Shengchun Piao

Space-frequency distribution of the vector field of broad-band sound in shallow water

An uniform distribution often appears in the space-frequency plane for broad-band, low-frequency sound intensity generated by a moving source in a shallow water environment. Waveguide invariant theory is used to interpret this phenomenon, and it has been applied for inverse problems in underwater acoustics. A vector sensor has advantages in providing both pressure and particle velocity information about the sound field simultaneously, and a higher signal-to-noise ratio signal than a traditional hydrophone. In the present paper, the space-frequency distribution of broad-band vector acoustic signals in a Pekeris waveguide is investigated using normal mode theory. A comparison between experimental and predicted space-frequency distributions is made for the Yellow Shark environment. The effects of the sound speed profile of the water column, the existence of a soft sediment layer, the change of sound speed or thickness of the soft sediment on the space-frequency distribution are examined. The possibility of using space-frequency distributions to invert for environmental parameters is discussed.

An Improved Hilbert-Huang Transform and its Application in Underwater Acoustic Signal Detection

The Hilbert-Huang transform (HHT) has the advantage in dealing with non-stationary signals, for its data driving. However there are some limitations of the original HHT. For getting more benefits in frequency resolution, an improved HHT is applied in this paper. In the improved version, the instantaneous harmonic retrieval method is applied to calculate the instantaneous frequencies of the intrinsic mode functions, which overcomes the limitation of the Hilbert transform. The simulation result shows, with the improved HHT, we can get more benefits in frequency resolution than the original HHT. At the same time the experimental data prove the improved version is effective for underwater acoustic signal detection.

Investigation of Interference Phenomena of Broadband Acoustic Vector Signals in Shallow Water

Although the ocean environment in shallow water is very complex, there still exists stable interference pattern for broadband low frequency sound propagation. The waveguide invariant concept is introduced to describe the broadband interference structure of the acoustic pressure field in a waveguide and now it is widely used in underwater acoustic signal processing. Acoustic vector sensor can measure the particle velocity in the ocean and provides more information for the underwater sound field. In this paper, the interference phenomena of broadband vector acoustic signals in shallow water are investigated by numerical simulation. Energy spatial‐frequency distributions are shown for energy flux density vector obtained by combination of pressure and particle velocity signals and they are analyzed according to normal mode theory. Comparisons of the interference structure between the scale acoustic field and vector acoustic field also have been made. The waveguide invariant concept is extended to describe the...

The classification of underwater acoustic target signals based on wave structure and support vector machine

The sound of propeller is a remarkable feature of ship-radiated noise, the loudness and timbre of which are usually applied to identify types of ships. Since the information of loudness and timbre is indicated in the wave structure of time series, the feature of wave structure can be extracted to classify types of various underwater acoustic targets. In this paper, the method of feature vector extraction of underwater acoustic signals based on wave structure is studied. The nine-dimension features are constructed via signal statistical characteristics of zero-crossing wavelength, peek-to-peek amplitude, zero-crossing wavelength difference, and wave train areas. And then, the support vector machine (SVM) is applied as a classifier for two kinds of underwater acoustic target signals. The kernel function is set radial basis function (RBF). By properly setting the penalty factor and parameter of RBF, the recognition rate reaches over 89.5%, respectively. The sea-test data shows the validity of target recognit...

This Submission is for Special Issue on Underwater Acoustics: Perfectly Matched Layer Technique for Parabolic Equation Models in Ocean Acoustics

In ocean waveguides, the ocean bottom is usually approximated as a half-space. Thus, there exist no reflection waves at the half-space bottom and condition of radiation at infinity should be satisfied. In numerical solutions like parabolic equation methods, the depth domain has to be truncated, which can generate reflection waves from the truncated ocean bottom. To reduce the effect of reflection waves and to simulate an unbounded ocean bottom accurately, an artificial absorbing layer (ABL) was used. As was demonstrated, an ABL meets well the demand of accuracy in sound field calculation. However, both the sea-bottom layer and the artificial absorbing layer are needed to be set quite thick by using an ABL technique. Fortunately, a PML with several wavelengths can keep similar calculation accuracy with an ABL with dozens of wavelengths. In this paper, perfectly matched layer (PML) techniques for three parabolic equation (PE) models RAM, RAMS and a three-dimensional PE model in underwater acoustics are presented. A key technique of PML “complex coordinate stretching” is used to truncate unbounded domains and to simulate infinity radiation conditions instead of the ABL in those models. The numerical results illustrate that the PML technique is of higher efficiency than the ABL technique at truncating the infinity domain with minimal spurious reflections in PE models.

