Underwater Acoustic Imaging Systems Research Articles

Research on Acoustic Reflection Characteristics of Typical Concave Body based on COMSOL

In the design of an underwater acoustic imaging system, to improve the signal-to-noise ratio (Snr) of the signal receiver and achieve good acoustic imaging results, it is necessary to discuss the acoustic focusing problem of the concave body. Based on COMSOL finite element analysis, this paper makes a comparative analysis of concave structures in several typical acoustic reflection modules, takes plane waves as the background sound field, analyses the acoustic reflection performance of typical concave structures at different wavelengths and angles of incidence, and studies the acoustic convergence effect under different concave structures, providing reference and help for further optimizing the design of acoustic imaging systems.

Underwater Acoustic Imaging via Online Bayesian Compressive Beamforming

Beamforming is extensively used in underwater acoustic imaging systems. As a high-resolution variety, a Bayesian compressive beamformer treats the acoustic echo of each snapshot independently, and achieves enhanced recovery performance. However, its associated computational cost is extremely high compared with conventional beamformers, and its iterative implementation makes it difficult to be used online. To overcome these obstacles, an online Bayesian compressive beamformer based on Kalman filtering (online-KSBL) is proposed in this work, which is non-iterative and computationally efficient for a continual working scenario, where a long-term imaging task is conducted underwater. Imposing independent assumptions on snapshots, the online-SBL approach can be derived from online-KSBL. The hyperparameters of the model are estimated recursively, following efficient procedures that are implemented approximately with a sawtooth lag scheme. For the underwater imaging scenario, the correlation among snapshots is exploited and inferred from online-KSBL, and it is found that online-SBL performs competitively with online-KSBL, and it is sufficient to employ online-SBL without considering the snapshot correlation, retaining low complexity, as demonstrated by experimental results.

A Generic Approach for Computation of Near-Field and Far-Field Pattern of Beamforming Acoustic Transducer Array

This paper addresses a mathematical formulation to determine the radiation performance of an acoustic beamforming array, which is valid for both near-field and far-field regions. The developed beamforming analysis is generic for application to any array configuration, with the array elements having a radiating aperture of canonical contours. The beamforming analysis presented in this paper for the acoustic transducer array is obtained by the process of vector summation of radiated fields from the elements of the acoustic transducer array with an array factor and beam steering phase factor. The initial validity of the beamforming analysis is examined by considering the far-field region as a particular case of the near-field region with the relevant distance criterion. The numerous simulation studies presented in this paper show that the proposed method produces excellent results with the distinct structure of sidelobes at a distance RNF ≥ 0.5D2/λ from the acoustic aperture of (7(1) linear and (7(7) planar array of acoustic transducers. If the distance between the transducer array and the point of observation is very low (RNF < 0.5D2/λ), simulation results indicate a less directive main beam. The two-dimensional near field beam steering method proposed in this paper finds the significant utility in many practical applications such as Underwater Acoustic Imaging (UAI) systems and Underwater Acoustic Communication (UAC).

Far-field acoustic subwavelength imaging with blind structured illumination

The blind structured illumination microscopy (blind-SIM) is an optical method to surpass the diffraction limit, a fundamental limitation to all wave-based imaging techniques. By illuminating the object with multiple structured illumination patterns, the spatial frequency mixing between the object and the illuminations converts evanescent waves to propagating waves that can be detected by sensors in the far field. In this work, we develop blind-SIM in the acoustic domain. The blind acoustic structured illuminations (ASI) are generated by randomly located scatterers with subwavelength spatial distributions. The image of the object can be reconstructed with a subwavelength resolution from multiple far-field measurements (each produced by an ASI pattern) utilizing a compressed sensing algorithm. We demonstrate subwavelength acoustic imaging resolution in both kilohertz airborne and megahertz underwater acoustic imaging systems. The developed blind-SIM method has great potential in medical imaging, non-destructive testing, and underwater acoustic imaging for improving the imaging resolution of objects far away from the sensors.

Analysis of High-Resolution Underwater Acoustic Imaging Technology in Multi-Subarray Beam Domain

Chu, P.; Zhang, L.; Huo, W.; Wang, K., and Chen, Y., 2020. Analysis of high-resolution underwater acoustic imaging technology in multi-subarray beam domain. In: Gong, D.; Zhang, M., and Liu, R. (eds.), Advances in Coastal Research: Engineering, Industry, Economy, and Sustainable Development. Journal of Coastal Research, Special Issue No. 106, pp. 518–521. Coconut Creek (Florida), ISSN 0749-0208.The development of ocean resources and the research of ocean engineering are inseparable from the acquisition and mastery of underwater scene information, and the most important way to obtain this information is underwater acoustic imaging technology. This paper is divided into two parts: theoretical research and model simulation. In the theoretical research, the principle of traditional the split-beam phase detection method and the basic principle of cross-correlation detection technology of the phase center in beam domain are analyzed. Then, the signal model is established, and the conclusion that the number of subarray elements is positively correlated with the resolution of the system is obtained. Finally, the simulation model of the seabed is established to verify the algorithm of multi-subarray beam domain seabed detection and the high-resolution underwater acoustic imaging system.

