Geometric distortions in side-scan sonar images: a procedure fortheir estimation and correction

A procedure is introduced for the estimation and correction of geometric distortions frequently observed in side-scan sonar images as a result of motion instabilities of the sonar towfish. This procedure estimates geometric distortions from the image itself, without requiring navigational or altitude measurements. Estimates of the local degree of geometric distortion are obtained by cross-correlating segments of adjacent lines of the image. A mathematical model for the distortions is derived from the geometry of the problem and is applied to these estimates to reconstruct the sampling pattern on the seabed, under the assumption of a planar bottom. The estimated sampling pattern is then used for resampling the image to correct the geometric distortions. The model parameters may also be used for calculating approximate estimates of the attitude parameters of the towfish. A simulation is employed to evaluate the effectiveness of this technique and examples of its application to high-resolution side-scan sonar images are provided     

The practical implementation of electronic stabilization for sectorscanning sonar displays

The natural motion of a ship at sea produces undesirable effects on sonar displays. To overcome this problem some form of stabilization is desirable. A practical sonar display system, which uses image processing techniques to stabilize the displayed sonar image against rotational motion is described. The stabilization method provides the capability of image enhancement as well as improved image interpretation due to an increased area of view and a geometrically correct conversion to polar display. Details of the three processing stages required in the system are provided. These processes are ship motion analysis, image integration, and polar display conversion. Stabilized results are given for simulated ship motion     

侧扫声纳图像增强技术

随着海洋探测技术的飞速发展,侧扫声纳的图像质量和定位精度迅速提高,其应用日益广泛和深入。在声纳图像判读分析时,常常需要进行辐射改正和均衡增强。根据海底地貌声纳图像的特点,归纳了几种有效、适用的图像增强方法,并建立了声纳图像分析软件。     

基于声呐图像的深海采矿车组合定位导航算法

实现高精度的定位导航是深海采矿车完成海底工作任务的基础条件。在采矿车行进过程中,声呐设备生成的图像信息能够反映海底场景的变化,从而体现采矿车本身的运动,由此建立了一种声呐图像里程计,并将其与轮式里程计和USBL测量数据相结合提出了一种深海采矿车组合定位导航算法。首先对多波束前视声呐图像进行预处理,然后使用Canny算法进行特征检测并对特征点云进行配准,再结合声呐成像原理构建了声呐图像里程计运动模型,最后通过轮式里程计运动模型推导预测方程、声呐图像里程计运动模型和USBL测量数据推导更新方程,利用EKF（extended Kalman filter）算法实现基于多传感器融合的定位与姿态估计。海试数据验证了该组合定位算法能实现轮式里程计、声呐里程计和超短基线在速度、位置、艏向角估计、定位速率的精度互补,具有一定的有效性和精确性,该算法为深海采矿车的定位与导航算法研发提供了参考。     

声纳记录失真分析

从侧扫声纳的工作原理出发，分析了侧扫声纳记录声图产生基本失真的原因，推导出基本失真的数学公式，所得结论对实际作用有重要的指导意义。     

Synthetic aperture sonar for seabed imaging: relative merits ofnarrow-band and wide-band approached

The advantages of using wideband sonar systems in underwater acoustical imaging by means of synthetic aperture (side-looking) sonars are described and illustrated through simulation examples. The simulations are conducted for two cases of sonar platform motion: perfect trajectory and disturbed trajectory. Several schemes used for wideband synthetic aperture processing are investigated and their relative merits (resolution and complexity) in the case of both disturbed and perfect trajectories are evaluated. Quantitative image evaluation is initially achieved through the evaluation of performance as regards resolution. The problems involved in the definition of the image quality are discussed     

Analysis of Phase Error Effects on Stripmap SAS

It is often of interest to consider how uncompensated platform motion can degrade the ideal point scatterer response (PSR) of a synthetic aperture sonar (SAS). This information can be used to shape the design of the sonar itself as well as that of the platform carrying it. Also, knowledge of how certain types of motion affect a SAS image can reduce the time spent in troubleshooting motion estimation and compensation schemes. In the field of spotlight-mode synthetic aperture radar (SAR), the effects of phase errors across the synthetic aperture are well documented (for example, Chapter 5 of Carrara , 1995). The counterpart problem for the stripmap mode is less well developed in the literature. This paper explores the effects of uncompensated phase errors on stripmap imagery and shows that, under certain conditions, they are similar to those for spotlight mode processing.      

Image projection and composition with a front-scan sonar system: methods and experimental results

This paper describes a complete set of methods for arranging acoustic images of the sea floor by projecting and interpolating data gathered with a novel front-scan sonar system, developed in the context of the EC-COSMOS project. Traditional sonar imaging systems for sea-floor analysis generate acoustic images during the motion of a ship; on the contrary, the front-scan sonar system not only provides information unreachable by traditional devices (blind region), but also does not need the ship motion to compose a whole image of the sea floor. Two different projection methods have been devised: a simpler analytical solution and a more precise ray-tracing approach. The development of an analytical solution is possible under the classical assumptions about a flat sea floor and a constant sound velocity profile; when these hypotheses are not realistic or a more precise image is required, a numerical solution obtained by a ray-tracing approach can be applied, which is based on some ad hoc solutions worked out for the front-scan sonar system. To move from the projection results to an image defined over a dense matrix of pixels, an interpolation stage is needed. To this end, an algorithm based on the generation of virtual-beam signals (only where necessary) has been tested and compared with more-traditional techniques. The potentials of the proposed projection and interpolation methods have been evaluated and some comparisons have been made, using real data gathered with the COSMOS sonar prototype during trials at sea.     

