Nuno Gracias

University of Girona

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J. Escartı́n

Laboratoire de Géologie de l’École Normale Supérieure

Rafael García

University of Girona

Aurélien Arnaubec

Ifremer

Klemen Istenič

University of Girona

Christine Deplus

Centre National de la Recherche Scientifique

Paraskevi Nomikou

National and Kapodistrian University of Athens

Jean‐Arthur Olive

Laboratoire de Géologie de l’École Normale Supérieure

Frédérique Leclerc

Géoazur

Mathilde Cannat

Centre National de la Recherche Scientifique

Nathalie Feuillet

Institut de physique du globe de Paris

Cooperative Institutions

Centre National de la Recherche Scientifique

University of Girona

Institut de physique du globe de Paris

Université Paris Cité

Sorbonne Paris Cité

International Ocean Discovery Program

Sorbonne Université

Consejo Superior de Investigaciones Científicas

Institut de Ciències del Mar

Institut Català de Ciències del Clima

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A noninvasive method for measuring the velocity of diffuse hydrothermal flow by tracking moving refractive index anomalies

Geochemistry Geophysics Geosystems (2010)

E. L. Mittelstaedt Anne Davaille Peter E. van Keken Nuno Gracias J. Escartı́n

Diffuse flow velocimetry (DFV) is introduced as a new, noninvasive, optical technique for measuring the velocity of diffuse hydrothermal flow. The technique uses images of a motionless, random medium (e.g., rocks) obtained through the lens of a moving refraction index anomaly (e.g., a hot upwelling). The method works in two stages. First, the changes in apparent background deformation are calculated using particle image velocimetry (PIV). The deformation vectors are determined by a cross correlation of pixel intensities across consecutive images. Second, the 2‐D velocity field is calculated by cross correlating the deformation vectors between consecutive PIV calculations. The accuracy of the method is tested with laboratory and numerical experiments of a laminar, axisymmetric plume in fluids with both constant and temperature‐dependent viscosity. Results show that average RMS errors are ∼5%–7% and are most accurate in regions of pervasive apparent background deformation which is commonly encountered in regions of diffuse hydrothermal flow. The method is applied to a 25 s video sequence of diffuse flow from a small fracture captured during the Bathyluck'09 cruise to the Lucky Strike hydrothermal field (September 2009). The velocities of the ∼10°C–15°C effluent reach ∼5.5 cm/s, in strong agreement with previous measurements of diffuse flow. DFV is found to be most accurate for approximately 2‐D flows where background objects have a small spatial scale, such as sand or gravel.

Particle tracking velocimetry

10.1029/2010gc003227

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Citations (16)

Underwater 3D Reconstruction from Video or Still Imagery: Matisse and 3DMetrics Processing and Exploitation Software

Journal of Marine Science and Engineering (2023)

Aurélien Arnaubec Maxime Ferrera J. Escartı́n Marjolaine Matabos Nuno Gracias

This paper addresses the lack of “push-button” software for optical marine imaging, which currently limits the use of photogrammetric approaches by a wider community. It presents and reviews an open source software, Matisse, for creating textured 3D models of complex underwater scenes from video or still images. This software, developed for non-experts, enables routine and efficient processing of underwater images into 3D models that facilitate the exploitation and analysis of underwater imagery. When vehicle navigation data are available, Matisse allows for seamless integration of such data to produce 3D reconstructions that are georeferenced and properly scaled. The software includes pre-processing tools to extract images from videos and to make corrections for color and uneven lighting. Four datasets of different 3D scenes are provided for demonstration. They include both input images and navigation and associated 3D models generated with Matisse. The datasets, captured under different survey geometries, lead to 3D models of different sizes and demonstrate the capabilities of the software. The software suite also includes a 3D scene analysis tool, 3DMetrics, which can be used to visualize 3D scenes, incorporate elevation terrain models (e.g., from high-resolution bathymetry data) and manage, extract, and export quantitative measurements for the 3D data analysis. Both software packages are publicly available.