SUPPORT VECTOR MACHINE AND DECISION TREE BASED CLASSIFICATION OF SIDE-SCAN SONAR MOSAICS USING TEXTURAL FEATURES

Abstract. The diversity and heterogeneity of coastal, estuarine and stream habitats has led to them becoming a prevalent topic for study. Woody ruins are areas of potential riverbed habitat, particularly for fish. Therefore, the mapping of those areas is of interest. However, due to the limited visibility in some river systems, satellites, airborne or other camera-based systems (passive systems) cannot be used. By contrast, sidescan sonar is a popular underwater acoustic imaging system that is capable of providing high- resolution monochromatic images of the seafloor and riverbeds. Although the study of sidescan sonar imaging using supervised classification has become a prominent research subject, the use of composite texture features in machine learning classification is still limited. This study describes an investigation of the use of texture analysis and feature extraction on side-scan sonar imagery in two supervised machine learning classifications: Support Vector Machine (SVM) and Decision Tree (DT). A combination of first- order texture and second-order texture is investigated to obtain the most appropriate texture features for the image classification. SVM, using linear and Gaussian kernels along with Decision Tree classifiers, was examined using selected texture features. The results of overall accuracy and kappa coefficient revealed that SVM using a linear kernel leads to a more promising result, with 77% overall accuracy and 0.62 kappa, than SVM using either a Gaussian kernel or Decision Tree (60% and 73% overall accuracy, and 0.39 and 0.59 kappa, respectively). However, this study has demonstrated that SVM using linear and Gaussian kernels as well as a Decision Tree makes it capable of being used in side-scan sonar image classification and riverbed habitat mapping.

Underwater acoustic imaging

Underwater acoustic imaging systems are useful in providing underwater observation and inspection capabilities whenever optical systems cannot. Underwater acoustic imaging is a part of the larger field of acoustic imaging, and as such, has its own peculiarities and attributes as compared with the rest of the acoustic imaging field. In this report, the general characteristics of underwater acoustic imaging are presented. Three basic techniques of acoustic imaging that are appropriate for underwater imaging are discussed (focused, beamformed, and holographic) and examples of each of them are described. Then the limitations and promise for the future in underwater acoustic imaging are explored.

Solid-liquid compound hydroacoustic lens of low aberration

With the intention of improving the resolving power of underwater acoustic imaging systems, hydroacoustic lenses of the solid-liquid compound type were investigated with special attention to their aberrations and F number. The solid and liquid materials used were, respectively, a plastic and a silicone fluid, thus forming an interface of higher refractivity than those between a plastic and water, or a silicone and water. The ray-tracing computations of the aberration showed that the meridional and sagittal aberrations of the solid-liquid compound lens of aspherical type designed and constructed in the present research were much reduced. The results of measurements on Fraunhofer diffraction patterns showed that an F number as small as F/1.0 was realized by the solid-liquid compound lens of aspherical type, and this in turn considerably improved the least resolvable separation on the focal plane of the lens for a given sound wavelength, and the resolving power of the lens for a given sound wavelength and a given focal length.

Experimental Hydroacoustic Imaging System

An underwater acoustic imaging system is described which has been developed especially for close-range, high-resolution (on the order of millimeters) viewing in very turbid water. The system operates at nominal 2.5-MHz frequency and has a range up to 10 m in water of suspended ocean sediment concentration as high as several thousand parts per million. Larger range capability is possible with lower sediment concentration. Real-time kinescope displays are presented by the system of the acoustic field of view, which is that insonified by a high-intensity sound transmitter of total beamwidth variable up to 30° in 10° increments. The sensitive (better than 10−10 W/cm2) piezoelectric image converter is augmented by a mechanical scanning system to provide in effect a 10 000-element pickup matrix. A Plexiglas lens moved by remote-control drive is employed for focusing. System parameters and performance are discussed, and some typical hydroacoustic images are displayed.

Information Rate, Resolving Power, and High-Resolution Sonar Systems

Ultrasonic visualization systems with very high resolution can best be described using concepts similar to those developed in the study of optical imaging systems. Such systems are thus quite naturally called acoustic imaging systems. The concepts of information rate, measured in resolution elements per radian second and resolving power, which is related to the spatial frequency bandwidth of a system, are developed and used as a basis for a historical review of some of the techniques that have been applied to underwater acoustic imaging. Unfortunately, a common characteristic of most systems developed in the past is their limited success. By considering designs that will simultaneously offer high resolving power and information rate, some new techniques that may result in more successful underwater acoustic imaging systems are introduced and discussed.

Basic Problems in Underwater Acoustic Imaging

The basic elements of an underwater acoustic imaging system are presented. Each element that introduces serious problems in the design and construction of a practical system is discussed and known ways of solving the problems are outlined. Problems of specular reflection, range-definition tradeoffs, size, weight, cost, lenses, deep submergence pressure, and data rate are presented. Techniques utilized in the HYDROCON system are used as illustrations of one practical embodiment of the principles of a system. Finally, new ideas are presented to suggest areas in which techniques now under development may provide solutions to some of our current problems.