Midfrequency "Through-the-Sensor" scattering measurements: a new approach

In this paper, a new paradigm for "through-the-sensor" remote sensing of the seafloor is presented. The methodology has been tailored for use with the AN/SQS-53C sonar found on many U.S. Navy destroyers. Sonar beamformer outputs are processed, and a point georeferenced database of signal attributes is constructed. Corresponding sonar settings and ship navigation information are also included for each database point. Database entries are then fused with environmental characteristics, such as bathymetry and sound speed information. These data may be derived from historical databases, on-site measurements, or a combination of the two. The database is then completed by ambiguity resolution and matching of modeled eigenray paths with database entries in order to associate signal attributes with specific propagation paths. Model inputs are derived from a customized version of the Comprehensive Acoustic System Simulation/Gaussian Ray Bundle eigenray propagation model (CASS/GRAB), which performs propagation estimates over incremental range/depth steps. Illustrations of how the point database may be filtered/constrained, gridded, and displayed are presented. An example of how bottom scattering strength can be derived from the database is presented, followed by an example of a technique for monostatic bottom loss estimation. Results indicate that the approach presented in this paper represents a viable method for conducting "through-the-sensor" measurements of seafloor scattering properties.     

多波束反向散射强度数据处理研究

在探讨多波束测深系统反向散射强度与海底底质类型的关系基础上,研究影响反向散射强度的各种因素,主要分析了海底地形起伏、中央波束区反射信号对反向散射强度的影响,并给出了消除这些影响的方法;将处理后的“纯”反向散射强度数据镶嵌生成海底声像图,为海底底质类型划分以及地貌解译提供了基础数据和辅助判读依据.     

Processing Multibeam Backscatter Data

A new highly precise source of data has recently become available using multibeam sonar systems in hydrography. Multibeam sonar systems can provide hydrographic quality depth data as well as high-resolution seafloor sonar images. We utilize the seafloor backscatter strength data of each beam from multibeam sonar and the automatic classification technology so that we can get the seafloor type identification maps. In this article, analyzing all kinds of error effects in backscatter strength, data are based on the relationship between backscatter strength and seafloor types. We emphasize particularly analyzing the influences of local bottom slope and near nadir reflection in backscatter strength data. We also give the correction algorithms and results of these two influent factors. After processing the raw backscatter strength data and correcting error effects, we can get processed backscatter strength data which reflect the features of seafloor types only. Applying the processed backscatter strength data and mosaicked seafloor sonar images, we engage in seafloor classification and geomorphy interpretation in future research.     

Processing Multibeam Backscatter Data

A new highly precise source of data has recently become available using multibeam sonar systems in hydrography. Multibeam sonar systems can provide hydrographic quality depth data as well as high-resolution seafloor sonar images. We utilize the seafloor backscatter strength data of each beam from multibeam sonar and the automatic classification technology so that we can get the seafloor type identification maps. In this article, analyzing all kinds of error effects in backscatter strength, data are based on the relationship between backscatter strength and seafloor types. We emphasize particularly analyzing the influences of local bottom slope and near nadir reflection in backscatter strength data. We also give the correction algorithms and results of these two influent factors. After processing the raw backscatter strength data and correcting error effects, we can get processed backscatter strength data which reflect the features of seafloor types only. Applying the processed backscatter strength data and mosaicked seafloor sonar images, we engage in seafloor classification and geomorphy interpretation in future research.     

Active impulsive echo discrimination in shallow water by mappingtarget physics-derived features to classifiers

One of the most difficult challenges in shallow-water active sonar processing is false-alarm rate reduction via active classification. In impulsive-echo-range processing, an additional challenge is dealing with stochastic impulsive source variability. The goal of active classification is to remove as much clutter as possible while maintaining an acceptable detection performance. Clutter in this context refers to any non-target, threshold-crossing cluster event. In this paper, we present a clutter-reduction algorithm using an integrated pattern-recognition paradigm that spans a wide spectrum of signal and image processing-target physics, exploration of projection spaces, feature optimization, and mapping the decision architecture to the underlying good-feature distribution. This approach is analogous to a classify-before-detect strategy that utilizes multiple informations to arrive at the detection decision. After a thorough algorithm evaluation with real active sonar data, we achieved over an order of magnitude performance improvement in clutter reduction with our methodology over that of the baseline processing     

