High-resolution beamforming by the Wigner-Ville distribution method

The Wigner-Ville distribution (WVD) function was originally proposed by Wigner in quantum mechanics and Ville applied it for signal analysis. This method has made it possible to represent a signal's power density spectrum in the time-frequency domain as a natural extension of the Fourier transform method (FTM). Recently, it has attracted great interest for its validity to analyze time-varying signals accomplished by the development of high-speed digital signal processing, and it is used for analyzing nonstationary signals. Conventionally, a sonar beamformer is constructed using delay lines, but the development of the high-speed processor has made it possible to apply the FTM for sonar beamforming. However, the bearing resolution of the beamformer is not enough for discriminating small underwater objects on the sea bottom by this method. To solve this problem, we aim to apply the WVD method, which can represent finer structure of signals as a natural extension of the FTM, for sonar beamforming to obtain sharper beam patterns than those of the beamforming method by FTM. Simulation results by computational calculations to clarify the resolution by the WVD method, which is presented in this paper, becomes approximately twice as high as by conventional FTM. The results of an experiment at sea also show the performance of this method     

适用于U型阵多波束系统的高分辨底检测算法

在多波束回声声纳系统中,高分辨处理算法例如MUSIC、ESPRIT,被广泛应用于海底地形的测绘。在应用高分辨算法时,一条均匀线阵是必要条件。然而,由于系统覆盖范围/分辨率的需求以及安装空间的限制,在多波束系统中经常会采用特殊形状的接收阵列,这使得高分辨算法无法直接应用。同时回波信号的短时平稳特性使得难以估计出协方差矩阵,这也增加了高分辨算法在多波束系统中的应用难度。本文首先介绍一种基于多角度子阵波束形成的ESPRIT算法,该算法能降低高分辨算法对信噪比、样本点数和计算能力的要求。仿真表明此算法能提供更好的分辨力。接着提出一种将基于多角度子阵波束形成的ESPRIT算法与虚拟阵列变换相结合的高分辨底检测算法,并针对高分辨底检测算法在U型阵上的应用进行了探讨。计算机仿真和试验数据处理结果验证了文章所提高分辨底检测算法的有效性。     

SAMI: a low-frequency prototype for mapping and imaging of theseabed by means of synthetic aperture

The objective of the Synthetic Aperture Mapping and Imaging (SAMI) project was to develop and to test at sea a wide-band synthetic aperture sonar prototype, capable of providing high-resolution seafloor images together with bathymetry maps. This system used the motion of a physically small array in order to synthesize a longer array, providing images with an across-track resolution independent of both range and transmit frequency. Such systems are clearly very relevant to the high-precision long-range (low-frequency) imaging of the sea bottom. The project has led to the construction of a prototype tested at sea on several well-known areas for comparison with existing images and maps. These areas included several types of sea bottom, depths, and geological structures. The results obtained in real time, on-board ship, have shown the relevance of the proposed wide-band techniques. The many profiles produced have provided high-resolution images and maps of various seafloors. Interpretation by geologists showed that the system was capable of providing the same or finer detail than a deep-sea short-range, high-frequency system and maintained a higher resolution over a wider swath. The sea data processed have shown that the system provided maps with a cubic meter voxel. The resolution cell is constant over the whole range (50 to 2500 m) thanks to the dynamic focusing of the synthetic aperture. Postprocessing of a part of the data stored during the experiments has been carried out in the laboratory. This work has shown that techniques such as autofocusing can give an increase in resolution (i.e., gain in contrast and resolution of about 3 dB). The results displayed in the paper show the relevance of the techniques developed to the provision of a complete high-performance imaging tool for the oceanographic community     

利用MVDR算法削弱多波束测深声纳的隧道效应

分析了海底多波束测深声纳中存在的隧道效应及其产生机理,指出旁瓣干扰是引起隧道效应的重要因素,隧道效应的出现导致多波束测深声纳把相对平坦的海底误测成凹面向上的水平半圆柱面海底地形。研究了基于GSC结构的自适应波束形成算法,推导提出了MVDR算法的连续自适应实现方案,并利用该算法对多波束测深声纳湖试数据中存在的隧道效应进行处理,结果表明该算法能够有效削弱多波束测深数据边缘波束中存在的旁瓣干扰。     

