Multibeam Echo Research Articles

CAGE13-1 Cruise Report: Ocean acidification in the Barents Sea cruise: West Barents Sea slope, Barents Sea, north Norway

From the morning of May 26th to the morning of June 1st 2013, the Department of Geology of the University of Tromsø, Norway, arranged a scientific cruise aimed at investigating the western Barents Sea margin and the western Barents Sea on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): the western Barents Sea slope to northwest of Bjørnøya, and the western Barents and north Norway. The scientific sampling was done within the framework of the ongoing project at the Department of Geology, University of Tromsø: “OAFS-Ocean Acidification Fram Strait”.
 A total of 3 gravity cores, 1 box-core, 5 multi-cores (with a total of 15 sub-cores retrieved), 13 plankton net casts and 13 CTD (conductivity-temperature-depth) casts were performed at the western Barents Sea slope, and the Barents Sea.
 >17 chirp profiles and multibeam echo sounder lines were acquired during transits and during surveys of potential core sites.
 The cruise may be known as: CAGE13_OA

CAGE13-2 Cruise Report: CAGE, OAFS and Paleo-CIRCUS cruise to Northwest Norway, Jan Mayen and East Greenland

From the morning of June 23rd to the morning of July 7th 2013, the Department of Geology of the University of Tromsø, Norway, arranged a scientific cruise aimed at investigating the western Northern Norwegian margin and the central Nordic seas and east Greenland margin| on R/V “Helmer Hanssen”. Investigated areas were (in order of visiting sites on the cruise): the northwestern Norwegian Sea margin to north of Jan Mayen, and the western Greenland Sea north and off Scorebysund. The scientific sampling was done within the framework of ongoing projects at the Department of Geology, University of Tromsø: “CAGE (CoE): Arctic Gas Hydrate, Environment and Climate”, hereunder the projects “OAFS-Ocean Acidification Fram Strait” and “Paleo-CIRCUS”.
 A total of 12 gravity cores, 5 piston cores and 9 multi-cores (with a total of 27 sub-coresretrieved), 15 plankton net casts and 20 CTD (conductivity-temperature-depth) casts were performed at the western Barents Sea slope, and the Barents Sea.
 >44 chirp profiles and multibeam echo sounder lines were acquired during transits and during surveys of potential core sites.
 -4 seismic lines using GI mini airgun
 The cruise may be known as: CAGE13_OA_PaleoCIRCUS

A Sample Enhancement Method Based on Simple Linear Iterative Clustering Superpixel Segmentation Applied to Multibeam Seabed Classification

Full-coverage and high-efficiency seabed sediment detection and identification are critical elements of digital marine construction that support the three-dimensional and thematic development of maritime spatial geographic information systems. With the development of multibeam echo sounder (MBES), the use of MBES backscatter intensity and bathymetry data to extract backscatter angular response (AR) features has increased. Using backscatter intensity features and seabed terrain features for classification is an effective way to achieve large-scale seabed sediment classification. However, it was still limited by small sample size problems and the poor stability of the classification model due to the complexity of performing seabed sediment sampling. In response to the above issues, this paper proposes a sample enhancement method based on simple linear iterative clustering (SLIC) superpixel segmentation to address the problems. First, a superpixel-based sample homogeneity expansion method is combined with multibeam backscatter intensity images to achieve adaptive sample range selection. Then, a Random Forest (RF)-based model is constructed using MBES backscatter intensity and seabed terrain features. To assess the model’s validity, the experiment uses data from an extensive MBES survey and field sampling information from the Celtic Sea, UK. It achieves accurate predictions for the area’s eight sediment types. The experimental results show that the proposed method expands 60 groups of original sample points to 9293 groups of valid sample points, and achieves an overall classification accuracy and kappa coefficient of 84.05% and 0.81 for the seabed sediment, respectively. In addition, the classification accuracy is significantly better compared to the traditional methods.

