Meter-Scale Seafloor Geodetic Measurements Obtained from Repeated Multibeam Sidescan Surveys

Abstract

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3?km by 20?km) on the port and starboard side of the ship are used to estimate vertical and across-track displacement. We present displacement measurements from a survey of the Ayu Trough southwest of the Marianas Trench using a 12?kHz multibeam. Vertical and across-track displacement errors for the 12?kHz multibeam sonar are typically 0–2?m with RMS uncertainties of 0.25–0.67 m in the across-track and 0.37–0.75 m in the vertical as determined by 3-way closure tests. The uncertainty of the range-averaged sound velocity is a major error source. We estimate that variations in the sound velocity profile, as quantified using expendable bathythermographs (XBTs) during data collection, contribute up to 0.3?m RMS uncertainty in the across-track direction and 1.6?m RMS uncertainty in the vertical direction.     

Perturbative inversion of geoacoustic parameters in a shallow waterenvironment

In many strategic shallow water areas, the geoacoustic properties of the sub-bottom are largely unknown. This paper demonstrates that inverse theory and measured data from a single hydrophone can be used to accurately deduce the geoacoustic properties of the sub-bottom, even when the initial background geoacoustic model is a highly inaccurate estimate. Since propagation in shallow water is very sensitive to the geoacoustic properties of the sub-bottom the inverse technique is a vitally important, practical, and inexpensive means to improve sonar performance prediction in a potentially hostile environment. To provide ground truth for the inverse technique, measured data collected during Project GEMINI were compared to the inverse solutions. Detailed, site-specific geoacoustic models were developed for two array locations and the finite-element parabolic equation (FEPE) model was used to estimate transmission loss (TL). The model estimates from FEPE compared well with the measured data and the detailed geoacoustic models were considered as “ground truth.” To test the efficacy of the technique, initial background geoacoustic models were constructed assuming no a priori information of the bottom. The resultant inverse solution was used to predict the geoacoustic properties at each of the sites. The final results were in excellent agreement with the measured data and the resulting TL estimates derived from the inverse technique were as good or better than the TL estimates obtained from the detailed, site-specific geoacoustic models     

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。     

Sensor position estimation and source ranging in a shallow water environment

Underwater acoustic transient signals are generated mechanically at known positions along a wharf. These signals are received by a wide aperture planar array of four underwater acoustic sensors, whose positions relative to the wharf are unknown. A method is described that enables the positions of the sensors to be estimated from accurate differential time-of-arrival measurements (with 0.1 /spl mu/s precision) as the signal wavefronts traverse the array. A comparison of the estimated positions with the nominal positions of the first three sensors, which form a 20-m-wide aperture horizontal line array, reveals a 2-cm displacement of the middle sensor from the line array axis. This slight bowing of the line array results in overranging (bias error of 3%) when the wavefront curvature method is used with the nominal collinear sensor positions to locate a static source of active sonar transmissions at a range of 59.2 m. The use of the spherical intersection method coupled with the estimated sensor positions of the line array provides an order of magnitude improvement in the range estimate (within 0.3% of the actual value). However, systematic ranging errors are observed when the sound propagation medium becomes nonstationary. Next, the differences in the arrival times of the direct path and boundary-reflected path signals at the middle sensor of the wide aperture line array are estimated using the differential phase residue of the analytic signal at the sensor output. These multipath delays are used to estimate the range and depth of the source. Although the average value of the multipath range estimates is within 0.5% of the actual value, the variance of the range estimates is 50 times larger when compared with the results of the spherical intersection and wavefront curvature methods. The multipath delay data are also processed to provide a reliable estimate of the temporal variation in the water depth enabling the tidal variation to be observed.     

XTF格式侧扫声纳数据质量评价

通过深入分析XTF格式侧扫声纳数据结构,选取了XTF格式侧扫声纳数据中的定位信息、时间信息、信号强度、船速、测线平直度及数据完整性6个要素作为质量评价指标,开发了XTF格式侧扫声纳数据质量评价软件系统,实现了对批量XTF格式侧扫声纳数据的计算机自动质量评价,具有较强的实用性。     

Optimal estimation of ship's attitudes for beampattern corrections in a coaxial circular array

A study is conducted to estimate the accurate attitude of a ship's motion and the estimation is used to arrive at the corrections required for a farfield pattern of a coaxial circular array. The relevant analytical expression is developed for the computation of moving average filter weights to estimate the ship's attitude for beamsteerings at different sea conditions. Farfield patterns are presented for different steered directions for the coaxial circular array of the additive type, and the multiplicative array technique is also used for beamsteering for higher steering angles to observe its control on the sidelobes as well as for the main beamwidths. In order to introduce an adaptive filter to estimate the accurate attitude for stable sonar platform, an adaptive parameter is examined for bathymetric applications where nonstationary ship movement prevails.     