Multibeam sonar backscatter data processing

Multibeam sonar systems now routinely record seafloor backscatter data, which are processed into backscatter mosaics and angular responses, both of which can assist in identifying seafloor types and morphology. Those data products are obtained from the multibeam sonar raw data files through a sequence of data processing stages that follows a basic plan, but the implementation of which varies greatly between sonar systems and software. In this article, we provide a comprehensive review of this backscatter data processing chain, with a focus on the variability in the possible implementation of each processing stage. Our objective for undertaking this task is twofold: (1) to provide an overview of backscatter data processing for the consideration of the general user and (2) to provide suggestions to multibeam sonar manufacturers, software providers and the operators of these systems and software for eventually reducing the lack of control, uncertainty and variability associated with current data processing implementations and the resulting backscatter data products. One such suggestion is the adoption of a nomenclature for increasingly refined levels of processing, akin to the nomenclature adopted for satellite remote-sensing data deliverables.     

Spatial and temporal pulse design considerations for a marinesediment classification sonar

A linear FM sonar system was developed to support the objective of remote acoustic classification of seafloor sediments. It is a calibrated, wideband, digital, frequency modulated sonar that provides quantitative, high-resolution, low-noise sub-bottom data. Since the linear sonar system can precisely transmit a specified waveform, the calibrated digitally recorded reflection data can be processed to estimate the acoustic impulse response of the seabed and sediment attenuation. An acoustic pulse with special frequency domain weighting characteristics is designed to provide low temporal sidelobe levels and a nearly constant resolution with depth even after passing through a sediment with high losses such as sand. After correlation processing, the wideband acoustic pulse yields an effective beam pattern with high spatial resolution and insignificant sidelobe levels. Data sets generated with the FM profiler indicate that the required temporal and spatial characteristics of the sonar are realized in practice     

Underwater vehicle obstacle avoidance and path planning using amulti-beam forward looking sonar

This paper describes a new framework for segmentation of sonar images, tracking of underwater objects and motion estimation. This framework is applied to the design of an obstacle avoidance and path planning system for underwater vehicles based on a multi-beam forward looking sonar sensor. The real-time data flow (acoustic images) at the input of the system is first segmented and relevant features are extracted. We also take advantage of the real-time data stream to track the obstacles in following frames to obtain their dynamic characteristics. This allows us to optimize the preprocessing phases in segmenting only the relevant part of the images. Once the static (size and shape) as well as dynamic characteristics (velocity, acceleration,…) of the obstacles have been computed, we create a representation of the vehicle's workspace based on these features. This representation uses constructive solid geometry (CSG) to create a convex set of obstacles defining the workspace. The tracking takes also into account obstacles which are no longer in the field of view of the sonar in the path planning phase. A well-proven nonlinear search (sequential quadratic programming) is then employed, where obstacles are expressed as constraints in the search space. This approach is less affected by local minima than classical methods using potential fields. The proposed system is not only capable of obstacle avoidance but also of path planning in complex environments which include fast moving obstacles. Results obtained on real sonar data are shown and discussed. Possible applications to sonar servoing and real-time motion estimation are also discussed     

海洋石油管线探测多波束剖面声纳设计

针对海洋石油管线探测多波束剖面声纳的高速、大容量的实时信号处理任务,以及声纳系统进一步扩展的需求,设计并实现了一种基于IP网络互连的、以并行DSP为处理节点的多波束剖面声纳系统.该系统应用于海底石油管线探测与定位,系统中的每个处理节点与数据采集转换部分采用TCP/IP网络连接,可以通过物理上添加一个或多个处理节点成倍的提高系统的信号处理能力,而在软件上无须大的改动.通过电联调与水槽试验,验证了系统的可行性和稳定性,该设计为高速水声阵列信号实时处理系统提供了一种全新的解决思路.     

