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

Extended Detection of Shallow Water Gas Seeps From Multibeam Echosounder Water Column Data

Multibeam echosounder water column data provides a three-dimensional image of features between the water surface and the seafloor. Although this swath of acoustic data can be collected over a wide range of angles, most of the data, at least beyond the range to the first seafloor return, is contaminated by noise created by receiver array sidelobe interference. As a result, the water column data beyond the minimum slant range commonly is excluded from analysis. This paper demonstrates a method to consistently filter and extract targets comprising a gas seep feature across the multibeam swath, including targets within the areas dominated by receiver array sidelobe interference. For each sample range, data are filtered based on the mean plus a certain number (k) of standard deviations of the sample values along that range. The filtering is coupled with a morphological classification to retain only targets of interest while excluding background data and noise. Data were collected over a shallow water artificial gas seep using two different flow rates and at three different vessel speeds. Using the proposed method, 119 of 124 test seeps were identified correctly. Seep targets were identified at all angles across the water column fan up to beam pointing angles of 55°, with 19 of 23 seeps being correctly identified at angles greater than 50°. This method demonstrates that features can be extracted and geolocated in the sidelobe noise when the interference is appropriately filtered. These results will improve the areal extent of multibeam surveys and increase the utility of acoustic data in capturing information on water column targets directly above the seafloor

Estimates of Methane Release From Gas Seeps at the Southern Hikurangi Margin, New Zealand

The highest concentration of cold seep sites worldwide has been observed along convergent margins, where fluid migration through sedimentary sequences is enhanced by tectonic deformation and dewatering of marine sediments. In these regions, gas seeps support thriving chemosynthetic ecosystems increasing productivity and biodiversity along the margin. In this paper, we combine seismic reflection, multibeam and split-beam hydroacoustic data to identify, map and characterize five known sites of active gas seepage. The study area, on the southern Hikurangi Margin off the North Island of Aotearoa/New Zealand, is a well-established gas hydrate province and has widespread evidence for methane seepage. The combination of seismic and hydroacoustic data enable us to investigate the geological structures underlying the seep sites, the origin of the gas in the subsurface and the associated distribution of gas flares emanating from the seabed. Using multi-frequency split-beam echosounder (EK60) data we constrain the volume of gas released at the targeted seep sites that lie between 1,110 and 2,060 m deep. We estimate the total deep-water seeps in the study area emission between 8.66 and 27.21 × 10 6 kg of methane gas per year. Moreover, we extrpolate methane fluxes for the whole Hikurangi Margin based on an existing gas seep database, that range between 2.77 × 10 8 and 9.32 × 10 8 kg of methane released each year. These estimates can result in a potential decrease of regional pH of 0.015–0.166 relative to the background value of 7.962. This study provides the most quantitative assessment to date of total methane release on the Hikurangi Margin. The results have implications for understanding what drives variation in seafloor biological communities and ocean biogeochemistry in subduction margin cold seep sites

Amélioration des méthodes de détection et de qualification des sondes pour les sondeurs multifaisceaux bathymétriques

