A New Method for Generation of Soundings from Phase-Difference Measurements

A desirable feature of bathymetric sonar systems is the production of statistically independent soundings allowing a system to achieve its full capability in resolution and object detection. Moreover gridding algorithms such as the Combined Uncertainty Bathymetric Estimator (CUBE) rely on the statistical independence of soundings to properly estimate depth and discriminate outliers. Common methods of filtering to mitigate uncertainty in the signal processing of both multibeam and phase-differencing sidescan systems (curve fitting in zero-crossing detections and differential phase filtering respectively) can produce correlated soundings. Here we propose an alternative method for the generation of soundings from differential phase measurements made by either sonar type to produce statistically independent soundings. The method extracts individual, non-overlapping and unfiltered, phase-difference measurements (from either sonar type) converting these to sonar-relative receive angle, estimates their uncertainty, fixes the desired depth uncertainty level and combines these individual measurements into an uncertainty-weighted mean to achieve the desired depth uncertainty, and no more. When the signal to noise ratio is sufficiently high such that the desired depth uncertainty is achieved with an individual measurement, bathymetric estimates are produced at the sonar’s full resolution capability. When multiple measurements are required, the filtering automatically adjusts to maintain the desired uncertainty level, degrading the resolution only as necessary. Because no two measurements contribute to a single reported sounding, the resulting estimated soundings are statistically independent and therefore better resolve adjacent objects, increase object detectability and are more suitable for statistical gridding methodologies

Optimizing Resolution and Uncertainty in Bathymetric Sonar Systems

Bathymetric sonar systems (whether multibeam or phase-differencing sidescan) contain an inherent trade-off between resolution and uncertainty. Systems are traditionally designed with a fixed spatial resolution, and the parameter settings are optimized to minimize the uncertainty in the soundings within that constraint. By fixing the spatial resolution of the system, current generation sonars operate sub-optimally when the SNR is high, producing soundings with lower resolution than is supportable by the data, and inefficiently when the SNR is low, producing high-uncertainty soundings of little value. Here we propose fixing the sounding measurement uncertainty instead, and optimizing the resolution of the system within that uncertainty constraint. Fixing the sounding measurement uncertainty produces a swath with a variable number of bathymetric estimates per ping, in which each estimate’s spatial resolution is optimized by combining measurements only until the desired depth uncertainty is achieved. When the signal to noise ratio is sufficiently high such that the desired depth uncertainty is achieved with individual measurements, bathymetric estimates are produced at the sonar’s full resolution capability. Correspondingly, a sonar’s resolution is no-longer only considered as a property of the sonar (based on, for example, beamwidth and bandwidth,) but now incorporates geometrical aspects of the measurements and environmental factors (e.g., seafloor scattering strength). Examples are shown from both multibeam and phase- differencing sonar systems

Sound Radiation of Seafloor-Mapping Echosounders in the Water Column, In Relation to the Risks Posed to Marine Mammals

