Sidescan Sonar Image Enchancement Using a Decomposition Based on Orthogonal Functions. Applications with Chebyshev Polynomials

A method is presented to remove from sidescan sonar images of the seafloor, artifacts that are clearly unrelated to the backscattering properties of the seafloor. A spectral analysis performed on a ping by ping basis proved to be well suited to the problem. The technique relies on a decomposition using Chebyshev polynomials. This stochastic method does not require a priori knowledge of deterministic parameters. It deals with the low spatial frequency components of the image whose wavelengths are not very small compared to the swath width. Applications to sidescan sonar images obtained with the SeaMARC LI system are presented

Differential Phase Estimation with the SeaMARC II Bathymetric Sidescan Sonar System

A maximum-likelihood estimator is used to extract differential phase measurements from noisy seafloor echoes received at pairs of transducers mounted on either side of the SeaMARC II bathymetricsidescan sonar system. Carrier frequencies for each side are about 1 kHz apart, and echoes from a transmitted pulse 2 ms long are analyzed. For each side, phase difference sequences are derived from the full complex data consisting of base-banded and digitized quadrature components of the received echoes. With less bias and a lower variance, this method is shown to be more efficient than a uniform mean estimator. It also does not exhibit the angular or time ambiguities commonly found in the histogram method used in the SeaMARC II system. A figure for the estimation uncertainty of the phasedifference is presented, and results are obtained for both real and simulated data. Based on this error estimate and an empirical verification derived through coherent ping stacking, a single filter length of 100 ms is chosen for data processing application

Sparse underwater acoustic imaging: a case study

International audienceUnderwater acoustic imaging is traditionally performed with beam- forming: beams are formed at emission to insonify limited angular regions; beams are (synthetically) formed at reception to form the image. We propose to exploit a natural sparsity prior to perform 3D underwater imaging using a newly built flexible-configuration sonar device. The computational challenges raised by the high- dimensionality of the problem are highlighted, and we describe a strategy to overcome them. As a proof of concept, the proposed approach is used on real data acquired with the new sonar to obtain an image of an underwater target. We discuss the merits of the obtained image in comparison with standard beamforming, as well as the main challenges lying ahead, and the bottlenecks that will need to be solved before sparse methods can be fully exploited in the context of underwater compressed 3D sonar imaging

Underwater acoustic imaging: sparse models and implementation issues

Projet ANR : ANR-09-EMER-001International audienceWe present recent work on sparse models for underwater acoustic imaging and on implementation of imaging methods with real data. By considering physical issues like non-isotropic scattering and non-optimal calibration, we have designed several structured sparse models. Greedy algorithms are used to estimate the sparse representations. Our work includes the design of real experiments in a tank. Several series of data have been collected and processed. For such a realistic scenario, data and representations live in high-dimensional spaces. We introduce algorithmic adaptations to deal with the resulting computational issues. The imaging results obtained by our methods are finally compared to standard beamforming imaging

Sparse reconstruction techniques for near-field underwater acoustic imaging

International audienceThe use of sparse priors has shown interesting potential in various acoustic or radar imaging applications. In this paper, sparse reconstruction is applied for underwater acoustic imaging using a newly built flexible sonar device. We investigate several models concerning the linear mapping between the image domain and the observation domain. In particular, we define a point-scatterer model in which the apparent back-scatter coefficient of a given reflector varies with respect to the specific emitter and receiver locations. To handle this problem, we adapt a multi-channel version of the orthogonal matching pursuit and we apply it on real data in order to obtain images of an underwater target placed at a small distance from the sonar. The techniques are shown to overcome bottlenecks that are apparent with more standard approaches that assume far-field conditions when building the image