Software suite

Georeference

10.3390/jmse11050985

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Citations (14)

The Kallisti Limnes, carbon dioxide-accumulating subsea pools

Scientific Reports (2015)

Richard Camilli Paraskevi Nomikou J. Escartı́n Pere Ridao Angelos Mallios

Abstract Natural CO 2 releases from shallow marine hydrothermal vents are assumed to mix into the water column and not accumulate into stratified seafloor pools. We present newly discovered shallow subsea pools located within the Santorini volcanic caldera of the Southern Aegean Sea, Greece, that accumulate CO 2 emissions from geologic reservoirs. This type of hydrothermal seafloor pool, containing highly concentrated CO 2 , provides direct evidence of shallow benthic CO 2 accumulations originating from sub-seafloor releases. Samples taken from within these acidic pools are devoid of calcifying organisms and channel structures among the pools indicate gravity driven flow, suggesting that seafloor release of CO 2 at this site may preferentially impact benthic ecosystems. These naturally occurring seafloor pools may provide a diagnostic indicator of incipient volcanic activity and can serve as an analog for studying CO 2 leakage and benthic accumulations from subsea carbon capture and storage sites.

Seafloor Spreading

Subsea

Caldera

10.1038/srep12152

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Citations (25)

Scale Accuracy Evaluation of Optical Based 3D Reconstruction Strategies Using Laser Photogrammetry

Klemen Istenič Nuno Gracias Aurélien Arnaubec J. Escartı́n Rafael García

Laser Scanning

3D Reconstruction

Source

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Globally aligned photomosaic of the Lucky Strike hydrothermal vent field (Mid‐Atlantic Ridge, 37°18.5′N): Release of georeferenced data, mosaic construction, and viewing software

Geochemistry Geophysics Geosystems (2008)

J. Escartı́n Rafael García O. Delaunoy J. Ferrer Nuno Gracias

We present a georeferenced photomosaic of the Lucky Strike hydrothermal vent field (Mid‐Atlantic Ridge, 37°18′N). The photomosaic was generated from digital photographs acquired using the ARGO II seafloor imaging system during the 1996 LUSTRE cruise, which surveyed a ∼1 km 2 zone and provided a coverage of ∼20% of the seafloor. The photomosaic has a pixel resolution of 15 mm and encloses the areas with known active hydrothermal venting. The final mosaic is generated after an optimization that includes the automatic detection of the same benthic features across different images (feature‐matching), followed by a global alignment of images based on the vehicle navigation. We also provide software to construct mosaics from large sets of images for which georeferencing information exists (location, attitude, and altitude per image), to visualize them, and to extract data. Georeferencing information can be provided by the raw navigation data (collected during the survey) or result from the optimization obtained from image matching. Mosaics based solely on navigation can be readily generated by any user but the optimization and global alignment of the mosaic requires a case‐by‐case approach for which no universally software is available. The Lucky Strike photomosaics (optimized and navigated‐only) are publicly available through the Marine Geoscience Data System (MGDS, http://www.marine‐geo.org ). The mosaic‐generating and viewing software is available through the Computer Vision and Robotics Group Web page at the University of Girona ( http://eia.udg.es/∼rafa/mosaicviewer.html ).

Georeference

Mosaic

10.1029/2008gc002204

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Citations (66)

Autonomous boundary inspection of Posidonia oceanica meadows using an underwater robot

Ocean Engineering (2023)

F. Di Ruscio Riccardo Costanzi Nuno Gracias Josep Quintana Rafael García

Monitoring provides important information for the planning and execution of marine environment preservation operations. Posidonia oceanica is one of the principal bioindicators in Mediterranean coastal areas and regular monitoring activities play a crucial role in its conservation. However, an efficient observation of vast areas colonised with P. oceanica is extremely challenging and it currently requires tedious and time consuming diving activities. Autonomous Underwater Vehicles (AUVs) endowed with optical sensors could represent a viable solution in carrying out visual inspection surveys. Nevertheless, AUVs are usually programmed to perform pre-defined trajectories, which are not effective for seagrass monitoring applications, as meadows may be fragmented and their contours may be irregular. This work proposes a framework based on machine learning and computer vision that enables an AUV equipped with a down-looking camera to autonomously inspect the boundary of P. oceanica meadows to obtain an initial estimate of the meadow size. The proposed boundary inspection solution is composed of three main modules: (1) an image segmentation relying on a Mask R-CNN model to recognise P. oceanica in underwater images, (2) a boundary tracking strategy that generates guidance references to track P. oceanica contours, (3) a loop closure detector fusing visual and navigation information to identify when a meadow boundary has been completely explored. The image segmentation model and the visual part of the loop closure detection module were validated on real underwater images. The overall inspection framework was tested in a realistic simulation environment, using mosaics obtained from real images to replicate the actual monitoring scenarios. The results show that the proposed solution enables the AUV to autonomously accomplish the boundary inspection task of P. oceanica meadows, therefore representing an effective tool towards the conservation and protection of marine environments.