Seafloor acoustic remote sensing with multibeam echo-sounders and bathymetric sidescan sonar systems

This paper examines the potential for remote classification of seafloor terrains using a combination of quantitative acoustic backscatter measurements and high resolution bathymetry derived from two classes of sonar systems currently used by the marine research community: multibeam echo-sounders and bathymetric sidescans sonar systems. The high-resolution bathymetry is important, not only to determine the topography of the area surveyed, but to provide accurate bottom slope corrections needed to convert the arrival angles of the seafloor echoes received by the sonars into true angles of incidence. An angular dependence of seafloor acoustic backscatter can then be derived for each region surveyed, making it possible to construct maps of acoustic backscattering strength in geographic coordinates over the areas of interest. Such maps, when combined with the high-resolution bathymetric maps normally compiled from the data output by the above sonar systems, could be very effective tools to quantify bottom types on a regional basis, and to develop automatic seafloor classification routines.     

基于CAATI技术的条带测深/侧扫声纳系统C3D-LPM

侧扫声纳及多波束测深仪是应用非常广泛的海底地形地貌调查仪器。通过对侧扫声纳和多波束测深优缺点的对比分析,介绍了基于CAATI技术将高分辨率的侧扫图像和高质量的测深数据完美结合,生成高质量的三维图像的条带测深/侧扫声纳系统C3D-LPM,特别介绍了条带测深/侧扫声纳系统C3D-LPM的构造和技术规格。仪器实地使用结果表明,该仪器完全能够满足海洋工程和研究的需求,并具有其独特的技术特点和使用优势。     

多波束与侧扫声纳海底目标探测的比较分析

侧扫声纳是目前常用的海底目标(如沉船、水雷、管线等)探测工具,在测深领域,多波束以全覆盖和高效率证明了它的优越性。由于多波束具有很高的分辨率,目前在工程上已经开始应用多波束进行海底目标物的探测。对多波束和侧扫声纳进行了比较分析,并着重探讨了影响多波束分辨率的各种因素。结果表明:多波束的最大优点在于定位精度高,但其适用范围不如侧扫声纳广泛,尤其受到水深和波束角的限制,多波束和侧扫声纳在探测海底目标时具有很好的互补性,同时应用可以提高目标解译的准确性。     

Automatic Registration of TOBI Side-Scan Sonar and Multi-Beam Bathymetry Images for Improved Data Fusion

Deep towed side-scan sonar vehicles such as TOBI acquire high quality imagery of the seafloor with very high spatial resolution but poor locational accuracy. Fusion of the side-scan sonar data with bathymetry data from an independent source is often desirable to reduce ambiguity in geological interpretations, to aid in slant-range correction and to enhance seafloor representation. The main obstacle to fusion is accurate registration of the two datasets.The application of hierarchical chamfer matching to the registration of TOBI side-scan sonar images and multi-beam swath bathymetry is described. This matches low level features such as edges in the TOBI image, with corresponding features in a synthetic TOBI image created by simulating the flight of the TOBI vehicle through the bathymetry. The method is completely automatic, relatively fast and robust, and much easier than manual registration. It allows accurate positioning of the TOBI vehicle, enhancing its usefulness as a research tool. The method is illustrated by automatic registration of TOBI and multi-beam bathymetry data from the Mid-Atlantic Ridge.     

基于回波时间的多波束测深与声纳数据匹配方法

以多波束精确的水深数据为参照源,采用原始回波时间对多波束测深数据与其同源声纳数据进行匹配,从而获得高精度和高分辨率的海底影像数据,并避免了传统声纳图像处理过程中斜距改正所带来的几何形变。匹配结果采用光照图输出,并与三维水深图、原始声纳图像和CARIS处理后的声纳图像进行比较分析。该方法有效地提高了多波束数据的利用率,增强了对海底地形的探测分辨率。     