Automated Detection of Hydrothermal Emission Signatures From Multibeam Echo Sounder Images Using Deep Learning

Seafloor massive sulfide deposits have attracted attention as a mineral resource, as they contain a wide variety of base, precious, and other valuable critical metals. Previous studies have shown that signatures of hydrothermal activity can be detected by a multi-beam echo sounder (MBES), which would be beneficial for exploring sulfide deposits. Although detecting such signatures from acoustic images is currently performed by skilled humans, automating this process could lead to improved efficiency and cost effectiveness of exploration for the seafloor deposits. Herein, we attempted to establish a method for automated detection of MBES water column anomalies using deep learning models. First, we compared the “Mask R-CNN” and “YOLO-v5” detection model architectures, wherein YOLO-v5 yielded higher F1 scores. We then compared the number of training classes and found that models trained with two classes (signal and noise) exhibited superior performance compared with models trained with only one class (signal). Finally, we examined the number of trainable parameters and obtained the best model performance when the YOLO-v5l model with a large trainable parameters was used in the two-class training process. The best model had a precision of 0.928, a recall of 0.881, and an F1 score of 0.904. Moreover, this model achieved a low false alarm rate (less than 0.7%) and had a high detection speed (20−25 ms per frame), indicating that it can be applied in the field for automatic and real-time exploration of seafloor hydrothermal deposits.

Detailed bathymetry and seafloor image datasets, using multi-beam echo sounder and side scan sonar, around Seonam, Ilsan beach, Ulsan

We obtained detailed bathymetry and seafloor image datasets using multi-beam echo sounder and side scan sonar around Seonam at Ilsan beach in Ulsan. The survey was conducted from May 9 to May 12, 2018, using a small vessel. The equipment used for the survey was R2Sonic's Sonic 2020 (Multi-beam Echo Sounder) and EdgeTech’s 4125 (Side Scan Sonar). The detailed bathymetry of the study area shows slightly different topographical features for each part around Seonam. The eastern and southern parts have steep slopes and the depth of the water in the areas deepens away from the surroundings of the submerged bedrock area adjacent to Seonam. The water depth in the western and northern part is shallower with a relatively gentle slope, compared to the eastern and southern parts. The rocky bottom zones are mainly distributed in the eastern part area with the water depth range of 3 to 9 m around the Seonam. Some rocky areas also appear in the western region. Seafloor backscatter image shows irregular, complex reflection patterns of acoustic intensity mainly in the eastern and western adjacent areas around Seonam, and in the northern and southern coastal waters, which are almost identical to the underwater rock zones with shallow water depth. The detailed bathymetry and seafloor image datasets around Seonam, using the multi-beam echo sounder and the side scan sonar, can be used as an essential data for environment habitat mapping.

Bedload estimation in large sand-bed rivers using Acoustic Mapping Velocimetry (AMV)

This paper describes the in-situ implementation of a recently developed bedload transport estimation method, called Acoustic Mapping Velocimetry (AMV). The method combines components and processing protocols from acoustic and image-based methods to provide velocity maps of bedforms migrating in the riverbed. The AMV implementation is illustrated with repeated Multibeam Echo Sounder (MBES) mappings that are acquired along longitudinal swaths over the entire transect of the study river sections. The paper exemplifies AMV implementation case studies leading to the estimation of bedload transport rates in two large sand-bed rivers: the Ohio and Mississippi rivers, in the USA. The analysis associated with these case studies emphasizes the importance of linking the characteristics of the bedform dynamics with the selection of the spatio-temporal parameters used during data acquisition for AMV processing (e.g., required length of the measurement section, and the effect of the time lag between the repeated surveys) for accurate quantification of bedform dynamics. The role of superimposed secondary small bedforms migrating over the large primary ones, in terms of their contribution to the sediment transport, is also discussed. The paper demonstrates the applicability of this novel technique, as a complement or stand-alone method for quantifying bedload transport in large sand-bed rivers, an area where the implementation of conventional methods is difficult and inadequate while the emerging measurement alternatives are not widely recognized yet.