Microsoft Word - Array_calib_web.doc

Abstract -A method to calibrate the elements of large arrays devoted to underwater applications is presented. The goal is to measure the sensitivity and directivity of the elements over their full bandwidth. The main constraint comes from the bounded geometry of the experimental setups that limits the duration of the time windows available for analyzing the received signals. Using short wideband pulse is detrimental to obtaining high signal-to-noise ratios. A classical method for handling this problem is time-delay spectrometry (TDS), which is based on the transmission of a linear frequency-modulated signal combined with a sliding frequency filter. An alternative, hybrid method based on the transmission of a sequence of time-frequency-limited signals is proposed. This hybrid method is shown to provide the same spectral density as TDS in the frequency scanning, but the filtering process is quite different. The transmitted signals are designed to take advantage of the coherent sums of the received signals to track the time of flight of the direct paths between the source and the elements. In addition, a fitting process based on the calibration geometry of data acquisition enables the boundaries of the interference-free time windows to be precisely delineated. An example of the application is described. Index Terms -calibration of array elements, time delay spectrometry, time-of-flight estimation. I. INTRODUCTION The correct tuning of underwater acoustical surveying tools (e.g., mine warfare sonar systems and multibeam echosounders) relies on the precise determination of the directivity and sensitivity of the transducer elements. Hence, many efforts have been made to improve the accuracy of the sonar calibrations achieved in a tank (e.g., [1] and [2]). This paper addresses the characterization of a receiving array using an external source. Adverse but common measurement environments are limited tank boundaries, large-sized arrays to be characterized, and limited telemetry. In such a context, a method is presented to derive the directivity and sensitivity of array elements over a wide angular sector and large bandwidth properly. The geometry of the experimental setup raises several practical difficulties. The size of the arrays used in underwater applications can be relatively large. Because it is advantageous to make the calibration in the far-field area, there must be a sufficient distance between the source and the array. It is also convenient to limit the parallax biases: the post-processing stage must compensate for these differences in the angles of view from the different elements of the array to the source. On the other hand, the source levels used for calibration purpose must be large enough to ensure accurate measurements. Hence, directional sources are more efficient than omnidirectional hydrophones. Although the source far-field condition is usually easy to meet, one expects also that the acoustic field would be as uniform as possible over the entire extent of the receiving area. Hence, the use of directional sources tightens the distance requirement. Given the distance between the source and the receiving array, there is a limited delay between the arrival of the signal through the direct path and the next arrival of the signal coming from the first reflection from the walls of the tank or the free surface. This delay decreases as the distance between the source and the receiver increases, thereby reducing the size of the time window available to process the direct signal without interference. Hence, measurement range and length of the time window are competing parameters. The accuracy in the estimation of the times of flight is critical whenever the calibration P. Cervenka is with th

Sonar frontal pour l'imagerie par synthese non-coherente et la bathymetrie

PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Sidescan Sonar Image Processing Techniques

A four-step processing sequence is described to produce image mosaics from the various segments of a sidescanned acoustic imaging survey of a given seafloor area. Starting with data consisting for each ping of acoustic backscatter levels versus horizontal range across-track, median prefiltering is used first to reduce the influence of outliers on subsequent linear processes. Artifacts that are clearly unrelated to the backscattering properties of the seafloor are then isolated on a ping by ping basis through a spectral analysis that relies on a decomposition using Chebyshev polynomials to filter the low spatial frequency components of the image. Contrast enhancement is then achieved through an original implementation of the classical gray level histogram equalization technique by balancing local versus global histogram contributions. Pixels are mapped on a geographic grid taking due account of the geometry of the measurement and of the spacing between pings to minimize along-track smearing of features. Examples of results obtained with these processing techniques are given for SeaMARC II data recorded during a complete survey of Fieberling Guyot (32°.5 N, 128° W

Postprocessing and Corrections of Bathymetry Derived from Sidscan Sonar Systems: Application with SeaMARC II

A procedure for postprocessing bathymetry data provided by a phase-measuring sidescan sonarsystem is presented. The data were collected with the SeaMARC II system, and are generally characterized by a high level of noise and uneven spatial sampling. Before any spatial filtering is applied, data are selected to remove most of the obvious artifacts and to retain instantaneous depth profiles whose slant ranges increase monotonically from a central location to the edges of the swath. An extrapolation scheme, patterned after a potential field, is proposed to fill gaps in the coverage or to extend the bathymetric swath to that of the corresponding sidescan image when regridding the data to a rectangular frame. To fill the near nadir gap typically found in these data, a specific interpolation methodology is developed that takes into account the slant range of the first bottom return as received by the sidescan sonar itself or by a shipboard echo-sounder. Spatial low-pass filtering is applied through convolutions with parabolic windows whose width is proportional to the footprint of the acoustic beam along track and roughly 1/8 of the swath width across track. Mismatches of contour lines between adjacent tracks are reduced through a statistical method design to correct systematic profile errors

Geoacoustic characterization by the image source method: a sensitivity study

A new method for measuring the sound speed profile of the seafloor has been recently proposed (JASA, vol. 128, pp. 1685-1693): the image source method. This method is based on a physical model of the acoustic field generated by a point source and reflected by a layered media. Under the Born approximation, the reflected signal can be modeled as a sum of contributions coming from image sources relative to the seabed layers. Consequently, the seabed geometry and sound speed profile can be recovered by exploiting the localization of these images. We present here a study about the sensitivity with the relative noise level and the map mesh size in the localization of the image sources