Multibeam echosounders (MBES) are complex sonar systems giving accurate bathymetric measurements over a wide angular sector by forming several hundreds of steered beams. This work deals with the topics of measurement qualification and the study of the possibility to take several soundings into account inside each formed beam. The definition of the quality is based on the classical estimation methods used in MBES to determine the water depth and on their variance derivation. This requires a good knowledge of the statistical properties of the acoustic signals according to the measurement conditions and the sounder settings, in order to derive the theoretical variance of the time of arrival inside each beam. Those developments define a quality estimator for each computed sounding in each beam, depending only on the acoustical signal properties. They were conducted specifically for the measurement of the time of arrival from the envelope of the amplitude signal, taking into account the impact of additive noise and the pulse length. The validation of the theoretical formulas on real data shows the relevance of the results. The study of the implementation of multidetection algorithms begins by underlining the loss of information caused by the selection of only one sounding per beam, and by observing the intrinsic measurement limitations caused by MBES antennas typical configuration. Several processing methods designed to implement a multidetection algorithm are then defined. These processes are based on the characterization of the nature of underwater target (isolated, one-dimensional, extended) and of their spectral properties. For those, a criterion relying on the "effective bandwidth" of the received signal makes it possible to distinguish real targets from noise. This finally resulted into the implementation of a multidetection algorithm which was be tested on several datasets acquired in complex underwater environments. The results obtained on real data show the interest residing in taking into account several soundings per beam. The results of this work contribute to the hydrographic performance improvement of MBES, giving a quality estimation of the acoustical measurement in real-time for each sounding and a better description of complex underwater targets.Les sondeurs multifaisceaux (SMF) bathymétriques sont des systèmes sonars permettant une mesure bathymétrique précise sur un large secteur angulaire par la formation de plusieurs centaines de faisceaux très fins dépointés de la verticale. Les thématiques abordées dans ces travaux de thèse sont le développement de méthodes de qualification des sondes fournies par ces sondeurs, et l'étude de l'implémentation d'algorithmes permettant la détection de plusieurs cibles à l'intérieur de chaque faisceau formé. La définition de la qualité associée à chaque sonde se base sur les méthodes d'estimation de la hauteur d'eau utilisées dans les SMF et sur le calcul de leurs variances. Ceci nécessite une connaissance approfondie de la statistique des signaux acoustiques en fonction des conditions de mesure et du paramétrage du sondeur, afin d'obtenir les formules théoriques décrivant la variance du temps d'arrivée de l'onde dans chaque faisceau. Les calculs aboutissent à la définition d'une qualité pour chaque sonde dépendant uniquement des caractéristiques du signal acoustique dans le faisceau considéré. Ils ont été menés en particulier pour la mesure du temps d'arrivée à partir de l'enveloppe du signal d'amplitude de l'écho, en prenant en compte l'influence du rapport signal à bruit et de la durée du signal émis. La validation des formules théoriques obtenues sur données réelles confirme la pertinence de l'approche retenue. L'étude de l'implémentation d'algorithme de détection multiple commence par une mise en évidence de la perte d'information liée à la détection d'une seule cible par faisceau et par la visualisation des limitations intrinsèques liées à la configuration actuelle des antennes de SMF. Par la suite, une série de traitements adaptés à la définition d'un algorithme de détection multiple est proposée. Ces traitements sont notamment basés sur une différenciation du type de cibles sous-marines (cibles linéiques, surfaciques, ponctuelles) et par leur caractérisation spectrale : pour cette dernière, un critère reposant sur la longueur de "bande effective" du signal reçu permet de séparer efficacement les cibles réelles du bruit. Ceci a permis d'implémenter un algorithme de multi-détection, qui a été testé sur différents jeux de données acquises dans des environnements complexes. Les résultats obtenus sur données réelles mettent en évidence l'intérêt de la prise en compte de plusieurs cibles temporellement disjointes dans chaque faisceau. Les résultats de ces travaux contribuent à l'amélioration des performances hydrographiques des SMF bathymétriques en permettant à la fois une qualification en temps réel de la mesure acoustique ainsi qu'une meilleure description des cibles sous-marines complexes

Maximum likelihood estimator based on Quality Factor for bathymetric multibeam echosounder

International audienceFor modern multibeam echosounders, the use of a robust and reliable quality estimator associated with each sounding is an absolute necessity. Indeed, due to the large volume of data acquired, a lot of time is lost, both during the survey and the post-processing. This is a costly problem for hydrographers. The definition of a quality estimator based on the characteristics of the beamformed signal gives an answer to this problem. It has been successfully implemented by several sonar manufacturers and its relevance in measuring the quality of each sounding has been demonstrated. Based on the Quality Factor and on the existing way of processing a sounding, we defined a new sounding estimator using a maximum likelihood approach. This new approach was developped under several hypothesis which are plausible under certain conditions. This new estimator was successfully tested and implemented on real data with good results

Extended scatterers model for fast sonar signal simulation

International audienceSonar signal simulation is an important area of research for industrials and scientific of the domain. Indeed, it can be used in many ways by sonar manufacturers, both for designing a system or in order to explain physical phenomenon on data. Therefore, by minimizing the expensive testing time at sea, a well designed simulator can be a very interesting financial asset. Traditional sonar signal simulation also contains problems to be dealt with. It is often necessary to find a balance between the computational power consumption, the needed accuracy of the physical phenomenon description and the scale at which simulated signals are relevant. In our simulation model we propose a solution based on a very dense grid (resolution of about the concerned wavelength) of extended scatterers spread on the seafloor. It is designed to describe efficiently the backscattering strength and the phase response of the seafloor for every type of sonar

Definition and application of a quality estimator for multibeam echosounders

International audienceFor modern multibeam echosounders, the use of a robust and reliable quality estimator associated with each sounding is an absolute necessity. Indeed, due to the large volume of data acquired, a lot of time is lost, both during the survey and the post-processing. This is a costly problem for hydrographers. The definition of a quality estimator based on the characteristics of the beamformed signal gives an answer to this problem. It has been successfully implemented by several sonar manufacturers and its relevance in measuring the quality of each sounding has been demonstrated. The defined Quality Factor can also be used directly in the sonar measurement process, such as in detection algorithm, in order to improve the performance of the systems. This makes it possible to enhance of existing systems at very little cost with success

Gas Bubble Forensics Team Surveils the New Zealand Ocean

International audienceAn international research group recorded the acoustic signatures of gas bubbles rising from a hydrothermal vent field to gather clues about greenhouse gases escaping into the atmosphere