Posidonia oceanica

10.1016/j.oceaneng.2023.114988

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Meadow fragmentation influences Posidonia oceanica density at the edge of nearby gaps

Estuarine Coastal and Shelf Science (2020)

Aina Barcelona Jordi Colomer Marianna Soler Nuno Gracias Teresa Serra

Fragmentation

Posidonia oceanica

Habitat Fragmentation

10.1016/j.ecss.2020.107106

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Citations (14)

Automatic segmentation of fish using deep learning with application to fish size measurement

ICES Journal of Marine Science (2019)

Rafael García Ricard Prados Josep Quintana Alexander Tempelaar Nuno Gracias

Abstract One of the leading causes of overfishing is the catch of unwanted fish and marine life in commercial fishing gears. Echosounders are nowadays routinely used to detect fish schools and make qualitative estimates of the amount of fish and species present. However, the problem of estimating sizes using acoustic systems is still largely unsolved, with only a few attempts at real-time operation and only at demonstration level. This paper proposes a novel image-based method for individual fish detection, targeted at drastically reducing catches of undersized fish in commercial trawling. The proposal is based on the processing of stereo images acquired by the Deep Vision imaging system, directly placed in the trawl. The images are pre-processed to correct for nonlinearities of the camera response. Then, a Mask R-CNN architecture is used to localize and segment each individual fish in the images. This segmentation is subsequently refined using local gradients to obtain an accurate estimate of the boundary of every fish. Testing was conducted with two representative datasets, containing in excess of 2600 manually annotated individual fish, and acquired using distinct artificial illumination setups. A distinctive advantage of this proposal is the ability to successfully deal with cluttered images containing overlapping fish.

Overfishing

10.1093/icesjms/fsz186

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Citations (119)

Large seafloor rupture caused by the 1956 Amorgos tsunamigenic earthquake, Greece

Communications Earth & Environment (2024)

Frédérique Leclerc Sylvain Palagonia Nathalie Feuillet Paraskevi Nomikou Danai Lampridou

In the Mediterranean Sea, the probability that a large earthquake-triggered tsunami will occur in the coming decades is high. Historical tsunami database informs us on their geographical occurrence but their sources, i.e., the faults that slipped during earthquakes and displaced the seafloor to generate tsunamis, are often unknown. Here we identify the submarine rupture of the Amorgos earthquake that on July 9, 1956, triggered the largest mediterranean tsunami in the past two centuries. Using submarines, we explored major normal faults in the epicentral area, and discovered a large surface rupture along the 75-km long Amorgos fault. The 9.8-16.8-m large seafloor offset is compatible with a Mw7.5 event. This finding prompts a reassessment of the largest (≥20 m) tsunami wave origin, previously attributed to earthquake-triggered submarine mass-wasting. It demonstrates that tsunami source can be determined several decades after an event, a key information to better assess future seismic and tsunami hazards. A surface rupture which offset the seabed by 9.8-16.8 m along the 75 km long Amorgos fault is probably the main cause of the largest tsunami in the Mediterranean Sea in the last two centuries, according to direct submarine observations.

Seafloor Spreading

10.1038/s43247-024-01839-0

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Citations (1)

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

Geochemistry Geophysics Geosystems (2017)

J. Escartı́n Catherine Mével Sven Petersen D. Bonnemains Mathilde Cannat

Abstract Microbathymetry data, in situ observations, and sampling along the 13°20′N and 13°20′N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high‐angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material ( apron ). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the along‐extension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension‐parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 13°20′N OCC, and gabbro and peridotite at 13°30′N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 13°30′N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 13°20′N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution.

Detachment fault

Slumping

Seafloor Spreading

Mass wasting

Lithology

Transform fault

Extensional fault

10.1002/2016gc006775

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Citations (116)