Simple method for multiple soundings extraction for wide beamwidth echosounders

Bathymetric multibeam echosounders (MBES) classically improve their bottom detection resolution by increasing the number of beams with narrower beamwidths. Many independent detections are thus obtained when extracting one sounding in each beam. However when the pulse footprint on the bottom gets narrower than the beamwidth, which often happens at high incidence angles, it is possible to extract several independent bottom detections within the same beam. These high-density soundings extraction methods present a still higher interest for MBES designed with rather “wide” beams (say 3° or more), so as to offer other benefits, such as compacity, low-cost, or low sidelobe levels as for fishery applications. This paper presents a simple way to derive multiple detections from wide beamwidth systems, which does not involve direct phase-ramp processing such as truncating, cleaning, smoothing and angle-crossing determination. Upon amplitude criteria all samples from a phase ramp are selected, and geographically positioned through their slant range and antenna relative angle, deduced from the raw phase values. Detections corresponding to seabed echoes are then selected through geometrical contiguity criteria and allocated to geographical grid nodes. Depth of each grid node is then taken as median depth of related detections. Results of such processing are presented upon characteristic seabeds, and compared to MBES-embedded high-density extraction. Resolution and accuracy issues of different strategies are discussed

Multispectral and multiangle measurements of acoustic seabed backscatter acquired with a tilted calibrated echosounder

A multispectral and multiangle analysis of seabed backscatter intensity has been conducted using data from a calibrated single-beam echosounder (SBES) with five frequency channels deployed over four homogeneous areas with different sediment types in the Bay of Brest (France). The SBES transducers were tilted at incidence angles from 0 degrees to 70 degrees to record the seafloor backscatter angular response at discrete frequencies ranging from 35 to 450 kHz. The recorded backscatter levels were analyzed for their angular dependence (average backscatter strength versus frequency and angle) as well as for their sample statistical distribution. The angle and frequency dependence of the seafloor backscatter obtained using a calibrated SBES can potentially be used to calibrate multibeam systems, and it can also help in elucidating the physical processes of backscatter controlled by the interaction between the acoustic wave characteristics and the sediment properties. Backscatter measurements for each area showed a consistent frequency dependence with little variation between the four sediment types

Fluid emissions at the Aquitaine Shelf (Bay of Biscay, France): A biogenic origin or the expression of hydrocarbon leakage?

Fishery acoustic surveys conducted in the Bay of Biscay and dedicated to monitoring and predicting pelagic ecosystem evolution reveal numerous active seeps on the Aquitaine Shelf, east of the shelf break, at water depths ranging from 140 to 185 m. Some acoustic anomalies recorded in the water column with hull-mounted single and multibeam echosounders are clearly caused by fluid escape at the seabed, most likely gases. These fluid emissions are associated at the seafloor with high backscatter subcircular small-scale mounds, on average less than 2 m high and a few metres in diameter. Based on near-bottom video and acoustic surveys, these mounds are interpreted to be by-products of gas seepage, possibly methane-derived authigenic carbonates. The spatial distribution of the seeps and related structures, based on water column acoustic gas flares and high backscatter seabed patches, appears to be relatively broad, with a North–South extension of ~65 km across the Parentis Basin and the Landes High, and a West–East extension along a few kilometres wide on the shelf. The seepage activity seems persistent through time at the annual scale, with acoustic evidence dating back to 1998. The spatial distribution of the fluid emissions at the Aquitaine Shelf may suggest possible sedimentary and tectonic controls in relation with the Pyrenean compression phase. The nature and the origin of the emitted fluids and seafloor mounds are unknown. The gases may correspond to biogenic methane from Late Pleistocene deposits or to thermogenic gases originating from deeper, Jurassic–Cretaceous levels. The oil province of the Parentis Basin raises questions regarding possible genetic links to the petroleum system

Did acoustic miss much spawning anchovy surface schools in the Bay of Biscay in spring 2019 ?

This study aims at estimating the anchovy biomass that was missed in the echosounder surface blind zone during the PELGAS survey, in an attempt to explain the relative difference between acoustic and DEPM biomass estimates in springtime 2019 in the Bay of Biscay. Acoustic data collected by a side-looking echosounder are analysed in order to estimate the negative bias introduced in acoustic biomass estimates by the acoustic blind zone. Corrected anchovy acoustic biomass estimates and maps are compared to DEPM ones, in an attempt to reconcile egg and acoustic estimates. Remaining discrepancies are discussed in the light of acoustic and egg biomass estimation assumptions. We showed that surface school biomass can be calculated while applying standard acoustic data analysis methodology to horizontal echosounder data, combined to broadband in-situ TS measurements. In spring 2019, the surface blind zone observation bias was not significant, as only 5% of the anchovy biomass was located in the 0-10m surface layer. The difference between acoustic and DEPM global biomass estimates observed in 2019 hence remains unexplained. Differences between acoustic biomass and egg counts maps were likely due to local differences in fecundity, probably caused, at least in the North Western area, by the presence of shallow schooling large anchovy displaying higher fecundity. This pilot study might pave the way to the routine combination of vertical and horizontal acoustic data to correct surface blind zone bias and improve small pelagic fish acoustic biomass estimates. DEPM and acoustic biomass estimates have to be calculated within post-stratification regions as homogeneous as possible, to reduce bias and improve estimate precision. The high spatial resolution of acoustic backscatter generally allows to delineate reasonibly homogeneous post-stratification regions. The coarser spatial resolution of the trawl hauls providing biological data used to calculate daily fecundity estimates in the DEPM method so far prevented to assess the spatial heterogeneity of fish fecundity. Daily Fecundity proxy maps derived from surveys such as PELGAS, where both egg and acoustic data are collected, might be used to assess the spatial heterogeneity of fish fecundity. Those new information could contribute to improve the precision of DEPM estimates, by improving post-stratification region delineation. Assessing the spatial and statistical distributions of daily fish fecundity might hence contribute to improve the precision of mean DF values used in DEPM, and ultimately better explain the discrepancies observed during some years between egg and acoustic indices

