Waveguide propagation effects are seen to be extremely important. Spatial correlation and modal cross-correlation statistics are estimated and used to study decorrelation of the sound field. Excellent agreement with the wave-number integration implementation by Schmidt and Kuperman is demonstrated for the case of rough interface scattering, with the modal approach requiring significantly less computation for low-frequency shallow-water scenarios. [ABSTRACT FROM AUTHOR]

The objective of the Trans-Arctic Propagation (TAP) experiment carried out in the spring of 1994 was to investigate the feasibility of using acoustic thermometry for inferring changes in the Arctic climate [P. Mikhalevsky, J. Acoust. Soc. Am. 94, 1760(A) (1993)]. An important issue is which acoustic property will provide the strongest climate change signature. The sensitivity of various properties of the acoustic field to changes in ice volume was investigated theoretically. A perturbation formulation for 3-D scattering from rough ice has been combined with the kraken normal-mode code to provide a model of the second-order statistics of the acoustic field. Such a hybrid model has been shown to provide excellent agreement with historical Arctic transmission loss data [LePage and Schmidt, J. Acoust. Soc. Am. 96, 1783-1795 (1994)]. We use this model to investigate the sensitivity of deterministic properties of the coherent field, such as modal loss, and modal phase and group velocity, to change in ice thickness and roughness statistics. In addition, the model has been use to investigate the sensitivity to changes in the ice cover of the higher-order statistics such as cross-modal correlation, and spatial correlation in general. Comparing these theoretical sensitivity measures, we discuss the potential performance of both deterministic and statistic acoustic thermometry in the Arctic. [ABSTRACT FROM AUTHOR]

A self-consistent perturbation method for three-dimensional acoustic scattering due to sound-speed inhomogeneities has been developed. This method allows calculation of the mean-field attenuation due to scattering, as well as the second-order statistics of the reverberant field. Propagation in a shallow ocean overlying a sediment bottom containing sound-speed inhomogeneities is considered as an application. A modal expansion of the acoustic field is assumed and (mean-field) modal attenuation coefficients are calculated. The attenuation coefficient calculation is straightforward and is suitable for use with standard normal-mode codes. Numerical results show this work is consistent with an earlier approach to plane-wave scattering [D. Tang and G. Frisk, J. Acoust. Soc. Am. 90, 2751-2756 (1991)], and illustrate the importance of scattering into the continuous spectrum. The spatial correlation and intensity of the reverberant field in the waveguide are calculated. [Work supported by ONR High Latitude Program.] [ABSTRACT FROM AUTHOR]

Rough surface scattering in ocean waveguides causes a loss of signal coherence and transfers energy between the guided modes of the waveguide. These effects are important for prediction and signal processing applications, and have been studied using techniques such as coupled mode theory. As an alternate approach, a self-consistent perturbational scattering theory developed by Kuperman and Schmidt [W. A. Kuperman and H. Schmidt, J. Acoust. Soc. Am. 57, 1511-1522 (1989)] is rewritten in terms of a normal mode representation of the pressure field. This approach is significantly less computationally intensive than the wave-number integration formulation used previously. The decay of the coherent field and statistics of the scattered field such as the cross-modal coherences can be calculated. The approach allows an arbitrary number of layers separated by rough interfaces and can be extended to elastic media as well. Numerical results are presented for a realistic shallow-water ocean environment. [Work supported by ONR.] [ABSTRACT FROM AUTHOR]