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     

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.      

Effective parameters for matched field geoacoustic inversion in range-dependent environments

Using the well-established technique of geoacoustic inversion, one can estimate a set of acoustic sea-bed parameters from sonar array data. Simultaneously, one can search for geometric parameters such as range, water depth, and hydrophone depth. When the technique is applied in a range-dependent environment, there is a potentially much larger set of parameters to match, unless one has perfect knowledge of the bathymetry. From the point of view of optimization, one needs to handle uncertainties in bathymetry without hugely increasing the amount of computation. A simple time-domain view (which is shown to be equivalent to the adiabatic approximation) suggests that it is sufficient to use a range-independent model with an empirical "effective" depth even when the bottom is not flat. In fact, there is a set of effective environments that will suffice; one can choose whichever is the most convenient. The success of this concept is demonstrated with some test cases from a recent Geoacoustic Inversion Techniques Workshop.     

The importance of habitat features in low-relief continental shelf environments

Benthic habitats can be loosely divided into low and high relief based upon gross bottom topography. While fish community studies in high relief environments such as coral reefs have been extensively studied using in situ visual analysis, most studies of low relief environments have utilized trawl-based methodologies. Such studies while valuable, are unable to detect small-scale variations in habitat availability and faunal responses to that variation. This paper presents the availability of other techniques, ranging from visual surveys to sonar and bathymetric analysis, which allow researchers to address questions of habitat utilization in low relief environments.     

Analysis of swath bathymetry sonar accuracy

The practical limitations of many bottom mapping sonars lie in their ability to accurately estimate the angle of arrival. This paper addresses the accuracy of angle estimation when employed to determine the location of an extended target such as the bottom. A Gaussian model is assumed for the bottom backscatter and the corresponding Cramer-Rao lower bound for the variance of the angle estimate is determined for multi-element linear arrays. The paper focuses on determining the performance of high-resolution swath bathymetry sonars and, therefore, concentrates on the ability to determine bottom location with short pulses. Two error mechanisms, footprint shift and uncorrelated noise, are identified as important contributors to measurement errors. The two-element interferometric sonar configuration is investigated in detail. It is shown through the use of probability distributions, the Cramer-Rao bound, and simulation that it is difficult to get a good estimate of performance through simulation alone. Performance enhancement through pre-estimation and post-estimation averaging of multiple snapshots and changes in performance with pulse length and pulse rise time are also considered. Bottom estimation performance employing multi-element arrays is compared and contrasted with that of the two-element interferometric array. It is determined that there is little benefit associated with the multi-element array in terms of angle estimation performance alone. However, when other considerations such as angle ambiguities, multiple angles of arrival, and physical shortcomings associated with practical arrays are taken into account, the multi-element array is favored.     

Shape from shading—a method of integration of sonar images and bathymetric data for ocean mapping

Shape from shading is one of the methods that derive geometric information of objects from analysis of monocular images. Application of this technique to underwater sonar images enables the conversion of imposed reflectance characteristics in sonar images to shape information, namely, slopes, about the seafloor surface. A combination of this shape information and available sparse distributed depth points results in improved dense bathymetric data.

The reconstruction of shape models of seafloor surfaces from sonar images is treated as an inverse problem and is solved by the regularization theory. Sparse gridded points are used for boundary constraints. The regularization is implemented as a relaxation procedure with a hierarchical structure of multiresolution grids.     

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     

Methods of estimating allowable motion perturbations in side-scansonar systems

This document describes the effects of nonideal hydrophone array motion on sonar image quality and derives the allowable motion errors for vehicles carrying side-scan sonar systems. Estimates of the quantity of motion that causes just noticeable sensor performance degradation as well as those that render the sensors useless are derived using two different methods. Using a special resolution target, examples of simulated image distortion resulting from several types of motion are presented. A method for quantitatively measuring the actual image distortion in situ is described     

Using McDaniel's model to represent non-Rayleigh reverberation

Reverberation in low-frequency active sonar systems operating in shallow water has often been observed to follow non-Rayleigh statistical distributions. McDaniel's model, generalized to allow noninteger valued parameters, has shown promise as being capable of accurately representing real data with a minimal parameterization. This paper first derives an exact analytical expression for the cumulative distribution function (CDF) of the generalized McDaniel model and then compares it with numerical inversion of the characteristic function. Both methods are seen to provide adequate and equivalent precision; however the characteristic function inversion method is significantly faster. The latter CDF evaluation technique is then applied to the analysis of simulated and real data to show that, when minimal data are available, McDaniel's model can more accurately represent a wide variety of non-Rayleigh reverberation than the K or Rayleigh mixture models. This result arises from the generality of McDaniel's model with respect to the K-distribution (i.e., the K-distribution P_fa estimate can be dominated by model mismatch error) and to its compact parameterization with respect to the Rayleigh mixture (i.e., the Rayleigh mixture model P_fa estimate is usually dominated by parameter estimation error)     