Channel Bed Adjustment of the Lowermost Yangtze River Estuary from 1983 to 2018: Causes and Implications

Deltaic channels are significant landforms at the interface of sediment transfer from land to oceanic realms. Understanding the dynamics of these channels is urgent because delta processes are sensitive to climate change and adjustments in human activity. To obtain a better understanding of the morphological processes of large deltaic channels, this study assessed the evolution and response mechanism of the South Channel and South Passage (SCSP) in the Yangtze Estuary between 1983 to 2018 using hydrology, multibeam echo sounding and historical bathymetry datasets. Decadal changes in riverbed volume and erosion/deposition patterns in the SCSP were assessed. The results showed that the SCSP experienced substantial deposition with a total volume of 26.90 × 107 m3 during 1983–2002, but significant bed erosion with a total volume of 26.04 ×107 m3 during 2003–2010. From 2011 to 2018, the estuarine riverbeds shifted from erosive to depositional, even though the deposition was relatively marginal (0.76 ×107 m3). We inferred that the SCSP have most likely changed from a net erosion phase to a deposition stage in response to local human activities including sand mining, river regulation project, and Deep Water Channel Regulation Project). The channel aggradation will possibly continue considering sea level rise and the ongoing anthropogenic impacts. This is the first field evidence reporting that the lowermost Yangtze River is reaching an equilibrium state in terms of channel erosion and, in fact, the Yangtze River Estuary channels are beginning to aggrade. The findings have relevant implications for the management of the Yangtze River and other lowland alluvial rivers in the world as global sea level continues rising and human intervention on estuarine systems persists.

О ПРИМЕНИМОСТИ МЕТОДА БЫСТРОГО ЦИФРОВОГО ФОРМИРОВАНИЯ ХАРАКТЕРИСТИК НАПРАВЛЕННОСТИ В СИСТЕМАХ ЛОКАЦИИ СО СЛОЖНЫМИ СИГНАЛАМИ

This work belongs to the field of hydroacoustics, namely to the problem of digitalbeamforming based on spatial fast Fourier transform (FFT) in a multibeam echo sounder(MLS) with baseband digital signal processing. The applicability of FFT-based beamformingwhen using relatively broadband signals is considered. The main attention is paid to the MLSwith a static beam pattern, at the same time, obtained results are also applicable to rangingsystems that provide dynamic beam control. The aim of the study is to establish the relatio nshipbetween the relative width of the signal spectrum and signal level in the spatial channelof the MLS with a given maximum beam steering. A qualitative condition for the applicabilityof FFT-based spatial beamforming for a probing signal with a given relative bandwidth isdetermined. An analytical expression linking the signal attenuation coefficient in the spatialchannel with the following signal and receiver characteristics was obtained: the number ofelements of the linear equidistant antenna array, the distance between array elements, beamsteering, the correlation function of the complex envelope of the signal, the window functionfor weighting the matched filter response in spectral domain. Presented results can be usefulin designing multibeam echo sounders, providing high resolution both in range and in angularcoordinates.

Sneaking into a Hotspot of Biodiversity: Coverage and Integrity of a Rhodolith Bed in the Strait of Sicily (Central Mediterranean Sea)

Habitat mapping, physical characteristics and benthic community of a rhodolith bed in the Pelagie Islands (Strait of Sicily, Mediterranean Sea) were studied through Multi–Beam Echo–Sounder (MBES), Remotely Operated Vehicle (ROV) and grab samples. The geomorphological analysis revealed an articulated and wide rhodolith bed; video inspections highlighted a bed with high coverage, few sandy patches and with a prevalence of the boxwork morphotype. A total of 207 taxa with 876 specimens were identified, and Polychaeta was the dominant taxon. Linguimaera caesaris, a Lessepsian benthic amphipod, was recorded in all sampling sites, and its presence represents an input to deepen the benthic assemblage research on the rhodolith bed. In terms of morphotype composition, dead/live ratio and species variability, the bed variability indicated a good status of health, although trawling signs were detected through ROV videos. The present study broadens the knowledge on Mediterranean rhodolith beds and supports the importance of survey and monitoring activities for the conservation and management of this important habitat.