Currently, more and more attention is focusing on the impact of anthropogenic sound sources on marine life, particularly marine mammals. Indeed, several unusual cetacean strandings linked to the use of high-power sonar have been observed over the past years. Hydrography and seafloor-mapping make extensive use of acoustic sources; this paper aims to present the order of magnitude of sound radiated by such echosounders, and hence estimate their potential impact on marine mammals. The paper begins with a presentation of the main issues related to sound-mediated risks to marine life and a reminder of echosounder characteristics and geometry. Next, the numerical results from several case studies are compared with currently accepted threshold values for marine mammal sound exposure. This comparison makes clear that, while echosounders may transmit at high sound pressure levels, the very short duration of their pulses and their high spatial selectivity make them unlikely to cause damage to marine mammal auditory systems, according to current knowledge. There remains a possibility that echosounders may affect marine mammal behaviour at ranges on the order of kilometres; however, the likelihood and biological effects of such behavioural responses to sound remain poorly understood at present.Actualmente, se dedica cada vez más atención al impacto de las fuentes sonoras antropogénicas en la vida marina, particularmente en los mamíferos marinos. Se han observado durante los últimos años varias varadas poco comunes causadas por cetáceos, vinculadas al uso de sonares de alta potencia. La hidrografía y la cartografía del fondo marino utilizan de forma considerable las fuentes acústicas; el objetivo de este artículo es presentar el orden de la magnitud del sonido radiado por similares sondas acústicas y por tanto estimar su impacto potencial en los mamíferos marinos. Este artículo empieza con una presentación de los principales temas relativos a los riesgos causados por el sonido a la vida marina y con un recordatorio de las características de las sondas acústicas y la geometría. Luego se comparan los resultados numéricos de varios casos prácticos con los valores de umbral corrientemente aceptados para la exposición al sonido de los mamíferos marinos. Esta comparación deja claro que, aunque las sondas acústicas pueden transmitir a niveles de presión de alta intensidad, la muy breve duración de sus impulsos y su alta selectividad espacial hacen que sea muy poco probable que causen daños a los sistemas auditivos de los mamíferos, según los conocimientos que se poseen actualmente. Queda la posibilidad de que las sondas acústicas puedan afectar al comportamiento de los mamíferos marinos en campos de cobertura del orden de kilómetros; sin embargo, actualmente siguen entendiéndose muy poco la probabilidad y los efectos biológicos de dichas reacciones del comportamiento.De plus en plus d‘attention est portée aujourd'hui à l‘impact du bruit d‘origine humaine sur la vie marine, et spécialement les mammifères marins. Un certain nombre d‘échouements accidentels de cétacés ont été, au cours des dernières années, reliés à l‘utilisation de sonars de forte puissance. L‘hydrographie et la cartographie des fonds marins font un large usage d‘émetteurs acoustiques ; cet article vise à présenter les ordres de grandeur des sons émis par ces sondeurs, et à estimer leur impact potentiel sur les mammifères marins. On présente d‘abord les grandes lignes décrivant les risques acoustiques pour la vie marine, et on rappelle les caractéristiques et la géométrie des sondeurs. Les résultats numériques pour plusieurs cas typiques sont ensuite comparés aux valeurs acceptées couramment pour les seuils d‘exposition sonore des mammifères marins. Cette comparai-son fait apparaître que, bien que certains sondeurs puissent émettre des signaux de forte intensité, la brièveté des émissions et leur forte directivité spatiale rendent improbables des lésions aux systèmes auditifs des mammifères marins, d‘après les connaissances actuelles. Il reste la possibilité que les sondeurs puissent affecter le comportement des mammifères marins, sur des distances kilométri-ques ; la possibilité et les conséquences biologiques des tels effets comportementaux sont encore peu connus

Streamlining Sound Speed Profile Pre-Processing: Case Studies and Field Trials

High rate sound speed profiling systems have the potential to maximize the efficiency of multibeam echosounder systems (MBES) by increasing the accuracy at the outer edges of the swath where refraction effects are at their worst. In some cases, high rate sampling on the order of tens of casts per hour is required to capture the spatio-temporal oceanographic variability and this increased sampling rate can challenge the data acquisition workflow if refraction corrections are to be applied in real-time. Common bottlenecks result from sound speed profile (SSP) preprocessing requirements, e.g. file format conversion, cast extension, reduction of the number of points in the cast, filtering, etc. Without the ability to quickly pre-process SSP data, the MBES operator can quickly become overwhelmed with SSP related tasks, potentially to the detriment of their other duties. A series of algorithms are proposed in which SSPs are automatically pre-processed to meet input criteria of MBES acquisition systems, specifically the problems of cast extrapolation and thinning are addressed. The algorithmic performance will be assessed in terms of sounding uncertainty through a series of case studies in a variety of oceanographic conditions and water depths. Results from a field trial in the French Mediterranean will be used to assess the improvement in real-time MBES acquisition workflow and survey accuracy and will also highlight where further improvements can be made in the pre-processing pipeline

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

Analyse statistique des données de sondeurs multifaisceaux

L'étude statistique d'images sonar enregistrées par un sondeur multifaisceaux haute résolution (95 kHz) démontre que la loi de Rayleigh n'est plus adaptée pour décrire la statistique des images texturées. Certaines modélisations de la réverbération permettent de comprendre les particularités des images haute résolution et justifient l'utilisation de distributions statistiques en loi K. Nous mettons à profit la géométrie des sondeurs multifaisceaux pour souligner le changement de statistique des images avec la taille de l'aire insonifiée instantanée et avec l'angle d'incidence. Une interprétation rigoureuse des propriétés statistiques et spectrales des images sonar passe par une prise en compte de la géométrie du sondeur

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