TOBI image processing-the state of the art

TOBI (Towed Ocean Bottom Instrument) is a deep-tow sidescan sonar vehicle from which sidescan sonar data are now routinely collected and archived. This paper describes the algorithms developed for detailed processing of TOBI data. Sonar imagery has a characteristic set of processing challenges and these are addressed. TOBI provides a very large sonar dataset, and to limit the difficulties of handling and processing these data, the raw data are subjected to a data reduction technique prior to further processing. Slant-range correction is improved by editing vehicle altitude data using a median filter. Noise on TOBI imagery can appear in two main forms; speckle noise and line dropouts. Speckle noise is removed by a small median difference kernel and line dropouts are removed using a ratio of two box-car filters, each with appropriate thresholding techniques. Precise geocoding of the imagery requires an accurate estimate of vehicle location, and a method of calculation is presented. Two optional processing algorithms are also; presented; deblurring of imagery to improve along-track resolution at far range, and the suppression of a surface reflection return which may occur when TOBI is operated in relatively shallow water. Several of the techniques presented can be transcribed and modified to suit other datasets     

一种多波束声呐回波信号时延的实时特征窗分析方法

本文介绍了一种多波束声呐回波信号时延的实时分析方法。通过实时提取信号的幅度及形状的综合特征计算出信号的时延。该方法具有较强的抗干扰能力,算法快速、准确,适合于多波束声呐系统的回波信号实时分析处理。     

Acoustic stereo images of the bottom surface

In order to enhance the efficiency of the interpretation of surface images obtained with a side scanning sonar, it is proposed to supplement the standard processing software with a program for obtaining acoustic stereo images. Examples of such images synthesized with this program using the data of a bottom sonar survey with a side scanning sonar and an echo sounder are presented. The cases are considered when the information on the bottom relief contained in sonar images obtained with a standard side scanning sonar or its modifications can be used instead of the data of an echo sounding survey.     

Processing of high-frequency multibeam echo sounder data for seafloor characterization

Processing simultaneous bathymetry and backscatter data, multibeam echosounders (MBESs) show promising abilities for remote seafloor characterization. High-frequency MBESs provide a good horizontal resolution, making it possible to distinguish fine details at the water-seafloor interface. However, in order to accurately measure the seafloor influence on the backscattered energy, the recorded sonar data must first be processed and cleared of various artifacts generated by the sonar system itself. Such a preprocessing correction procedure along with the assessment of its validity limits is presented and applied to a 95-kHz MBES (Simrad EM 1000) data set. Beam pattern effects, uneven array sensitivities, and inaccurate normalization of the ensonified area are removed to make possible further quantitative analysis of the corrected backscatter images. Unlike low-frequency data where the average backscattered energy proves to be the only relevant feature for discriminating the nature of the seafloor, high-frequency MBES backscatter images exhibit visible texture patterns. This additional information involves different statistical distributions of the backscattered amplitudes obtained from various seafloor types. Non-Rayleigh statistics such as K-distributions are shown to fit correctly the skewed distributions of experimental high-frequency data. Apart from the effect of the seafloor micro-roughness, a statistical model makes clear a correlation between the amplitude statistical distributions and the signal incidence angle made available by MBES bathymetric abilities. Moreover, the model enhances the effect of the first derivative of the seafloor backscattering strength upon statistical distributions near the nadir and at high incidence angles. The whole correction and analysis process is finally applied to a Simrad EM 1000 data set.     

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     

基于Kongsberg EM多波束的Snippet数据处理方法

多波束声呐记录的海底后向散射片段(Snippet)数据处理成角度响应曲线和地理编码(Mosaic)图像可以 帮助识别海底底质类型和反映地貌形态,这一过程包括辐射校正、角度响应改正(AVG)和几何地理编码,但不同的多波束系统硬件在辐射校正和角度响应改正方法上存在差异且传统处理方法忽略了声呐系统本身的指向性模型随时间变化的事实。以声呐方程为基础,针对Kongsberg EM 多波束系统提出了一套完整的Snippet数据处理流程,并分析了各步骤中存在的可变性,给出了每一步的处理建议,最后将此方法应用于EM2040浅水多波束实测数据,并验证了该方法的有效性和实用性。     