Organizing and visualizing point clouds with continuous levels of detail

Point clouds contain high detail and high accuracy geometry representation of the scanned Earth surface parts. To manage the huge amount of data, the point clouds are traditionally organized on location and map-scale; e.g. in an octree structure, where top-levels of the tree contain few points suitable for small scale overviews and lower levels of the tree contain more points suitable for large scale detailed views. The drawback of this solution is that it is based on discrete levels, causing visual artifacts in the form of data density shocks when creating the commonly used perspective views. This paper presents a method based on an optimized distribution of points over continuous levels, avoiding the visualization shocks. The traditional distribution ratio’s of data amounts over discrete levels of raster or vector data is considered the reference. How to convert this to point clouds with continuous levels (still benefiting from the proven advantages of the data distribution in discrete levels for efficient access at a wide range of scales)? In our solution, for each point a cLoD (continuous Level of Detail) value is computed and added as dimension to the point. A SFC (Space Filling Curve)-based nD data clustering technique can be used to organize the points, so that they can be efficiently queried. It should be noted that also other multi-dimensional indexing and clustering techniques could be applied to realize continuous levels based on the cLoD value. Besides the mathematical foundation of the approach also several implementations are described, varying from a 3D web-browser based solution to an augmented reality point cloud app in a mobile phone. The cLoD enables interactive real-time visualization using perspective views without data density shocks, while supporting continuous zoom-in/out and progressive data streaming between server and client. The described cLoD based approach is generic and supports different types of point clouds: from airborne, terrestrial, mobile and indoor laser scanning, but also from dense matching optical imagery or multi-beam echo soundings.

Application of Remote Sensing Techniques to Identification of Underwater Airplane Wreck in Shallow Water Environment: Case Study of the Baltic Sea, Poland

Multibeam echo sounders (MBES), side-scan sonars (SSS), and remotely operated vehicles (ROVs) are irreplaceable devices in contemporary hydrographic works. However, a highly reliable method of identifying detected wrecks is visual inspection through diving surveys. During underwater research, it is sometimes hard to obtain images in turbid water. Moreover, on-site diving operations are time-consuming and expensive. This article presents the results of the remote sensing surveys that were carried out at the site of a newly discovered wreck, in the southern part of the Baltic Sea (Poland). Remote sensing techniques can quickly provide a detailed overview of the wreckage area and thus considerably reduce the time required for ground truthing. The goal of this paper is to demonstrate the process of identification of a wreck based on acoustic data, without involving a team of divers. The findings, in conjunction with the collected archival documentation, allowed for the identification of the wreck of a Junkers Ju-88, a bomber from World War II.

Pre-rig mobilization hazard evaluation in offshore oil and gas prospect drilling: A case study of TM field, offshore Niger Delta