Adaptive port-starboard beamforming of triplet sonar arrays

For a low-frequency active sonar (LFAS) with a triplet receiver array, it is not clear in advance which signal processing techniques optimize its performance. Here, several advanced beamformers are analyzed theoretically, and the results are compared to experimental data obtained in sea trials. Triplet arrays are single line arrays with three hydrophones on a circular section of the array. The triplet structure provides the ability to solve the notorious port-starboard (PS) ambiguity problem of ordinary single-array receivers. More importantly, the PS rejection can be so strong that it allows to unmask targets in the presence of strong coastal reverberation or traffic noise. The theoretical and experimental performance of triplet array beamformers is determined in terms of two performance indicators: array gain and PS rejection. Results are obtained under several typical acoustic environments: sea noise, flow noise, coastal reverberation, and mixtures of these. A new algorithm for (beam space) adaptive triplet beamforming is implemented and tuned. Its results are compared to those of other triplet beamforming techniques (optimum and cardioid beamforming). These beamformers optimize for only one performance indicator, whereas in theory, the adaptive beamformer gives the best overall performance (in any given environment). The different beamformers are applied to data obtained with an LFAS at sea. Analysis shows that adaptive triplet beamforming outperforms conventional beamforming algorithms. Adaptive triplet beamforming provides strong PS rejection, allowing the unmasking of targets in the presence of strong directional reverberation (e.g., from a coast) and at the same time provides positive array gain in most environments.     

Eivissa slides, western Mediterranean Sea: morphology and processes

After obtaining full-coverage swath bathymetry data in 1995 and very high-resolution acoustic profiles in 2002, four slides at the Balearic Margin of the Eivissa Channel in the western Mediterranean Sea were revisited in 2004 when side-scan sonar data were collected using a MAK-1M deep-towed acoustic system. These new findings, higher in resolution than those for the swath bathymetry, show two main features previously undetected within these submarine landslides: (1) a series of step-forming inclined and detached slabs oriented perpendicular to the slide movement and located in the uppermost part of the slides, and (2) arcuate regular positive ridges oriented also normal to the slide movement and located in the depositional lobes of some of the slides. The former are interpreted as extensional ridges, suggesting a retrogressive post-failure evolution of the slides. The latter are interpreted as compression ridges, related to plastic deformation of the sediment before movement freezing. Moreover, the new data show that fluid escape features are even more widespread in the Eivissa Channel than previously thought, dozens of new pockmarks less than 20 m in diameter having been identified.     

Autonomous bathymetry survey system

This paper describes the Autonomous Bathymetry Survey System (AutoSurvey), a system that provides automation of swath sonar bathymetric surveys. This system enables faster surveying of an area through environmentally adaptive techniques while ensuring adequate coverage and data quality. AutoSurvey assesses data quality and coverage in real time and generates next-trackline waypoints based on actual system performance. The need for real-time performance assessment is discussed. A primary factor considered is the effect of the environment on swath bathymetry system performance, which is difficult to predict a priori. The system's features, design, and implementation are discussed in this paper. Simulation and sea trial results are presented, as well as an analysis of the system's ability to reduce survey time     

基于L型阵列的二维CAATI算法高分辨DOA估计

将多角测深侧扫声纳（MSBSS）中应用的计算到达角瞬态成像（CAATI）技术扩展到三维空间,提出一种基于L型阵列的二维CAATI高分辨DOA估计方法,同时解决了信号的参数配对问题。该算法具有运算量和快拍数均较小的优点,为水下三维高分辨探测提供了可能。     

基于多源水深数据融合的海底高精度地形重建

本文在研究多源水深数据构建技术的基础上,分析了张力样条插值算法和“移去-恢复”法的多源水深数据融合处理技术,基于该方法选取实验区,利用多波束、单波束、历史海图等多源水深数据进行高精度海底地形融合试验,并针对多源水深融合技术缺少误差评估的现状,利用split-sample方法对融合结果进行水深不确定性评估,形成融合结果的可靠性空间分布。结果表明该方法无论是在数据稀疏区还是高密度区都达到了较好的融合效果,既保留了高分辨率水深数据的细节信息,又较真实的反映了研究区海底地形特征,且构建的海底地形精度可靠,误差百分比集中在0.5%。本文整套数据融合和结果评估方法可为多源水深数据融合的海底高精度地形构建提供借鉴和参考。     

Fault scarp identification in side-scan sonar and bathymetry images from the Mid-Atlantic Ridge using wavelet-based digital filters