Digital preprocessing techniques for GLORIA II sonar images

Image processing techniques are discussed that correct distortions in GLORIA II side scan sonar imagery including water column offset, slant-range distortion, multiple returns, aspect ratio, speckle noise, striping, and cross-track power drop-off. The software operates within NASA's ELAS image processing system and is applied to the original 12-bit GLORIA II data. Procedures are discussed for generating large scale mosaics and three-dimensional overlays with sea floor bathymetry. The results are shown in four sonographs acquired off the southern coast of California.     

Detection of objects on the sea bottom using backscatteringcharacteristics dependent on the observation point

Sector-scanning sonar systems image the sea bottom to detect objects that can be distinguished from the background structure of the sea bottom. In current systems, images are displayed and discarded as new image data become available, In this paper, a method for improving sonar detection by utilizing all images in a sequence is investigated. The proposed method requires that sonar data are acquired with a sector-scanning sonar in a side-looking configuration. It is demonstrated that these data can be used to detect observation-point-dependent changes in sea-bottom backscattering characteristics. These changes provide additional cues for discrimination that can improve the detection of objects on the sea bottom. Results of applying the method to experimental data are presented     

A method for estimating the sub-wavelength sway of a sonar towfish

The essentially random motion of the towfish in a side scan sonar system limits the fidelity of the resultant image. Although corrective techniques have been developed to deal with the gross departures from a set of regular amplitude versus range samples along a straight locus as well as the gross rotational errors, sub-wavelength displacement errors are mostly ignored. This paper suggests a technique to estimate the appropriate displacement errors from the statistics of the measured data when there are few or no strong scatterers on an otherwise featureless sea floor. As it happens, the proposed technique is somewhat similar to an existing optical technique but is new to underwater sonar. We show in a series of simulated trials that it is possible to estimate the resultant sub-wavelength horizontal departure from a straight locus under some (hopefully) realistic operating conditions     

多波束声呐图像条带中央和边缘残差处理方法

多波束声呐图像是进行海底底质分类的主要数据源之一,由于受海洋噪声、声波散射和混响、仪器设备等因素影响,其经各项常规改正后仍存在明显残差,突出表现在中央波束区和条带重叠区,难以形成高质量的声呐图像。文中分析了多波束声呐图像残差的成因及影响,提出了一种基于多条带最小二乘拟合的多波束声呐图像残差处理方法。首先,得到相邻声脉冲(ping)信号中央区域、重叠区域以及整体趋势的拟合函数;然后,通过拟合函数计算得到中央和重叠区域的残差改正系数;最后,通过改正系数进行残差改正。实验分析表明,该方法在保留原始细节的基础上,有效削弱了残差对声呐图像的影响,对多波束声呐图像处理具有参考和应用价值。     

Curvelet变换域侧扫声纳图像增强算法

针对海底侧扫声纳图像对比度低、纹理弱、噪声严重等问题,提出了一种基于第二代Curvelet变换的声纳图像增强算法。首先对原始声纳图像进行多尺度、多方向的Curvelet变换分解,得到低频子带和高频子带;然后引入非线性S型函数对低频系数进行处理,提高图像整体的对比度;采用一种可以避免过度增强的新型非线性函数对各尺度的高频子带系数进行处理,提高图像整体的对比度,增强图像边缘和纹理细节,并通过估计噪声水平设定阈值进行阈值降噪。最后经Curvelet逆变换得到增强图像。实验表明,该方法不仅改善了海底侧扫声纳图像对比度低的问题,而且降低了噪声,突出了声纳图像的边缘和纹理细节。     

Robust tracking of multiple objects in sector-scan sonar imagesequences using optical flow motion estimation

The fast update rate and good performance of new generation electronic sector scanning sonars is now allowing practicable use of temporal information for signal processing tasks such as object classification and motion estimation. Problems remain, however, as objects change appearance, merge, maneuver, move in and out of the field of view, and split due to poor segmentation. This paper presents an approach to the segmentation, two-dimensional motion estimation, and subsequent tracking of multiple objects in sequences of sector scan sonar images. Applications such as ROV obstacle avoidance, visual servoing, and underwater surveillance are relevant. Initially, static and moving objects are distinguished in the sonar image sequence using frequency-domain filtering. Optical flow calculations are then performed on moving objects with significant size to obtain magnitude and direction motion estimates. Matches of these motion estimates, and the future positions they predict, are then used as a basis for identifying corresponding objects in adjacent scans. To enhance robustness, a tracking tree is constructed storing multiple possible correspondences and cumulative confidence values obtained from successive compatibility measures. Deferred decision making is then employed to enable best estimates of object tracks to be updated as subsequent scans produce new information. The method is shown to work well, with good tracking performance when objects merge, split, and change shape. The optical flow is demonstrated to give position prediction errors of between 10 and 50 cm (1%-5% of scan range), with no violation of smoothness assumptions using sample rates between 4 and 1 frames/s     

