Unsupervised calibrated sonar imaging for seabed observation using hidden Markov random fields

International audienceThis paper deals with seabed imaging issued from sonar systems. Such imaging systems produce images of backscattering (BS) strength relative to physical seabed characteristics. However, these Bs measurements are not only seabed-related but also dependent on the incident angle. Therefore, to enhance the quality of such seabed imaging systems, we develop an unsupervised approach to compensate for these seabed-related angular dependencies. Our approach combines robust estimation and hidden Markov random fields. Results on real data demonstrate the relevance of our approach to improve seabed observation

A Quality Estimator of Acoustic Sounding Detection

Swath sonar bathymetry accuracy depends on the intrinsic performance of acoustic signal processing. We propose here a quality factor, quantifying the accuracy associated with every sounding computation. This descriptor is derived from simple models either for amplitude (variance of the centre-of-gravity instant of a fluctuating bell-shaped envelope) or for interferometric phase (local variance for a number of processed samples). The purpose is to attach to each individual sounding an objective quality level that is sonar independent, and directly applicable in bathymetry processing, either in data editing, or as an input parameter to statistical post-processing. This concept is illustrated by examples from experimental data.La exactitud de la batimetría obtenida por sonar de sector depende del rendimiento intrínseco del procesado de señales acústicas. Proponemos aquí un factor de calidad, cuantificando la exactitud asociada al cálculo de cada sondeo. Este descriptor se deriva de modelos sencillos para la amplitud (variación del instante del centro de gravedad de una envoltura fluctuante campaniforme) o para una fase interferométrica (variación local para un número de muestras procesadas). El objetivo es atribuir a cada sondeo individual un nivel de calidad objetivo que sea independiente del sonar y directamente aplicable en el procesado de la batimetría, al editar los datos o bien como un parámetro de entrada para el posprocesado estadístico. Este concepto está ilustrado mediante ejemplos de datos experimentales.La précision des sonars bathymétriques dépend des performances intrinsèques du traitement des signaux acoustiques. Nous proposons ici un facteur de qualité, quantifiant la précision associée à chaque calcul de sonde. Ce descripteur est obtenu à partir de modèles simples soit pour l’amplitude (variance du centre de gravité d’une enveloppe fluctuante) soit pour la phase Interférométrique (variance locale pour un nombre donné d’échantillons). L’objectif est d’affecter à chaque sonde individuelle un niveau objectif de qualité valide quel que soit le sonar, et applicable directement dans le traitement bathymétrique, soit pour l’édition des données, soit comme paramètre d’entrée d’un post-traitement statistique. Ce concept est illustré par des exemples de données expérimentales

"La sensación de la soledad" en las Rimas de Bécquer (Aproximación a la poesía metafísica)

In our article we try to approximate to the topic of Spanish romanticism and the cosmovision of Gustavo Adolfo Bécquer in particular. We evaluate the role of universal romantic symptom – the feeling of loneliness in Bécquer’s Rimas and observe how his poetic world is linked to the existensial vision of Miguel de Unamuno.En nuestro artículo intentamos acercarnos al tema del romanticismo español, y a la visión romántica de Gustavo Adolfo Bécquer en particular. Dentro de ello investigamos el papel del síntoma universal del romanticismo – la sensación de la soledad en las Rimas de Bécquer y observamos cómo está vinculado su mundo romántico con la visión existencialista de Miguel de Unamuno

