Application of Mellin Transform in Wideband Underwater Acoustic Signal Processing

According to the features of the wideband underwater acoustic signals,an algorithm for the wideband ambiguity function is put forward based on Mellin transform.The wideband acoustic signal processing using the fast Mellin transform is also explored.The theoretical analysis and simulation results show that the algorithm has not only high computation efficiency but also good concentration in wideband ambiguity domain.It suits for the wideband underwater acoustic signal processing.

A standing wave tube method for measuring acoustic and vibration characteristics of underwater acoustic materials

A measurement method based on standing wave tube is proposed to get the acoustic and vibration characteristic parameters of underwater acoustic materials under hydrostatic pressure.A low-frequency emission transducer with pneumatic compensation is designed as the sound source for standing wave tube.Through a water column,the planar piston vibration of the transducer is coupled to the sample under the water surface.An accelerometer is installed on the backing of the tested sample to measure the vibration velocity of the sample backside.Meanwhile,a pair of hydrophones embedded in the tube wall is used to measure the sound pressure of the water column.Based on the measured data,the vibration velocity and sound pressure of the sample surface of incidence are derived.The vibration velocity transfer coefficient(decoupling coefficient)between the front and back surfaces of the sample,the input impedance,and the sound pressure reflection coefficient of the sample are then calculated.A low-frequency standing wave tube test system is constructed with a tube size of 300 mm inner diameter,600 mm outer diameter,and 3 m in height,with a frequency range of 100 Hz to 1000 Hz.The experimental studies are conducted using water layer samples and multi-layer perforated rubber samples.The results show good agreement with the simulation data,thus validating the effectiveness of the measurement method.

Method and application of wavelet shrinkage denoising based on genetic algorithm

Genetic algorithm (GA) based on wavelet transform threshold shrinkage (WTS) and translation-invariant threshold shrinkage (TIS) is introduced into the method of noise reduction, where parameters used in WTS and TIS, such as wavelet function, decomposition levels, hard or soft threshold and threshold can be selected automatically. This paper ends by comparing two noise reduction methods on the basis of their denoising performances, computation time, etc. The effectiveness of these methods in-troduced in this paper is validated by the results of analysis of the simulated and real signals.

Study on applicability of modal analysis of thin finite length cylindrical shells using wave propagation approach

Donnell’s thin shell theory and basic equations based on the wave propagation method discussed in detail here, is used to investigate the natural frequencies of thin finite length circular cylindrical shells under various boundary conditions. Mode shapes are drawn to explain the circumferential mode number n and axial mode number m, and the natural frequencies are cal-culated numerically and compared with those of FEM (finite element method) to confirm the reliability of the analytical solution. The effects of relevant parameters on natural frequencies are discussed thoroughly. It is shown that for long thin shells the method is simple, accurate and effective.

Wave mode computing method using the step-split Padé parabolic equation

Models based on a parabolic equation(PE)can accurately predict sound propagation problems in range-dependent ocean waveguides.Consequently,this method has developed rapidly in recent years.Compared with normal mode theory,PE focuses on numerical calculation,which is difficult to use in the mode domain analysis of sound propagation,such as the calculation of mode phase velocity and group velocity.To broaden the capability of PE models in analyzing the underwater sound field,a wave mode calculation method based on PE is proposed in this study.Step-split Pade PE recursive matrix equations are combined to obtain a propagation matrix.Then,the eigenvalue decomposition technique is applied to the matrix to extract sound mode eigenvalues and eigenfunctions.Numerical experiments on some typical waveguides are performed to test the accuracy and flexibility of the new method.Discussions on different orders of Padéapproximant demonstrate angle limitations in PE and the missing root problem is also discussed to prove the advantage of the new method.The PE mode method can be expanded in the future to solve smooth wave modes in ocean waveguides,including fluctuating boundaries and sound speed profiles.

Seismic and geological evidence of hidden faults in the Yinpan Reservoir area based on a dense seismic array

1.Introduction.On November 23,2017,an M_(S)5.0 earthquake occurred in Wulong,Chongqing,China,resulting in tens of casualties and considerable economic loss.Geologically,the epicenter of this earthquake was located in the transition zone from the Central Chongqing Fold Belt to the southeast,characterized by a low tectonic loading rate.