小波函数对侧扫声纳图像滤波效果的影响分析

侧扫声纳技术应用日益广泛，已成为海洋测量的重要工具，而去除噪声处理是对侧扫声纳图像进行正确判读的前提。利用小波函数滤波处理的方法，分别采用Haar、Daubechies、Coiflets、Symlets、Discrete Meyer、Biorthogonal、Reverse Biorthogonal等小波函数与中值滤波函数对侧扫声纳图像进行处理，并以平滑指数和边缘保持指数为评价指标，对滤波效果进行定量比较。试验表明，小波函数可以有效地平滑声纳图像，并能保持其较好的边缘效果。     

Pulse-length-tolerant features and detectors for sector-scan sonar imagery

This paper presents a neural-network-based system to detect small man-made objects in sequences of sector-scan sonar images created using signals of various pulse lengths. The detection of such objects is considered out to ranges of 150 m by using an experimental sector-scan sonar system mounted on a vessel. The sonar system considered in this investigation has three modes of operation to create images over ranges of 200, 400, and 800 m from the vessel using acoustic pulses of a different duration for each mode. After an initial cleaning operation performed by compensating for the motion of the vessel, the imagery is segmented to extract objects for analysis. A set of 31 features extracted from each object is examined. These features consist of basic object size and contrast features, shape moment-based features, moment invariants, and features extracted from the second-order histogram of each object. Optimal sets of 15 features are then selected for each mode and over all modes using sequential forward selection (SFS) and sequential backward selection (SBS). These features are then used to train neural networks to detect man-made objects in each sonar mode. By the addition of a feature describing the sonar's mode of operation, a neural network is trained to detect man-made objects in any of the three sonar modes. The multimode detector is shown to perform very well when compared with detectors trained specifically for each sonar mode setting. The proposed detector is also shown to perform well when compared to a number of statistical detectors based on the same set of features. The proposed detector achieves a 92.4% probability of detection at a mean false-alarm rate of 10 per image, averaged over all sonar mode settings.     

基于数学形态学的侧扫声呐图像轮廓自动提取

侧扫声呐图像特征自动提取的难点在于特征地貌边缘检测较困难,依据图像灰度突变检测得到的边缘比较粗糙、不连续,而且有断口和小洞。本文在对图像进行预处理和阈值化的基础上,采用数学形态学方法对图像进行处理,即用具有一定形态的结构元素去量度和提取图像中的对应形状,得到连续化、粗化、圆滑的特征区域边缘填充目标内部阴影且消除背景噪声。基于数学形态学的侧扫声呐图像特征自动提取的主要步骤为:首先对侧扫声呐图像进行预处理,然后进行灰度阈值化,接着采用数学形态学方法进行处理,最后对处理后的图像进行边缘检测,提取出特征地貌边缘。实验表明,采用数学形态学方法进行处理后,错断、离散的海底目标物变得连续,背景噪声大大减少,自动提取结果准确可靠。     

基于MRF与引导滤波的声纳图像去噪方法

声纳具有对大范围水下场景探测的能力,一直以来都是水下设备感知外界环境的重要手段,但由于声纳图像分辨率低,海洋环境噪声干扰较为复杂,所以在诸多方面的应用都受到了限制。文中提出了一种基于马尔可夫随机场和引导滤波的声纳图像去噪与增强方法,使用马尔可夫随机场对声纳图像进行预分割,然后采用中值滤波方法对原始图像进行简单滤波处理,最后将该图像作为引导图像对马尔可夫随机场(Markov Random Field,MRF)分割后的图像进行引导滤波实现了对声纳图像的去噪与增强。该方法有效地去除了背景和影子内的噪声,对目标区域内部噪声起到了很好的抑制作用,消除了MRF分割产生的伪轮廓效应,具有较好的边界保持和增强效果。     

GeoSwath与Klein 3000的综合对比及实验分析

GeoSwath Plus系统利用相干原理能同时得到水深数据和声纳图,与传统侧扫声纳存在一定差异。本文对GeoSwath Plus 125 kHZ系统和Klein 3000系统从工作原理、技术参数、数据采集和后处理平台以及实测数据等方面进行了对比。结果表明:二者接收数据存在差异,导致两者的用途也存在差异;GeoSwath Plus系统相比Klein 3000更加复杂,但适用水深范围较小;GeoSwath Plus采集软件较少,对后处理软件的要求更高,而Klein 3000数据采集和后处理软件都有多种选择;通过实测数据的比较发现,GeoSwath Plus的定位精度更高,但由于发射波束水平开角大于Klein 3000,加之采用悬挂式安装,导致得到的声纳图像对比度和分辨率比传统侧扫声纳Klein 3000要差一些。