Comparison of methods employed to extract information contained in seafloor backscatter

International audienceSeabed maps are based on quantities extracted from measurements of the seafloor‘s acoustic response by sonar systems such as single-beam echo-sounders (SBES), multibeam echo-sounders (MBES) or sidescan sonars (SSS). In this paper, a comparison of various strategies to estimate the backscattering strength (BS) from recorded time-series, i.e. seabed echoes extracted from pings, is presented. The work hypotheses are based on processed data from a SBES designed to be tilted mechanically. Ideal survey conditions are taken into account and the seafloor is supposed to be rough so that BS is assumed to be equivalent to the Rayleigh probability density function parameter. Classical methods such as averaging corrected (sonar equation) backscattered single values over a set of pings to estimate BS are compared to other methods exploiting several time-samples being part of pings. Simulated data is considered to estimate BS in different situations (several estimators, natural/squared values, number of samples and pings). The best estimator to reach a 0.1dB uncertainty is proposed, and a formula governing the number of time-samples and pings needed to reach an accurate BS estimation according to the measurement conditions is derived

Caractérisation précise de la réponse acoustique du fond marin pour l’amélioration de l’identification des fonds

Echosounders are commonly used in hydrography to measure bathymetry; they can also provide seabed reflectivity information based on the measurement of the seafloor acoustic response. The aim of the PhD is to provide a detailed description of the seafloor acoustic response metrology based on both theoretical and practical states of the art. Empirical methods to compute the backscattering strength, developed in operations by users, softwares or echosounder manufacturers, are confronted to theoretical statements from literature. The link between the two domains is made using a stochastic model based on the bathymetric processing of singlebeam and multibeam echosounders (respectively SBES and MBES). From this model, an accurate definition of the seafloor acoustic response is proposed and estimators of the backscattering strength are derived. The best estimator is then identified by comparing their bias, variances, and speeds of convergence. Finally, a measure of the accuracy of backscattering strength acquisitions with SBES or MBES is proposed, based on the uncertainty of the measurements. This uncertainty can be used as an a priori information about the quality of a planned survey, or as an a posteriori indication of precision of the seafloor response measurements.Les échosondeurs sont aujourd’hui largement utilisés en hydrographie pour mesurer la bathymétrie ; ils fournissent une information de réflectivité du fond à partir de la mesure de la réponse acoustique du fond marin. L’objectif de cette thèse est de proposer une description détaillée de la métrologie de la réponse acoustique des fonds fondée sur les états de l’art théoriques et opérationnels. Les méthodes empiriques pour calculer l’index de rétrodiffusion, développées en opération par les utilisateurs, logiciels, ou les fabricants d’échosondeurs, sont confrontées aux considérations de la littérature. Le lien entre les deux domaines est fait en utilisant un modèle stochastique se basant sur le traitement bathymétrique des sondeurs mono- et multi-faisceaux. À partir de ce modèle, une définition précise de la réponse acoustique du fond marin est proposée et des estimateurs de l’index de rétrodiffusion sont calculés. Le meilleur estimateur est ensuite identifié par comparaison des biais, variances et rapidité de convergence. Enfin, une mesure de la précision de l’acquisition des index de rétrodiffusion avec des sondeurs mono- et multi-faisceaux est proposée, basée sur l’incertitude de la mesure. Cette incertitude peut être utilisée en tant qu’information a priori sur la qualité d’un levé planifié, ou comme une indication a posteriori sur la précision de la mesure de la réponse du fond marin

Study of underwater biodiversity in Lake Guerlédan (France) using acoustic systems: A student project at ENSTA Bretagne

International audienceSeit 2016 nehmen jedes Jahr Studierende der ENSTA Bretagne, die auf Hydrographie/Ozeanographie und Meeresrobotik spezialisiert sind, an einem Feldprojekt am Guerlédan-See in der Bretagne (Frankreich) teil. Eines der Themen, an denen sie arbeiten, ist die Untersuchung der Artenvielfalt im See mit Hilfe akustischer Systeme. In Zusammenarbeit mit Forschungsinstituten wie Ifremer und unter der Leitung ihrer Beraterin Irène Mopin setzen sie professionelle Echolote ein, um die Echos aus der Wassersäule in Tiefen bis zu 40 m zu analysieren. Jedes Jahr werden die Ergebnisse der vorherigen Gruppe genutzt, um die Verarbeitungstechniken zu verbessern, und es kommen neue Besonderheiten hinzu, wie zum Beispiel die Beschallung der Daten.Each year since 2016, students of ENSTA Bretagne specialised in hydrography/oceanography and marine robotics participate to a field project on Lake Guerlédan in Brittany (France). One of the subjects they work on is the study of biodiversity in the lake using acoustic systems. In collaboration with research institutes such as Ifremer and directed by their advisor Irène Mopin, they are using professional echo sounders in order to implement analyses of water column echoes in depths up to 40 m. Each year, results of the previous group are used to improve processing techniques and new specificities are added such as the sonification of data

Le Furieux : Analyse acoustique d’une épave semie-enfouie en petit fond.