An Optical Fiber Hydrophone Using Equivalent Phase Shift Fiber Bragg Grating for Underwater Acoustic Measurement

An optical fiber hydrophone based on equivalent phase shift fiber Bragg grating （EPS-FBG） with temperature compensation package provides an improvement of sensitivity in underwater acoustic measurement at wide frequency range, from 2.SkHz to 12kHz. The acoustic pressure is transduced into elastic vibration of a circle metal disk, resulting in an intensity modulation of the reflected light wave back from fiber Bragg grating （FBG）. Experiment shows that the 500 EPS-FBG hydrophone has a minimum detectable acoustic pressure of about at 5 kHz and achieves about 18-dB improvement of acoustic pressure sensitivity compared with a regular apodized FBG hydrophone.

The parametric source method for measuring characteristics of underwater acoustic materials in a pressure vessel

A truncated broadband parametric array with a primary frequency of 500 kHz and difference frequency range of 1 kHz to 30 kHz was designed as a sound source of the underwater acoustic material measurement system.By analyzing the theoretical calculation and actual measurement results in array directivity of the truncated broadband parametric source at typical frequencies,we observed that the curves of the two results were basically consistent,which proved that the calculation model was correct.Application of bell-shaped short-duration pulse to achieve broadband measurement for characteristics of underwater acoustic materials was beneficial to reduce the effects of diffraction from the panel edges.The measurement system was established for measuring the sound pressure reflection coefficients,sound pressure transmission coefficients and absorption coefficients of the large panel sample in the pressure vessel.The size of this tank isφ4 m×12 m,the maximum hydrostatic pressure is 4.5 MPa,and the corresponding measuring frequency range is from 1 kHz to 30 kHz.The measured curves had a good agreement with theoretical curves,which verified that the parametric source measurement method was feasible.Then,the sound absorption properties of the rubber plate sample were measured under different hydrostatic pressures.The studying results could show that the parametric source measurement method had the potential application in the limited space water,such as the pressure vessel.

Traveling wave tube measurements for low-frequency properties of underwater acoustic materials

A traveling wave tube measurement technique for measurmg acoustic properties of underwater acoustic materials was developed. Water temperature and pressure environments of the ocean can be simulated in a water-filled tube, and the acoustic parameters of samples of underwater acoustic materials are measured in the range of low-frequency. A tested sample is located at central position of the tube. A pair of projectors is separately located at both ends of the tube. Using an active anechoic technique, the sound wave transmitting the tested sample is hardly reflected by the surface of secondary transducer. So the traveling sound field is built up in the tube. By separately calculating the transfer functions of every pair of double hydrophones in the sound fields from the both sides of the sample, its reflection coefficients and transmission coefficients are obtained. In the measurement system, the inside diameter of the tube is 0208 mm, the working frequency range is from 100 to 4000 Hz, the maximum pressure is 5 MPa. The reflection coefficients and transmission coefficients of a water layer and a stainless steel layer samples are measured actually and calculated theoretically. The results show that the measured values are in good agreement with the values calculated, and the measurement uncertainty is not greater than 1.5 dB.

Detection of number of sources and DOA estimation based on the combined information processing of pressure and particle velocity using acoustic vector sensor array

In order to solve the problem of DOA （direction of arrival） estimation of underwater remote targets, a novel subspace-decomposition method based on the cross covariance matrix of the pressure and the particle velocity of acoustic vector sensor arrays （AVSA） was proposed. Whereafter, using spatio-temporal virtual tapped-delay-line, a new eigenvector-based criteria of detection of number of sources and of subspace partition is also presented. The theoretical analysis shows that the new source detection and direction finding method is different from existing AVSA based DOA estimation methods using particle velocity information of acoustic vector sensor （AVS） as an independent array element. It is entirely based on the combined information processing of pressure and particle velocity, has better estimation performance than existing methods in isotropic noise field. Computer simulations with data from lake trials demonstrate, the proposed method is effective and obviously outperforms existing methods in resolution and accuracy in the case of low signal-to-noise ratio （SNR）.

Self-reciprocity calibration for spherical transducers based on acoustic reflection from the inside wall of a spherical shell

B ased on the emission of pulsed spherical acoustic waves from a spherical transducer and their reflection from the inside wall of a concentric spherical shell,the electromechanical reciprocity theorem is applied to derive a reciprocity coefficient for the calibration of a spherical transducer in a spherical acoustic field.First,a self-reciprocity calibration method using a closed spherical shell reflector is proposed to obtain the five fundamental electroacoustic parameters of the spherical transducer from a single measurement.Further,a conversion factor is given for converting the voltage sensitivity obtained by the current method to the standard sensitivity defined in plane wave fields.To determine this conversion factor,a method is developed for measuring the scattering coefficient of the transducer.Calibration of a 10-mm-radius spherical transducer was performed in the frequency range 25 kHz to 63 kHz.The results showed that the standard uncertainty of the measured voltage sensitivity was up-bounded by 0.9 dB.

