Characteristic Analysis of Underwater Acoustic Scattering Echoes in the Wavelet Transform Domain

Underwater acoustic scattering echoes have time–space structures and are aliasing in time and frequency domains. Different series of echoes properties are not identified when incident angle is unknown. This article investigates variations in target echoes of monostatic sonar to address this problem. The mother wavelet with similar structures has been proposed on the basis of preprocessing signal waveform using matched filter, and the theoretical expressions between delay factor and incident angle are derived in the wavelet domain. Analysis of simulation data and experimental results in free-field pool show that this method can effectively separate geometrical scattering components of target echoes. The time delay estimation obtained from geometrical echoes at a single angle is consistent with target geometrical features, which provides a basis for object recognition without angle information. The findings provide valuable insights for analyzing elastic scattering echoes in actual ocean environment.

Optical polarization imaging for underwater target detection with non-scatter background

For conventional optical polarization imaging of underwater target,the polarization degree of backscatter should be pre-measured by averaging the pixel intensities in the no target region of the polarization images,and the polarization property of the target is assumed to be completely depolarized.When the scattering background is unseen in the field of view or the target is polarized,conventional method is helpless in detecting the target.An improvement is to use lots of co-polarization and cross polarization detection components.We propose a polarization subtraction method to estimate depolarization property of the scattering noise and target signal.And experiment in a quartz cuvette container is performed to demonstrate the effectiveness of the proposed method.The results show that the proposed method can work without scattering background reference,and further recover the target along with smooth surface for polarization preserving response.This study promotes the development of optical polarization imaging systems in underwater environments.

An Efficient Acoustic Scattering Model Based on Target Surface Statistical Descriptors for Synthetic Aperture Sonar Systems

Acoustic scattering as the perturbation of an incident acoustic field from an arbitrary object is a critical part of the targetrecognition process in synthetic aperture sonar(SAS)systems.The complexity of scattering models strongly depends on the size and structure of the scattered surface.In accurate scattering models including numerical models,the computational cost significantly increases with the object complexity.In this paper,an efficient model is proposed to calculate the acoustic scattering from underwater objects with less computational cost and time compared with numerical models,especially in 3D space.The proposed model,called texture element method(TEM),uses statistical and structural information of the target surface texture by employing non-uniform elements described with local binary pattern(LBP)descriptors by solving the Helmholtz integral equation.The proposed model is compared with two other well-known models,one numerical and other analytical,and the results show excellent agreement between them while the proposed model requires fewer elements.This demonstrates the ability of the proposed model to work with arbitrary targets in different SAS systems with better computational time and cost,enabling the proposed model to be applied in real environment.

Pentamode-Based Coding Metasurface for Underwater Acoustic Stealth

Coding metasurface draws amounts of research interests due to its potential for achieving sophisticated functions in wave manipulation by using simple logical unit cells with out-of-phase responses. In this paper, we present a novel acoustic coding metasurface structure for underwater sound scattering reduction based on pentamode metamaterials. The metasurface is composed of two types of hexagonal pentamode unit cells with phase responses of 0 and π respectively. The units are arranged in random 1-bit coding sequence to achieve low-scattering underwater acoustic stealth effect. Full-wave simulation results are in good accordance with the theoretical expectation. The optimized arrangement resulted in the distribution of scattered underwater acoustic waves and suppression of the far field scattering coefficient over a wide range of incident angles. We show that pentamode-based coding metasurface provides an efficient scheme to achieve underwater acoustic stealth by ultrathin structures.

Experimental researches on underwater target scattering properties in non-incident direction

In the bistatic case, theoretical analysis and experimental researches on underwater acoustic scattering properties of some submarine model are made in this paper. When sourcet target and receiver have complicated triangular configuration, the relationships among target strength, incidence angle and bistatic angle are obtained. The validity of this theory is verified by theoretical calculations and tank experiments. The research results can be directly used in bistatic or multistatic underwater acoustic detection systems.

Sound scattering from underwater elastic spherical shell covered with multilayered medium approximated acoustic cloak

The anechoic performance and mechanism of underwater elastic spherical shell covered with coating are studied at low frequencies.The acoustic cloak is anisotropic material,which can be designed with homogeneous isotropic materials on the basis of effective medium approximation theory.The analytic expression of scattering acoustic field from the shell covered with multilayered medium is formulated and the scattering form function,resonance mode,acoustic field distribution are computed,the scattering characteristics and mechanism of transmission are analyzed.The results show that the direction of sound transmission inside the multilayered medium is changed,the acoustic field is deflected gradually,and the acoustic energy flux is guided around the target,which reduces the scattering intensity at low frequencies,the acoustic intensity of target＇s surface is very weak.Excepting the first resonance peak in spectrum produced by the zero order partial wave,the other resonance modes of elastic spherical shell are not excitated and the multilayered medium can suppress the resonance of the spherical shell effectively.

The resonance scattering of underwater elastic cylinders

If the frequency of the incident sound wave coincides with one of the eigenfrequencies of the underwater elastic cylinder, the corresponding eigenvibration will be excited by incident sound wave and strongly reradiate sound wave towards surronding water. It has been revealed by previous investigations that the amplitude of backscattering sound appears to be minimum at the eigenfrequencies of the underwater metallic cylinders because of the destructive interference between reradiated wave of the eigenvibration and the geometrical reflected wave from surface of the cylinders. In this paper, a new phenomenon has been revealed. The amplitude of backscattering sound appears to be maximum at the eigenfrequencies of a cylinder made from nonmetallic material in which the velocity of elastic transverse wave is less than the sound velocity in water.

Study on the optical system for the record of the pulsed acoustic field and the scattering of the cylindrical-like objects buried in underwater sand

The response of the two working mode of the optical systems, the schlieren mode and shadowgraph mode, for taking of optical photograph of the pulsed acoustic field in liquid are studied. It shows that, the response of the optical intensity on the screen to the acoustic pressure is linear for shadowgraph mode and nonlinear for the schlieren mode. Because the function of shadowgraph mode has no limit on working frequency, it is suitable for the studies of the acoustic field of laboratory model of the seabed or the buried objects at low ultrasonic range. The ultrasonic pulse scattered by the cylindrical-like objects buried in underwater sand was studied experimentally by the shadowgraph mode at low ultrasonic frequency. There are five kinds of the scattered waves from the half-buried object and three kinds of the waves from the full-buried objects were recorded. The two kinds of creeping waves (the longitudinal wave and the shear wave of the object) appear in both of the two cases.