Backscatter calibration of high-frequency multibeam echosounder using a reference single-beam system, on natural seafloor

The calibration of multibeam echosounders for backscatter measurements can be conducted efficiently and accurately using data from surveys over a reference natural area, implying appropriate measurements of the local absolute values of backscatter. Such a shallow area (20-m mean depth) has been defined and qualified in the Bay of Brest (France), and chosen as a reference area for multibeam systems operating at 200 and 300 kHz. The absolute reflectivity over the area was measured using a calibrated single-beam fishery echosounder (Simrad EK60) tilted at incidence angles varying between 0° and 60° with a step of 3°. This reference backscatter level is then compared to the average backscatter values obtained by a multibeam echosounder (here a Kongsberg EM 2040-D) at a close frequency and measured as a function of angle; the difference gives the angular bias applicable to the multibeam system for recorded level calibration. The method is validated by checking the single- and multibeam data obtained on other areas with sediment types different from the reference area

Characterization of sound scattering layers in the Bay of Biscay using broadband acoustics, nets and video

Sound scattering layers (SSLs) are observed over a broad range of spatio-temporal scales and geographical areas. SSLs represent a large biomass, likely involved in the biological carbon pump and the structure of marine trophic webs. Yet, the taxonomic composition remains largely unknown for many SSLs. To investigate the challenges of SSL sampling, we performed a survey in a small study area in the Northern Bay of Biscay (France) by combining broadband and narrowband acoustics, net sampling, imagery and video recordings. In order to identify organisms contributing to the observed SSLs, we compared measured frequency spectra to forward predicted spectra derived from biological data. Furthermore, to assess the confidence in SSL characterization, we evaluated uncertainties in modeling, acoustical and biological samplings. Here, we demonstrate for the first time that SSL backscattering intensity in the Bay of Biscay can be dominated in springtime by resonant gas bearing organisms below 100 kHz, namely siphonophores and juvenile fishes and by pteropods at higher frequencies. Thus, we demonstrate the importance of broadband acoustics combined to nets, imagery and video to characterize resonant backscatterers and mixed mesozooplankton assemblages

Cartographie des Herbiers de Zostères. Guide technique

Les phanérogames marines sont devenues ces dernières années des "espèces phares". Habitat à forte valeur écologique et patrimoniale, les herbiers bénéficient de mesures de gestion et de protection à différentes échelles : locale, nationale et internationale. Il y a donc un contexte favorable pour que les connaissances sur les herbiers progressent et que leur dynamique spatio-temporelle soit surveillée à une échelle globale. Depuis quelques années, et en lien avec les exigences des directives européennes, les phanérogames marines ont servi de support pour le développement et la valorisation des outils et techniques de télédétection pour la planification d'une gestion et d’une stratégie de surveillance efficace. En France, les méthodes pour évaluer les paramètres relatifs à leur distribution sont diverses. L’imagerie optique (aéroportée et satellite) est souvent utilisée pour cartographier les herbiers en zone intertidale. En zone subtidale, la technologie acoustique est une alternative pour accéder aux herbiers submergés. Cependant, les résultats peuvent varier en fonction des techniques de cartographie utilisées (GPS, imagerie optique ou acoustique, etc.), des méthodes de délimitation (traitement automatique ou photo-interprétation), des seuils retenus par les opérateurs pour délimiter les herbiers (indice spectraux), et de la nature des objets mesurés (surface végétalisée ou enveloppe de l’herbier, degré de fragmentation, etc.).  Afin d’assurer l’homogénéité des rendus et permettre l’élaboration des bilans à l’échelle nationale, les efforts doivent converger vers une standardisation des protocoles de cartographie des herbiers et du  suivi de leur dynamique. Elaboré dans le cadre de la convention Ifremer-AQUAREF-AFB 2016-2018, le présent document constitue ainsi un guide technique pour la cartographie des herbiers de zostères qui complète les protocoles de suivi stationnel existants dans le cadre de la DCE (Directive Cadre sur l’Eau). Il pourra également être utilisé pour répondre aux objectifs des autres directives : Directive Habitats Faune et Flore (DHFF) et Directive Stratégie milieu Marin (DCSMM)

CSWOT2023 - testing the potential of a (fancy) unmanned surface vehicle for oceanography / measuring sea level in situ ... or at least trying to !

During the C-SWOT-2023 campaign, two experimental approaches were tested in SWOT zones in the northwestern Mediterranean Sea. An unmanned surface vehicle (USV) recorded meso- and sub-mesoscale processes in the northern current off Toulon. GNSS systems were deployed on various platforms (ship, buoys, USV) to map sea surface height. Au cours de la campagne C-SWOT-2023, deux approches expérimentales ont été testées dans les zones de SWOT au nord-ouest de la mer Méditerranée. Un drone  de surface (USV) a enregistré les processus méso et sous-mésoscales dans le courant nord au large de Toulon. Des systèmes GNSS ont été déployés sur différentes plates-formes (navire, bouées, USV) pour cartographier la hauteur de la surface de la mer

A method for controlled target strength measurements of pelagic fish, with application to European anchovy (Engraulis encrasicolus)

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