TM Field, located offshore Niger Delta in the Gulf of Guinea (Nigeria), has been evaluated for potential drilling hazards prior to the drilling of a proposed oil well. This is in a region where potential hazards that could inhibit successful drilling exercise have not been adequately studied. The study adopted the modern technique of offshore geohazard evaluation that relies mainly on suites of geophysical and shallow geological investigations which include ocean bathymetry using multi-beam echo sounder, side-scan sonar, sub-bottom profiler, magnetic, and 2D-high resolution seismic reflection surveys. The results were integrated to draw inferences about the risk potential of the field. From the bathymetric survey, water depths were found to range from 345 m to 650 m LAT, with seabed relief being 305 m. Water depth at the proposed well location was found to be approximately 450 m and the seabed was found to slope in the northwestern direction. Seafloor gradient was computed as 0.05 across the whole area. Two shallow sub-surface stratigraphic units, labelled A and B, were delineated. Unit A was directly beneath the seafloor and seemed to be composed mainly of clayey/silty sand. Its thickness ranged from 6 m to 70 m across the study area. Unit B appeared to be a layer of sand and its total thickness was not fully delineated. Observable kinks and displacements at the flanks of seabed undulations/depressions on sub-bottom profiler sections suggested that the depressions may be fault-controlled. On the map generated from the marine magnetic data, a magnetic fault was delineated and this appeared to be coincident with the shallower of the two buried faults on the cross-line seismic section cutting through the proposed well-head location. Interpreted seismic reflection data presented this fault as being inactive. Most of the faults mapped, especially the buried ones, appeared to be inactive except a few. Regions with mapped amplitude anomalies are to be avoided during rig-mobilization operations. Though engineering conditions are expected to vary vertically since layers occur as intercalation, significant lateral variation is not expected within uniform layers of bed. Risk of punch-through associated with clayey formation is expected to be low because the geologic layers are not uniformly and entirely clay. The proposed well-head location was found to be free of potential hazard sources.

On spreading of Antarctic Bottom Water in fracture zones of the Mid-Atlantic Ridge at 7–8°N

A Data-Interpolating Variational Analysis in n-dimensions was used to describe a potential temperature distribution in the bottom layer of the fracture zones of the Mid-Atlantic Ridge at 7–8°N. This analysis was based on a new digital terrain model obtained by supplementing the STRM15+ bathymetry data with multibeam echo sounding data from the 33rd cruise of the research vessel
 Akademik Nikolaj Strakhov (2016) and oceanological data from the World Ocean Database, supplemented with CTD profiles and reversing thermometer data measured in scientific cruises of the Shirshov Institute of Oceanology, Russian Academy of Sciences in 2014–2016. A 2D model of near-bottom potential temperature distribution in the study area was calculated based on the analysis. The model allows us to propose the Antarctic Bottom Water propagation pattern through the Doldrums, Vernadsky, and Pushcharovsky fracture zones. It is shown that bottom water warms up when passing fracture zones from 1.4°C in Pushcharovsky Fracture Zone up to 1.6–1.7°C in Vernadsky Fracture Zone. Bottom water from Pushcharovsky and Vernadsky fractures propagates in two directions. Northernly, it propagates to the Doldrums Fracture Zone, where its temperature reaches about 1.9–2.0°C. Easterly, it flows along Pushcharovsky Fracture Zone and raising the temperature up to 1.8–2.0°C. We propose the absence of Antarctic Bottom Water's overflow with a temperature less than 1.8°C to the East Atlantic in the study area.

There is no geophysical evidence for the Mahuika Crater on the continental shelf southwest of New Zealand

ABSTRACT This study examines new bathymetric and geophysical evidence that questions the existence of the Mahuika Crater: a proposed impact depression 20.2 km wide lying on the continental shelf ∼250 km SW of New Zealand. Multibeam echo sounder and sub-bottom profiler data across the published location of the crater were collected and synthesised with existing bathymetric data for the wider region to assess the validity of such a claim. We conclude that there is no geophysical basis for the existence for the Mahuika impact crater in the region, the initial presumption of a crater was unsubstantiated, and is a myth propagated in the literature.

Mesophotic Reefs of the Largest Brazilian Coastal Protected Area: Mapping, Characterization and Biodiversity

