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Estimating the sound speed profile by matching modeled acoustic intensity striations to those that are measured with a mounted horizontal line array is investigated. Due to constructive and destructive interference between normal modes, the acoustic intensity manifests a range-frequency striation structure in shallow water, which can be described by the waveguide invariant. Using environmental measurements from the SW'06 experiment; the effects of sound speed profile (SSP) fluctuation on the acoustic intensity interference pattern are simulated. The simulation shows that the changes in the waveguide invariant are determined by isotherm thickness. The acoustic data recorded in an experiment conducted in the South China Sea in June 2010 are analyzed to invert for the sound speed profiles. The sound speed profile is represented by Empirical Orthogonal Functions (EOFs) to reduce the unknown parameters. The estimates agree well with the measured sound speed profiles. Most of the standard deviations are less than 1 m/s. [ABSTRACT FROM AUTHOR]

Internal waves (IW) can significantly affect sound propagation in shallow water. When arrival times of normal modes are used for ocean tomography, IW-caused variations of the modal arrival time should be taken into account. The sensitivities of the normal mode arrival time to solitary IWs are analyzed by using the SWARM'95 environments. It is shown through theoretical analysis that acoustic modes with more energy distribution in the thermocline are more sensitive to the internal waves. The arrival times of these normal modes fluctuate greatly with thermocline depth variation. Moreover, the sensitive mode and insensitive mode can be the same mode, but with different frequencies. By contrast, the modes with smaller grazing angles and propagating mainly below the thermocline have stable arrival times. The results are meaningful for source depth location and frequency selection in shallow water tomography. [ABSTRACT FROM AUTHOR]

Matched-field tomography (MFT) has been proven to be an efficient method for retrieving the sound-speed profile (SSP) in several shallow water experiments. In this paper the effect of source depth on MFT for SSPs in shallow water is examined by using airgun signals at different depths in SWARM95 experiment. To reduce the unknown parameters, the SSPs are represented by empirical orthogonal functions (EOFs) in MFT. As the acoustic signal propagation path was nearly perpendicular to the direction of the internal wave, the SSP along the acoustic signal propagation path was almost range-independent. The inverted mean SSPs are compared with the measurements. The results show that the inversions using the sources above the thermocline obtain a better performance than those using sources below the thermocline. The explanation is that under conditions of negative velocity gradient, the shallower source excites more energy of higher-order normal modes, which have larger grazing angles and carry more information about the whole water column. [ABSTRACT FROM AUTHOR]

In a realistic application of matched field processing for source localization, the ability to localize an acoustic source is strongly affected by the uncertainty of the environment. However, accurate measurements of the environment are extremely difficult to obtain in large regions of the ocean. Two general approaches, referred to here as one-step focalization and multistep focalization, are used to overcome mismatch and accurately estimate source location with limited a priori environmental information. Focalization maximizes the posterior probability density over the unknown source and environmental parameters. One-step focalization directly estimates all parameters at a single step, while multistep focalization reduces the inversion problem to a sequence of smaller subsets that have fewer parameters to optimize at each stage. This greatly improves the efficiency of the optimization and gives the less sensitive parameters more chances to contribute to the objective function. The broadband signals recorded by a vertical line array during a Yellow Sea experiment are used to verify the validity of the approach. [ABSTRACT FROM AUTHOR]

Research on ocean reverberation has practical and scientific significance. Much progress has been made in the past three decades to improve our understanding of reverberation. However, there remain important unanswered questions and a real scarcity of high-quality basic research data sets. New progress on the reverberation modeling and the low-frequency (LF) seabed scattering characterization in shallow water (SW) requires three essential conditions: 1). A reliable reverberation model using a physics-based seabed scattering function, 2). Carefully calibrated broadband reverberation data, and 3). A ground truth about the seabed geoacoustic model. Some related work on these topics is introduced in this paper. The energy flux method for SW reverberation is briefly introduced. Integration of this method with physics-based seabed scattering models directly and intuitively results in a general expression for SW reverberation. A simple relationship between the classic scattering cross-section and the modal scattering matrix is derived. Some basic research data sets, including the reverberation level/vertical coherence as a function of time, frequency, depth/hydrophone separation and sea state, are reported. Reverberation data and model predictions are in good agreement, which results in some inversion results. The HF seabed scattering models and the energy flux method-derived reverberation model are validated using LF reverberation broadband data. [ABSTRACT FROM AUTHOR]