The horizontal correlation of long range bottom reverberation in shallow water with inclined sea floor

The performance of active sonar system is seriously influenced by bottom reverberation in shallow water waveguide. It is important to understand the horizontal correlation of bottom reverberation for active towed-array processing techniques in shallow sea. However, little work had been done for the research on horizontal correlation of distant bottom reverberation. In this paper, a coupled mode reverberation model was applied for the horizontal correlation, and it was investigated as a function of receiving position, time and frequency. Calculations show that transverse correlation is greater than the longitudinal correlation in horizontal space for distant bottom reverberation. The adiabatic mode solution is introduced to derive the mathematic mode for horizontal correlation in the range-dependent waveguide with varying depth and the numerical results indicate that the influence of inclined sea floor on the horizontal correlation should be considered.

Two new DOA estimation methods based on orthogonal joint diagonalization

Two new DOA (Direction Of Arrival) estimation methods based on orthogonal joint diagonalization using Jacobi rotation are proposed. Through processing spatio-temporal correlation matrices and high-order cumulant matrices by the new technique, the spatial spectrum can be redefined by the joint diagonalization matrix and a set of diagonal matrices. At the same time, these methods are also justified to be effective to process the coherent sources. Finally, the simulated and experimental results justify the proposed methods to be useful methods to DOA estimation and show that they have higher angle resolution, lower RMSE (Root Mean Squared Error), and better robustness than the conventional DOA estimation methods.

Combination of WVD with TRM on detection of LFM signal in inhomogeneous medium

On the basis of time-reversal focusing theory, time frequency analysis combining with time reversal mirror (TRM) on detection of linear frequency modulation (LFM) signal in inhomogeneous medium is studied. The simulation experiment on time frequency analysis is carried out in a lab water tank. Time reversal is manifested by taking the time-domain scattered fields received on an array, reversing them, and then propagating them back into the same medium. The re-transmitted signal is received near the source and Wigner-Ville Distribution(WVD) analysis is performed. The time varying spectrum of time frequency analysis gives more useful information to recognize different types of target signals. The simulation results show that the new method can suppress multi-scattering. The approach is effective and feasible.

Analysis of Surface Sound Duct in the Northern Shelf of the South China Sea

The northern shelf of the South China Sea (NSSCS) is characterized by surface low-salinity water due to discharge from the Pearl River. In such an environment, the surface sound duct (SSD) is the most important duct for near-surface sonar applications. Nevertheless, the mechanism of SSD formation is very complicated and is influenced by salinity, temperature at the air-sea interface, and various additional marine phenomena. In this study, an 8-year conductivity-temperature-depth (CTD) profile of the NSSCS was used to analyze the SSD formation. An advanced diagrammatic method is proposed to provide a quantitative analysis of the contribution of salinity, temperature, and hydrostatic pressure on SSD formation. Large salinity gradient (0.25 psu/m) was shown to play a crucial role in SSD formation when a mixed layer exists. As representative examples, the sea under cold surges, typhoon genesis, and low-salinity lenses were studied. Conversely, the absence of SSDs in low-salinity water was also observed in upwelling regions. This study further showed that highly negative temperature gradients affect SSD formation even in low-salinity water. Furthermore, although the duct depth of a low-salinity SSD is usually less than 10 meters, it still can serve as an effective duct for acoustic propagation.

Time delay estimation based on cross power spectrum with Doppler compensation

Time delay estimation is an important topic in underwater acoustic field. However, the Doppler phenomenon caused by relative movement between the source and receivers can arise errors in time delay estimation. In order to deal with this problem, a time delay estimation method based on cross power spectrum with Doppler compensation in frequency domain is proposed in this paper. The proposed method separates the Doppler compensation from the time delay estimation rather than estimates them jointly. The results of simulations and experimental data processing show that this method can improve the estimation accuracy of time delay with the influence of Doppler phenomenon.