Motion-Compensation Improvement for Widebeam, Multiple-Receiver SAS Systems

The effect that uncompensated motion errors have on synthetic aperture sonar (SAS) imagery can be severe. Time-domain beamforming SAS reconstruction is able to compensate arbitrary track errors, but the more efficient frequency-domain reconstruction algorithms, such as the range-Doppler, chirp-scaling, and wave number (aka range migration or Stolt-mapping) algorithms do not allow for simple compensation, especially for widebeam sonars. Data processed via these block algorithms is usually compensated before azimuth compression in a computationally inexpensive preprocessing step. Unfortunately, this compensation assumes a narrowbeam geometry, causing blurring in high-resolution images collected with widebeam sonars. In this paper, we demonstrate a new technique for compensation of large, but known, motion errors in data collected with widebeam geometry sonars. The technique relies on obtaining angle-of-arrival information from the multiple-receiver array configuration typical in high-resolution SAS systems. The new method of compensating for motion errors was found to significantly outperform the previous techniques in a simulation of point-reflector imagery.      

Seafloor profiling by a wideband sonar: simulation,frequency-response optimization, and results of a brief sea test

An ahead-looking probe of some kind, optical or acoustic, is critical when one is attempting seafloor exploration from a mobile platform. A single-frequency, split aperture sonar system can be used for this purpose, but a wideband monopulse sonar offers many advantages. It computes a running estimate of the vertical directional cosine of the source of the echo, and can thus reveal the positions of multiple wave scatterers as long as their echoes can still be time resolved. Theoretical studies of its performance have been made previously, but were directly applicable only to extremely simple seafloor geometries. A new time-domain digital simulation that largely circumvents this limitation has been developed. The simulation also provides a means for testing the theory and optimizing system parameters. The reverberation model does not account for some features of acoustic backscattering such as diffraction, but it is believed to be adequate for the investigation of most signal processing aspects of the sonar system. The theory of the simulation is developed and several examples are presented and discussed. In addition, some preliminary results are presented from a sea test that used the air-sea interface as a surrogate seafloor     

In Situ Alignment Calibration of Attitude and Doppler Sensors for Precision Underwater Vehicle Navigation: Theory and Experiment

This paper reports the development and experimental evaluation of two in situ least squares techniques for estimating the alignment matrix of Doppler sonars commonly used for precision navigation of oceanographic submersibles. Most previously reported methods addressed the problem of single degree-of-freedom heading alignment using bottom-lock Doppler sonar data and global positioning system (GPS) navigation data. This paper reports and evaluates two techniques for three degree-of-freedom calibration of attitude and Doppler sonar sensors using sensor data available to vehicles at full ocean depth. The first technique provides a general linear least squares estimate of the alignment matrix. The second technique results in a least squares alignment matrix estimate constrained to the group of rotation matrices. The performance of these estimates is evaluated with a laboratory remotely operated vehicle (ROV) and a field-deployed autonomous underwater vehicle (AUV). Experimental results are reported which demonstrate that Doppler navigation employing the reported alignment calibration techniques significantly improves navigation precision. The experiments show that the latter technique provides calibration estimates that improve Doppler navigation precision not only on the calibration data set itself, but also provide improved precision over a wide variety of vehicle trajectories other than the calibration data set.     

Relative Height Estimation by Cross-Correlating Ground-Range Synthetic Aperture Sonar Images

The relative height of the seafloor can be estimated by using two vertically displaced receivers. In this paper, we propose techniques to improve the accuracy of the estimated height. Our results are based on the use of synthetic aperture sonar (SAS) imaging, which implies coherent addition of complex images acquired from a moving platform. The SAS processing improves the along-track (or azimuth) resolution, as well as the signal-to-noise ratio (SNR), which in turn improves the estimated height accuracy. We show that the shift of the effective center frequency induced by coherent, frequency-dependent scattering affect the time-delay estimates from complex cross correlations, and we propose a correction technique for broadband signals with uneven magnitude spectra. To reduce the effect of geometrical decorrelation and increase the coherence between the images, we beamform the sonar images onto an a priori estimate of the seafloor height before correlating. We develop a mathematical model for the imaging geometry. Finally, we demonstrate our proposed estimators by providing relative seafloor height estimates from real aperture and SAS images, obtained during the InSAS-2000 experiment at Elba Island in Italy. In particular, we demonstrate that the SAS image quality is significantly improved by inclusion of the height estimates as a priori information.     

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