Xavier Lurton

An Introduction to Underwater Acoustics

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

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Swath bathymetry using phase difference: theoretical analysis of acoustical measurement precision

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.

A Measurement Quality Factor for Swath Bathymetry Sounders

The phase difference principle is widely applied nowadays to sonar systems used for sea floor bathymetry. The apparent angle of a target point is obtained from the phase difference measured between two close receiving arrays. Here we study the influence of the phase difference estimation errors caused by the physical structure of the backscattered signals. It is shown that, under certain current conditions, beyond the commonly considered effects of additive external noise and baseline decorrelation, the processing may be affected by the shifting footprint effect: this is due to the fact that the two interferometer receivers get simultaneous echo contributions coming from slightly shifted seabed parts, which results in a degradation of the signal coherence and, hence, of the phase difference measurement. This geometrical effect is described analytically and checked with numerical simulations, both for square- and sine-shaped signal envelopes, its relative influence depends on the geometrical configuration and receiver spacing; it may be prevalent in practical cases associated with bathymetric sonars. The cases of square and smooth signal envelopes are both considered. The measurements close to nadir, which are known to be especially difficult with interferometry systems, are addressed in particular.

Analysis of multibeam echo-sounder signals from the deep seafloor

The quality estimation associated with individual soundings measured and computed by swath bathymetry sonars is a paramount issue which is most often imperfectly addressed today by sonar manufacturers. In this paper, a unified definition is proposed for a quality factor usable for all swath bathymetry sonars; the depth-relative error is directly estimated from the signal characteristics used in the sounding computation. The basic algorithms are presented for both phase-difference (oblique incidence) and amplitude (normal incidence) detection, and can be readily implemented in any swath bathymetry system using these detection principles. This approach gives a direct access to the objective bathymetric performance of individual soundings, and hence avoids the explicit estimation of intermediate quantities such as the signal-to-noise ratio (SNR). It makes possible intercomparisons between different systems (multibeam sounders versus interferometric sonars) and processing principles (phase difference versus amplitude); it is expected to provide a useful input to bathymetry postprocessing software using quality estimation of individual soundings.

Backscattering from buried sediment layers: The equivalent input backscattering strength model

The new generation low-frequency echo-sounders, covering the full angular range, are also able to record acoustic images comparable to side-scan sonars. Possibilities of using these systems for sea-floor type identification are currently being investigated. Experimental echoes obtained with a 13 kHz multibeam echo-sounder have been collected and processed for a variety of sea-floor types and water depths. The results of two different analysis methods are presented. The backscattering strength of the sea-floor and its angular variations are known to be strongly dependent on the seabed type. They may be obtained as a function of incident angle from the measured levels averaged in narrow tilted beams, after correcting for the insonified area, the refraction by the sound-speed profile and the sea-floor local slopes. The results of this method make clear large differences between the various investigated seabed classes. The spectral features of backscattered time signals have previously been shown to be a useful tool for identification by side-scan sonars. The authors present the first results obtained by this method for deep sea-floor multibeam echoes: notable discrepancies between the spectral parameters corresponding to various seabeds are shown, enhancing the possibility of using such a technique for low-frequency echo-sounders.<<ETX>>

Automated Sea-bed Classification System For Echo-Sounders

Signals received by low-frequency multibeam echosounders are strongly affected by sound penetration inside the upper sediment layers and by backscattering from buried layers down to depths of a few meters; this may lead to serious ambiguities and misinterpretations of experimental data. These phenomena are modeled here using a concept of equivalent input backscattering strength (EIBS), based on a combination of classical models of local backscattering strength and propagation inside fluid layered media. The local backscattering strength at a buried interface is expressed first to account for the impedance adaptation due to the overlying layers, for the angular refraction effects due to the velocity profile, and for the layered structure of the underlying medium. It is then transferred to the upper water–sediment interface, accounting for propagation inside the layered stack; the transfer coefficient is obtained from the classical theory of plane wave propagation in layered media. The volume backscattering...

Recommendations for improved and coherent acquisition and processing of backscatter data from seafloor-mapping sonars

A method is proposed for real-time identification of the continental shelf sea-bottom type, using signal time envelopes from standard echo- sounders for fisheries and bathymetry. The method consists in comparing the reverberated signals with a set of references computed for a given sounder and a variety of water heights and sea bottom types. The basic method is at first presented. The experimental setup used for in-situ measurements is then described; the results of at-sea campaigns are finally presented and commented. 1. ECHO-SOUNDERS BACKSCATTERED SIGNALS

Applications of an image segmentation technique to multibeam echo-sounder data

Multibeam echosounders are becoming widespread for the purposes of seafloor bathymetry mapping, but the acquisition and the use of seafloor backscatter measurements, acquired simultaneously with the bathymetric data, are still insufficiently understood, controlled and standardized. This presents an obstacle to well-accepted, standardized analysis and application by end users. The Marine Geological and Biological Habitat Mapping group (Geohab.org) has long recognized the need for better coherence and common agreement on acquisition, processing and interpretation of seafloor backscatter data, and established the Backscatter Working Group (BSWG) in May 2013. This paper presents an overview of this initiative, the mandate, structure and program of the working group, and a synopsis of the BSWG Guidelines and Recommendations to date. The paper includes (1) an overview of the current status in sensors and techniques available in seafloor backscatter data from multibeam sonars; (2) the presentation of the BSWG structure and results; (3) recommendations to operators, end-users, sonar manufacturers, and software developers using sonar backscatter for seafloor-mapping applications, for best practice methods and approaches for data acquisition and processing; and (4) a discussion on the development needs for future systems and data processing. We propose for the first time a nomenclature of backscatter processing levels that affords a means to accurately and efficiently describe the data processing status, and to facilitate comparisons of final products from various origins.

The feasibility of a very-long baseline acoustic positioning system for AUVs

A segmentation algorithm has recently been presented for seafloor characterization from multibeam echosounder sonar images, accounting for angular variations of the backscattered signals obtained from training zones. This approach allows one to suppress the image effects due to seafloor topography and angle dependance. It is applied here to some real cases both in deep and shallow water, with data from two different echo-sounders at 13 kHz and 95 kHz. For every case the raw image, the angular backscattering patterns obtained on training zones, and the segmentation results are presented. The various limitations of the method are finally discussed: difficulties around the vertical of the sonar, too low number of training data, high dependence of the results on the quality of the echo-sounder processes and corrections. It is shown that such a segmentation is really effective despite non optimal data and difficult geological configurations.