Region based variational approach for the segmentation textured sonar images

We propose a new region-based segmentation of textured sonar images with respect to seafloor types. We characterize sea-floor types by a set of empirical distributions estimated on texture responses to a set of different filters and we introduce a novel similarity measure between sonar textures in this attribute space. Our similarity measure is defined as a weighted sum of Kullback-Leibler divergences between texture features. The texture similarity measure weight setting is twofold : first we weight each filter, according to its discrimination power, the computation of these weights are issued from the margin maximization criterion. Second, we add an additional weighting, evaluated as an angular distance between the incidence angles of the compared texture samples, to cope with the problem related to the sonar image acquisition process that leads to a variability of the backscattered (BS) value and the texture aspect with the incidence angle range. Our segmentation method is stated as the minimization of a region-based functional that involves the similarity between region texture based statistics and prototype ones and a regularization term that imposes smoothness and regularity on region boundaries. The proposed approach is implemented using level-set methods, and the functional minimization is done using shape derivative tools.Nous proposons une nouvelle méthode formulée au niveau région pour la segmentation texturale d’images sonar haute résolution. Nous caractérisons les différents types de fonds marins par des descripteurs de texture sous forme de distributions de leurs réponses à un ensemble de filtres, estimées sur la globalité des régions et nous définissons une nouvelle mesure de similarité adaptée à la discrimination entre fonds marins dans l’espace de ces descripteurs. Notre mesure de similarité est une somme doublement pondérée de divergences de Kullback-Leibler entre les descripteurs de textures : la première pondération permet la sélection des filtres les plus pertinents pour la discrimination entre textures et la deuxième pondération est angulaire et elle permet de tenir compte de la variation des descripteurs de texture en fonction des angles d’incidence. La méthode de segmentation est formulée dans un cadre variationnel. La fonctionnelle d’énergie associée fait intervenir deux termes. Le premier est un terme qui évalue l’homogénéité des régions selon la mesure de similarité pondérée entre les statistiques estimées sur les différentes régions de l’image et les prototypes relatifs aux différents types de fonds marins. Le deuxième terme contraint la régularité des frontières entre régions. La minimisation de la fonctionnelle est effectuée par descente du gradient et exploite les outils de dérivation de forme et la méthode est implantée selon la technique des ensembles de niveaux

Electric sampling of soot particles in spreading non-premixed flames: methodology and influence of gravity

Finer strategies of spacecraft fire mitigation require more experimental data related to fire detection. Fire detection systems developed on Earth rely massively on the optical detection of soot particles, which are present in the smoke. To detect the fire correctly, it is thus important to know how the optical properties of these particles are affected in reduced gravity. With different transport processes and increased residence time, soot in reduced gravity can be different from those produced at normal gravity. As their optical properties are related to their morphological properties, a better understanding about the evolution of soot particle morphology in flames under microgravity conditions is required. Within this context, a novel technique of soot sampling using electric field is applied to a spreading non-premixed flame at normal and micro-gravity. The soot particles sampled are observed subsequently under Transmission Electron Microscopy (TEM). Density, soot particle projected area, radius of gyration, fractal dimension, and primary particle size are extracted and the influence of gravity is investigated with the evolution of these morphological properties within the flame. Though the present study cannot be conclusive in itself, the similarity between the evolution of the optical density measured throughout the flame just before the electric perturbation required by the sampling technique and the evolution of the amount of soot deposited due to the electric perturbation along the sampling plates supports the future works that need to be devoted to further assess the consistency of the technique

Control of the repeatability of high frequency multibeam echosounder backscatter by using natural reference areas

The increased use of backscatter measurements in time series for environmental monitoring necessitates the comparability of individual results. With the current lack of pre-calibrated multibeam echosounder systems for absolute backscatter measurement, a pragmatic solution is the use of natural reference areas for ensuring regular assessment of the backscatter measurement repeatability. This method mainly relies on the assumption of a sufficiently stable reference area regarding its backscatter signature. The aptitude of a natural area to provide a stable and uniform backscatter response must be carefully considered and demonstrated by a sufficiently long time-series of measurements. Furthermore, this approach requires a strict control of the acquisition and processing parameters. If all these conditions are met, stability check and relative calibration of a system are possible by comparison with the averaged backscatter values for the area. Based on a common multibeam echosounder and sampling campaign completed by available bathymetric and backscatter time series, the suitability as a backscatter reference area of three different candidates was evaluated. Two among them, Carré Renard and Kwinte, prove to be excellent choices, while the third one, Western Solent, lacks sufficient data over time, but remains a valuable candidate. The case studies and the available backscatter data on these areas prove the applicability of this method. The expansion of the number of commonly used reference areas and the growth of the number of multibeam echosounder controlled thereon could greatly contribute to the further development of quantitative applications based on multibeam echosounder backscatter measurements