Mesophotic reefs are poorly known worldwide despite their great ecological relevance and management importance for coral reef conservation strategies. To aid in filling this gap, we conducted a pioneering, large-scale survey, covering a total of around 315 miles in length, in the largest Brazilian coastal Marine Protected Area (MPA) Costa dos Corais. From the digital bathymetry model (30 to 50 m depth) generated by a multibeam echo sounder, we selected areas of greater geomorphological diversity for a detailed investigative expedition of mesophotic ecosystems. Various sampling techniques were used: single-beam echo sounders for detailing the relief, a remotely operated underwater vehicle (ROV) for habitat type investigation, baited remote underwater video (BRUV) for collecting images of the fish community, and scuba diving to perform transects describing the benthic and fish community. We analyzed reef environments from 20 to 68 m deep. As a result, we present the mapping and geomorphological characterization of two compartments of mesophotic reefs at 21–45 m depth and an image library of mesophotic ecosystems with the species description and indications of whether it is a new record in the region. Biodiversity data were collected covering eight species of sponges, with greater abundance for Ircinia spp., Aplysina spp., and Xestospongia muta; eight from corals, mainly Siderastrea spp. And Montastrea cavernosa; and 68 species of reef fish, with the Labridae family (including Scarinae—11 species) being the richest. Our results demonstrate the importance of mesophotic reefs for MPA Costa dos Corais reef biodiversity and, with that, the need to protect these areas through the application of local conservation strategies, such as the creation of “no-take zones”.

A Case Study of Completely Buried Wind-Power Cable Detection Using 3D Acoustic Imaging

As offshore wind power is renewable energy produced through the installation and operation of large-scale offshore infrastructure, risk management is crucial in power platforms. Safety accidents caused by external factors during the operation of submarine power cables can lead to enormous costs, thus necessitating the monitoring of burial depth and route information of cables. In this study, we developed a 3D acoustic imaging method that obtains information on the route and depth of completely buried power cables. An acoustic source-based engineering ocean seismic 3D (EOS3D) system has been used to detect buried objects in the subsurface because conventional sonars, such as multi-beam echo sounder (MBES) and side-scan sonar (SSS), which are used to analyze seafloor characteristics, have limitations in detecting completely buried cables in the subsurface. Field data were obtained as 8-channel data using a chirp source (2–8 kHz) designed to obtain a 25 × 25 cm horizontal spatial resolution from real-time kinematic (RTK) positioning. The image stack method was proposed to effectively detect buried cables, with the vertical gradient analyzed using signals decomposed into representative bin sizes and low-mid-high-frequency components. The acoustic anomalies of buried objects, identified as export cables and protectors, were processed into images using the proposed image stack method and gradient analysis. This case study of buried wind power cables using 3D acoustic imaging could be utilized in burial assessment survey (BAS)-data acquisition, processing/analysis processes, and operation and management of buried cables.

Accelerated Deconvolved Imaging Algorithm for 2D Multibeam Synthetic Aperture Sonar.

High-accuracy level underwater acoustical surveying plays an important role in ocean engineering applications, such as subaqueous tunnel construction, oil and gas exploration, and resources prospecting. This novel imaging method is eager to break through the existing theory to achieve a higher accuracy level of surveying. Multibeam Synthetic Aperture Sonar (MBSAS) is a kind of underwater acoustical imaging theory that can achieve 3D high-resolution detecting and overcome the disadvantages of traditional imaging methods, such as Multibeam Echo Sounder (MBES) and Synthetic Aperture Sonar (SAS). However, the resolution in the across-track direction inevitably decreases with increasing range, limited by the beamwidth of the transducer array of MBES. Furthermore, the sidelobe problem is also a significant interference of imaging sonar that introduces image noise and false peaks, which reduces the accuracy of the underwater images. Therefore, we proposed an accelerated deconvolved MBSAS beamforming method that introduces exponential acceleration and vector extrapolation to improve the convergence velocity of the classical Richardson-Lucy (R-L) iteration. The method proposed achieves a narrow beamwidth with a high sidelobe ratio in a few iterations. It can be applied to actual engineering applications, which breaks through the limitation of the actual transducer array scale. Simulations, tank, and field experiments also demonstrate the feasibility and advantages of the method proposed. 3D high-accuracy level underwater acoustical surveying can be achieved through this 2D MBES transducer array system, which can be widely promoted in the field of underwater acoustical remote sensing.