Digital filters designed using wavelet theory are applied to high resolution deep-towed side-scan sonar data from the median valley walls, crestal mountains, and flanks of the Mid-Atlantic Ridge at 29°10 N. With proper tuning, the digital filters are able to identify the location, orientation, length, and width of highly reflective linear features in sonar images. These features are presumed to represent the acoustic backscatter from axis-facing normal faults. The fault locations obtained from the digital filters are well correlated with visual geologic interpretation of the images. The side-scan sonar images are also compared with swath bathymetry from the same area. The digitally filtered bathymetry images contain nine of the eleven faults identified by eye in the detailed geologic interpretation of the side-scan data. Faults with widths (measured perpendicular to their strike) of less than about 150 m are missed in the bathymetry analysis due to the coarser resolution of these data. This digital image processing technique demonstrates the potential of wavelet-based analysis to reduce subjectivity and labor involved in mapping and analyzing topographic features in side-scan sonar and bathymetric image data.     

Simultaneous operation of the Sea Beam multibeam echo-sounder andthe SeaMARC II bathymetric sidescan sonar system

An experiment aboard the Scripps Institution of Oceanography's RV Thomas Washington has demonstrated the seafloor mapping advantages to be derived from combining the high-resolution bathymetry of a multibeam echo-sounder with the sidescan acoustic imaging plus wide-swath bathymetry of a shallow-towed bathymetric sidescan sonar. To a void acoustic interference between the ship's 12-kHz Sea Beam multibeam echo-sounder and the 11-12-kHz SeaMARC II bathymetric sidescan sonar system during simultaneous operations, Sea Beam transmit cycles were scheduled around SeaMARC II timing events with a sound source synchronization unit originally developed for concurrent single-channel seismic, Sea Beam, and 3.5-kHz profile operations. The scheduling algorithm implemented for Sea Beam plus SeaMARC II operations is discussed, and the initial results showing their combined seafloor mapping capabilities are presented     

基于四阶累积量的被动测向声纳高分辨力算法研究

利用高阶累积量的阵列扩展和抑制高斯噪声特性,提出1种基于四阶累积量的逆波束形成算法,并通过滑动平均进一步改善了算法。仿真结果和实验数据表明:逆波束形成算法较常规波束形成方法在空间方位分辨力和噪声抑制能力上明显提高,可以更有效地应用于被动测向声纳中。     

多波束声纳计量测试系统设计

为满足多波束声纳量值溯源与传递需求,设计多波束声纳关键参数的计量测试系统。基于大比尺原型深水港池和多维运行控制机构,通过水下横向测距代替垂向测深的方式对多波束声纳进行了测深准确度与有效条带宽度的计量测试。阐述了测试系统的主要组成和测试方法,给出了本测试系统测深结果的扩展不确定度。参考标准值与示值(或标称值)比对结果表明,被检多波束声纳测深示值误差小于0.2%,条带宽度误差小于7%。为规范水下声纳设备技术指标校准,指导多波束声纳计量标准建设提供参考。     

Do equilibrium conditions exist during sediment transport studies on continental margins? An example from the East China Sea

Analysis of high-resolution (<1 m) GeoPulse seismic reflection and Chirp sonar data from the continental margin of the East China Sea reveals the presence of a thin (1- to 5-m-thick) sediment veneer over a highly reflective transgressive surface. Due to reverberation of the output signal and the effect of the bubble pulse, this veneer may erroneously be interpreted as a laterally continuous feature. Sonar data, which does not suffer from these ghosting effects, reveals that the sea bottom is often an erosional surface that exposes older, relict sediments, which were deposited under a radically different hydrodynamic regime than exists in these locations today.     

An efficient architecture for real-time narrowband beamforming

A short finite impulse response (FIR) filter architecture for narrowband beamforming is presented that is well suited for active sonar applications. By applying the technique of adaptive modeling, a short FIR filter can be designed to carry out any required narrowband constant phase shift. A 4-tap FIR filter designed in this way can generate constant phase shift within bandwidths of 0.04 (relative to the sampling frequency), with an error of less than 1°. Thus a beam can be formed with a bank of short FIR filters, each filter corresponding to one sensor. Due to the light computational load of this method, it is rather convenient for digital signal processors (DSP) to implement beamforming in real time. Satisfactory sonar beam patterns are shown to result from a TMS320C25-based emulation of the architecture