International audienc

Caractérisation précise de la réponse acoustique du fond marin pour l’amélioration de l’identification des fonds

Echosounders are commonly used in hydrography to measure bathymetry; they can also provide seabed reflectivity information based on the measurement of the seafloor acoustic response. The aim of the PhD is to provide a detailed description of the seafloor acoustic response metrology based on both theoretical and practical states of the art. Empirical methods to compute the backscattering strength, developed in operations by users, softwares or echosounder manufacturers, are confronted to theoretical statements from literature. The link between the two domains is made using a stochastic model based on the bathymetric processing of singlebeam and multibeam echosounders (respectively SBES and MBES). From this model, an accurate definition of the seafloor acoustic response is proposed and estimators of the backscattering strength are derived. The best estimator is then identified by comparing their bias, variances, and speeds of convergence. Finally, a measure of the accuracy of backscattering strength acquisitions with SBES or MBES is proposed, based on the uncertainty of the measurements. This uncertainty can be used as an a priori information about the quality of a planned survey, or as an a posteriori indication of precision of the seafloor response measurements.Les échosondeurs sont aujourd’hui largement utilisés en hydrographie pour mesurer la bathymétrie ; ils fournissent une information de réflectivité du fond à partir de la mesure de la réponse acoustique du fond marin. L’objectif de cette thèse est de proposer une description détaillée de la métrologie de la réponse acoustique des fonds fondée sur les états de l’art théoriques et opérationnels. Les méthodes empiriques pour calculer l’index de rétrodiffusion, développées en opération par les utilisateurs, logiciels, ou les fabricants d’échosondeurs, sont confrontées aux considérations de la littérature. Le lien entre les deux domaines est fait en utilisant un modèle stochastique se basant sur le traitement bathymétrique des sondeurs mono- et multi-faisceaux. À partir de ce modèle, une définition précise de la réponse acoustique du fond marin est proposée et des estimateurs de l’index de rétrodiffusion sont calculés. Le meilleur estimateur est ensuite identifié par comparaison des biais, variances et rapidité de convergence. Enfin, une mesure de la précision de l’acquisition des index de rétrodiffusion avec des sondeurs mono- et multi-faisceaux est proposée, basée sur l’incertitude de la mesure. Cette incertitude peut être utilisée en tant qu’information a priori sur la qualité d’un levé planifié, ou comme une indication a posteriori sur la précision de la mesure de la réponse du fond marin

Détection et quantification par acoustique des organismes zooplanctoniques à l’embouchure de l’Elorn

International audienc

Dimensionnement du système SCAMPI pour AUV et détection d’objets enfouis

International audienc

HF radar spectrum comparison method for current maps analysis

International audienceOceanographic HF radar allows todescribe the hydrodynamics of a coastal area by providing a synoptic surface current map at high frequency and regular time steps (10mn). Its interest lies in its wide cover,the high frequency of acquisition and the regularity of the measurements (regardless of weather conditions). Nevertheless, the estimation of surface currents is not straightforward and requires processing of the data acquired by radarin order to extractsurface radial current. The methods involving a beamforming or methods known as direction finding are the most used in this scope. Unfortunately, these methods do not provide exactly unambiguous estimations. In addition, HF radar can be associate to others sensors (ADCP, satellites, etc.) or methods (numerical models) that also provide relevant information about the area of interest. Thus, the analysis of hydrodynamics behavior at various time scales (from tidal to pluriannual time scale) requires the analysis of all these sources of information in order to better catch and possibly improve the knowledge of this key area in between the bay of Biscay and the Celtic Sea.In this presentation, a method which intends to compare a current map to HF radar datawill be introduced. The surface current could come from any sensors that provides surface current map or from numerical simulations. This comparison is directly performed with the low level data acquired by the radars ; we would rather avoid the comparison to geophysical data (current themselves) in order to prevent from information loss due to the estimation process or due to geometrical configurations of the radars (geometrical dilution of precision). Indeed, this comparison is performed thanks to an analysis of the radar Doppler spectrum. Hence the comparison is done by radar and therefore only the radial component is studied at the same time. This presentation will focus on the impact of the met-oceanic (wind, waves, tides) conditions and will highlight the main differences observed from current map estimating from HF radar itself and from a high resolution numerical simulation. The area of interest for this study is the Iroise Sea on the west of Brittany in France where data have been acquiring since 2006. This large amount of data covering a wide span of environmental conditions has been fundamental to investigatethis area in a wide variety of hydrodynamical situations

Amélioration de la connaissance de la mesure des courants de surface par radar HF

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