Optimized simulated annealing algorithm for thinning and weighting large planar arrays

This paper proposes an optimized simulated annealing(SA) algorithm for thinning and weighting large planar arrays in 3D underwater sonar imaging systems.The optimized algorithm has been developed for use in designing a 2D planar array(a rectangular grid with a circular boundary) with a fixed side-lobe peak and a fixed current taper ratio under a narrow-band excitation.Four extensions of the SA algorithm and the procedure for the optimized SA algorithm are described.Two examples of planar arrays are used to assess the efficiency of the optimized method.The proposed method achieves a similar beam pattern performance with fewer active transducers and faster convergence ability than previous SA algorithms.

Centroid-based sifting for empirical mode decomposition

A novel sifting method based on the concept of the 'local centroids' of a signal is developed for empirical mode decomposition (EMD), with the aim of reducing the mode-mixing effect and decomposing those modes whose frequencies are within an octave. Instead of directly averaging the upper and lower envelopes, as suggested by the original EMD method, the proposed technique computes the local mean curve of a signal by interpolating a set of 'local centroids', which are integral averages over local segments between successive extrema of the signal. With the 'centroid'-based sifting, EMD is capable of separating intrinsic modes of oscillatory components with their frequency ratio ν even up to 0.8, thus greatly mitigating the effect of mode mixing and enhancing the frequency resolving power. Inspection is also made to show that the integral property of the 'centroid'-based sifting can make the decomposition more stable against noise interference.

Robust minimum-variance time-reversal beamforming for target detection and localization in uncertain environment

The target detection and localization in uncertain environment with robust time reversal （TR） technique was investigated. TR is a physical process as well as a method of signal processing. Therefore, it is natural to perform beamforming via TR, i.e., TR beamforming （TRBF）. To reduce the effects of environmental uncertainty on TR spatio-temporal focusing, transmitting TRBF with modeled instead of physical probe source was studied and robust minimum-variance receiving TRBF with diagonal loading was put forward. Both of them were applied to the detection and distance estimation of the target, which was dealt with in a waveguide experiment. The experimental results show the validity of the methods in uncertain environment.

A unified algorithm for target detection and tracing based on data of array sensors

A unified method for target detection and tracing based on data from sensors of array is presented in order to improve detection and tracking abilities of the weak targets with low signal-to-noise ratio. Assuming that the multiple targets are uncorrelated each other and the number of the targets is known a priori, the status of the targets can be estimated with the maximum a-posteriori （MAP） method directly through the sensors data. The proposed method is different from the classical method, by which it can detect and track targets simultaneously by adding the target＇s signal energy information besides its direction of arrivM（DOA） information. Simulated and sea trial data results show that the detection and tracing capabilities of weak targets can be improved and wrong tracing and missing tracing problems, which exist in the classical tracing method when it is faced with the crossing targets, can be resolved by the proposed method.

Wideband imaging of extended targets with the decomposition of the time reversal operator

The weighted wideband imaging approach of full signal subspaces is proposed based on the decomposition of the time reversal operator(DORT). Although each singular vector of nonzero singular values does not correspond to one of the extended targets any more, the conventional approach of selective time reversal focusing still chooses one of the signal subspaces for imaging. Simultaneously, the time-reversal MUSIC imaging is carried out at a single frequency for wideband signal. The imaging of both methods has a high background fluctuation.In order to overcome these drawbacks, the number of signal subspaces is determined by the singular values of the time reversal operator, and then DORT imaging of full signal subspaces and entire bandwidth is achieved using the generalized reflectivity coefficients as the weighted parameters. The experimental result shows that the sidelobe level of this approach is extremely decreased.

Time reversal imaging of suspended target in strong reverberation

The weighted wideband time-reversal imaging approach of full signal subspaces was proposed.The extended target is modeled as an infinite number of independent point-like scatterers,and the imaging is constructed by accumulating all time-reversal images over all signal subspaces and the entire bandwidth in the region of suspected targets.On the bottom of a laboratory water waveguide,a cylindrical shell was used to produce reverberation.Two experiments were carried out.The first experiment is that an extended target was suspended near the cylindrical shell,whose echo and the reverberation reflected by the cylindrical shell were in different time windows.The second experiment is that a point-like target was suspended above the cylindrical shell,thus the echo and the reverberation were in the same time window.The experimental results show that the imaging quality of the proposed method is better than that of the conventional time reversal imaging methods for weak suspended targets in a strong reverberation background.