Sidescan Only Neural Bathymetry from Large-Scale Survey.

Sidescan sonar is a small and low-cost sensor that can be mounted on most unmanned underwater vehicles (UUVs) and unmanned surface vehicles (USVs). It has the advantages of high resolution and wide coverage, which could be valuable in providing an efficient and cost-effective solution for obtaining the bathymetry when bathymetric data are unavailable. This work proposes a method of reconstructing bathymetry using only sidescan data from large-scale surveys by formulating the problem as a global optimization, where a Sinusoidal Representation Network (SIREN) is used to represent the bathymetry and the albedo and the beam profile are jointly estimated based on a Lambertian scattering model. The assessment of the proposed method is conducted by comparing the reconstructed bathymetry with the bathymetric data collected with a high-resolution multi-beam echo sounder (MBES). An error of 20 cm on the bathymetry is achieved from a large-scale survey. The proposed method proved to be an effective way to reconstruct bathymetry from sidescan sonar data when high-accuracy positioning is available. This could be of great use for applications such as surface vehicles with Global Navigation Satellite System (GNSS) to obtain high-quality bathymetry in shallow water or small autonomous underwater vehicles (AUVs) if simultaneous localization and mapping (SLAM) can be applied to correct the navigation estimate.

Seagrass Habitat Suitability Models using Multibeam Echosounder Data and Multiple Machine Learning Techniques

Seagrass beds are important habitats in the marine environment by providing food and shelter to dugongs and sea turtles. Protection and conservation plans require detail spatial distribution of these habitats such as habitat suitability maps. In this study, machine learning techniques were tested by using Multibeam Echo Sounder System (MBES) and ground truth datasets to produce seagrass habitat suitability models at Redang Marine Park. Five bathymetric predictors and seven backscatter predictors from MBES data were used to representing topography features and sediment types in the study area. Three machine learning algorithms; Maximum Entropy (MaxEnt), Random Forests (RF), and Support Vector Machine (SVM) were tested. The results revealed that MaxEnt and RF models achieved the highest accuracy (93% and 91%, respectively) with SVM produced the lowest (67%). Depth was identified as the most significant predictor for all three models. The contributions of backscatter predictors were more central for SVM model. High accuracy models showed that suitable habitat for seagrass is distributed around shallow water areas (<20 m) and between fringing reef habitats. The findings highlight that acoustic data and machine learning are capable to predict how seagrass beds are spatially distributed which provide important information for managing marine resources.

Seasonal Variation of Catenary-Bead Dunes in the Yangtze River Estuary: Causes and Implications

Low-angle lee-side slopes of dunes are commonly developed on the world’s riverbeds, and dune migration associated with sediment transport exert a major influence on riverine processes. However, the catenary-bead dune has been identified in the Yangtze River (YR) Estuary, featuring a higher lee-side angle. To date, the morphological variation and formation reasons of catenary-bead dunes in the YR Estuary remain uncharacterized. In this study, we used a multibeam echo system (MBES) to investigate the bedforms of the YR estuary during 2014–2015, as well as to discuss the seasonal variation of catenary-bead dunes. The results indicate that the catenary-bead dunes of the YR Estuary are characterized by growth during the flood season and extinction during the dry season. The lee-side angle is typically ~16.7°, which is larger than that of other dune types (3.7–8°) in the estuary; moreover, the catenary-bead dunes are higher than other dune types of the same length in the YR Estuary. The relationship between the dune height (H) and length (L) was found to be H = 0.1667L0.603 (R2 = 0.38), while the other dune types yielded the relationship of H = 0.0845L0.758 (R2 = 0.52). Strong runoff superimposing the ebb tide led to the development of catenary-bead dunes. Furthermore, the higher coarse sediment content (69.9–72%) and lower clay content (6.3–6.7%) of the riverbed sediment are favorable for their formation, while the higher curved crest-lines are favorable for the formation of the associated elliptical pits.