西沙北部海域海洋环境噪声频谱特性

Ambient noise is very important in the prediction system of a sonar performance, because it determines the detection ranges always in a passive sonar and usually in an active sonar. In the uncertainty issue for the so-nar performance, it is necessary to know this factor＇s statistical characteristics that are only obtained by data processing from the underwater ambient noise measurements. Broad-band ambient noise signals from 16 hydrophones were amplified and recorded for 2 min every 1 h. The results show that the ambient noise is essentially depth independent. The cross correlation of the ambient noise levels （1, 6 and 12 h average） with a wind speed is presented. It was found that the correlation is excellent on the upper frequency band and the noise levels correlate better with high wind speed than with low wind speed.     

Efficient processing of water wave records via compressive sensing and joint time-frequency analysis via harmonic wavelets

A methodology is proposed for efficient processing of sea wave field data via compressive sensing (CS), and joint time-frequency analysis via harmonic wavelets (HWs) based evolutionary power spectrum (EPS) estimation. In this regard, it is possible to record and store relatively long wave data sequences, whereas the commonly adopted in-practice assumption of stationary data is abandoned. Currently, most wave records are measured by buoys, which acquire data for a time interval representative of stationary time series. Next, following a Fourier transform processing, only few spectral parameters are stored. Thus, detailed information about localized-in-time phenomena are completely lost. Herein, it is shown that CS can be adopted for efficiently compressing and reconstructing wave data, while retaining localized information. For this purpose, CS is used in conjunction with a HW basis for processing long time series. In particular, storage capacity demands are drastically decreased as only the HW coefficients need to be saved. These are determined from a randomly-sampled record by invoking a L_1/2 norm minimization procedure. The resulting reconstructed record, being longer than conventional wave time series, can no longer be regarded as stationary; thus, a HW based EPS estimate is employed for describing the joint time-frequency features of the record. Finally, the reliability of the methodology is assessed by analyzing wave field data measured at the Natural Ocean Engineering Laboratory (NOEL) of Reggio Calabria. Specifically, comparisons between original and reconstructed records demonstrate a satisfactory agreement regarding the time-histories, and the estimated EPS and relevant statistical quantities, even for up to 60% missing/removed data.     

Range and Doppler information from fourth-order spectra

In an active sonar system setting, a novel method is evaluated that extracts range and Doppler information from a Doppler-spread active sonar echo. The Doppler spreading is based on H. Van Trees' (1971) Gaussian amplitude modulating model, and the new method is based on the Fourier transform of a special case of the fourth-order cumulant. Specifically, from the envelope of the Gaussian amplitude modulated echo of a transmitted coded pulse train, the second-order spectrum and the Fourier transform of a special case of the fourth-order cumulant are derived and analyzed for this ability to extract range and Doppler information. It is shown that the method can theoretically extract range and Doppler information without degradation. The reason for this result is that a special case of the fourth-order cumulant is independent of the covariance of the Gaussian amplitude modulating function. These methods are also simulated and compared with the simulated results of the range-Doppler ambiguity function. This shows that the ambiguity function and the second-order spectrum are degraded due to the Gaussian amplitude modulation. The results are further demonstrated by simulating the three range-Doppler extraction methods for the received echo in noise     

A processing requirement and resolution capability comparison ofside-scan and synthetic-aperture sonars

The processing requirements and resolution capabilities of both side-look sonar (SLS) and synthetic-aperture sonar (SAS) systems are outlined. Side-look sonar is presented as a real-beam imaging technique along with expressions for relevant system- and image-related parameters. Synthetic-aperture sonar is discussed, and the limitations imposed by the speed of sound in the ocean environment are identified. A specific side-look system (SeaMARC I) is presented under two configurations and comparable SAS designs are proposed. Based on the examples provided by the SeaMARC I system and the hypothetical SAS designs, it is shown that single-beam SAS systems can be designed to achieve area coverage rates comparable to single-beam side-scan systems, yet with improved azimuth resolution