Displays

GM6.7

The ocean floor hosts a tremendous variety of forms that reflect the action of a range of tectonic, sedimentary, oceanographic and biological processes at multiple spatio-temporal scales. Being able to map the form and shape of the seabed and to understand the processes that shape it is a major prerequisite to ocean and coastal management, nature conservation and hazard assessment as well as a key objective of national and international research programmes and IODP expeditions.

High quality seafloor maps are integral to submarine geomorphic investigations. Acoustic remote-sensing technologies (singlebeam, multibeam, sidescan, interferometric and synthetic-aperture sonars), deployed on various platforms, are fundamental to seafloor mapping. In relatively shallow and transparent waters, optical methods such as aircraft and satellite-based remote sensing and LIDAR are being employed with increasing success. Seafloor maps, especially when combined with sub-seafloor and/or seabed measurements, provide an exciting opportunity to integrate the approaches of geomorphology and geophysics, and to extend quantitative geomorphology offshore. 3D seismic reflection data has also given birth to the discipline of seismic geomorphology, which has provided a 4D perspective to continental margin evolution. Innovative processing and classification software, image analysis, machine and deep-learning applications are advancing developments in seabed-recognition techniques.

The aim of this interdisciplinary session is two-fold: (i) to highlight recent advances in seabed mapping and classification, and (ii) to improve the understanding of the causes and consequences of geomorphic processes shaping underwater landscapes, including submarine erosion and depositional processes, submarine landslides, sediment transfer and deformation, volcanic activity, fluid migration and escape, faulting and folding, among others. Contributions to this session can include work from any physiographic region, ranging from shallow coastal settings to abyssal plains and deep-sea trenches. Datasets of any scale, from satellite-predicted depth to ultra-high resolution swath bathymetry, sub-surface imaging and sampling, are anticipated.

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Co-organized by OS4/SSP3, co-sponsored by IAG
Convener: Aaron Micallef | Co-conveners: Markus Diesing, H. Christian Hass, Sebastian Krastel, Alessandra Savini, Maria Judge, Kim Picard, Anne-Cathrin WölflECSECS
Displays
| Wed, 06 May, 14:00–18:00 (CEST)

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Session summary Download all presentations (142MB)

Chat time: Wednesday, 6 May 2020, 14:00–15:45

Chairperson: Sebastian Krastel
D1038 |
EGU2020-4242
Character, spatial and temporal variation of turbidity currents on a source-to-sink scale.
(withdrawn)
Kate Heerema, Peter Talling, Matthieu Cartigny, Gwyn Lintern, Cooper Stacey, Randy Enkin, Sophie Hage, Claire McGhee, Ye Chen, Dan Parsons, Steve Simmons, and Mike Clare
D1039 |
EGU2020-3849
Yael Sagy, Oz Dror, Michael Gardosh, and Moshe Reshef

The progradation of the Nile River Delta and the thick (~1500m) Sinai-Israel shelf since the Pliocene provide a world class source to sink system feeding a deep (>1.5 km) siliciclastic basin.  The general agreement that the Pliocene-to-Recent succession originates from the Nile Delta dispersing sediments via a system of counterclockwise currents does not reveal how the sediments were transported to the deep basin. Particularly, how sediments originating from the Nile Delta could have bypassed the ~50 km wide Sinai-Israeli shelf. Here, we examine the various sources that contributed to the accumulation of the Pliocene-to-Recent succession in the deep Levant basin, and the temporal and spatial contribution of each source. The analysis of a unique seismic data set covering the shelf, slope and deep basin enable us to track submarine sediment transport systems.

Following attribute analysis of the seismic volumes we map channel sets, analyze their morphological features and interpret their erosional and depositional patterns. Direction flow maps indicate that sediments sources vary from eastward remnant Arabian drainage network at the onset of the Pliocene, to direct Nilotic origin during the Pliocene. Since the Late Pleistocene reworked sediments, deriving from the Israeli shelf and northern Sinai provide a major source to the deep basin. Furthermore, our results demonstrate an increase in channel’s complexity since the Early Pliocene to Recent suggesting a gradual transition from sporadic submarine flow events, carrying fewer sediments to the deep basin at the Early Pliocene, to more frequent events during the Late Pleistocene-to-Recent characterized by an increase in sediment load. The gradual increase of channel complexity from Pliocene-to-Recent is discordant to the general trend of sea-level fluctuation, suggesting that sea-level has a minor effect on sediment accumulation in the deep basin. We propose that the balance between the northward prograding Nile Cone and the longshore currents building the Sinai-Israeli shelf dictate siliciclastic accumulation in the southeastern Mediterranean basin as well as the paleogeography of its margin.

D1040 |
EGU2020-10812
Ingo Pecher, Bryan Davy, Jess Hillman, Lowell Stott, Richard Coffin, Anna Prestage, Paula Rose, and Joerg Bialas

An area of the seafloor of >50,000 km2 on the Chatham Rise and Bounty Trough east of New Zealand’s South Island is covered by seafloor depressions.  Distribution and type of these depressions seem to be bathymetrically controlled, with smaller depressions occurring between ~500-700 m water depth and larger ones in water depths of >800 m.  Formation of these features is enigmatic.  The smaller features display typical features of pockmarks caused by sudden escape of fluids and gas.  Echosounder and seismic data furthermore reveal wide-spread buried pockmarks that appear to have been formed repeatedly near glacial-stage maxima.  Some of the buried pockmarks appear to be stacked, often at a slight offset, underlain by positive-polarity reflections, and aligned with structures that promote fluid escape.  These patterns are compatible with repeated release of fluids from deep sources and precipitation of authigenic material.  Some of the larger seafloor depressions appear to involve interaction with the Southland Current.  These depressions have been interpreted as contouritic mounds although alternative hypotheses have been proposed and they may be linked to deeply rooted fluid migration.

Pronounced Δ14C anomalies during the last glacial termination, around the time of formation of the most recent pockmarks, indicate release of significant amounts of geologic carbon.  The pockmark fields coincide with the extent of the flat-subducted Hikurangi Plateau.  We hypothesize formation of the pockmarks is linked to repeated release of CO2 that originates from carbonates on top of the Hikurangi Plateau.  We will discuss this hypothesis, open questions in particular related to the “valve” mechanism controlling repeated release and pockmark formation, as well as alternative mechanisms for possible formation of seafloor depressions in the study area.

D1041 |
EGU2020-12332
Jasper Hoffmann, Jens Schneider von Deimling, Jan Schröder, Mark Schmidt, Philipp Held, Jan Scholten, Gareth Crutchley, and Andrew Gorman

Submarine groundwater discharge into coastal areas is a common global phenomenon and is rapidly gaining scientific interest due to its influence on marine biology and the coastal sedimentary environment, and it's potential as a future freshwater resource. We conducted an integrated study of hydroacoustic surveys combined with geochemical porewater and water column investigations at a well-known freshwater seep site in Eckernförde Bay (Germany).

The location and distribution of pockmarks in this area have been the focus of many studies since their discovery in 1966 including numerous investigations of their geochemical, geological and geophysical behavior. Despite several intense and extensive research campaigns (e.g. Sub-GATE/CBBL) their internal morphology and structure presented in this study were poorly constrained to date. With recent advances in shallow high-frequency multibeam echosounder methods combined with highly accurately positioned sediment cores, we can provide new insights on the influence of shallow gas and freshwater on the formation and internal morphology of the pockmarks. We show that high-frequency multibeam data can be used to detect free shallow gas in areas of enhanced freshwater advection in muddy sediments. Intra-pockmarks, forming due to ascending gas and freshwater, pose a new form of ‘eyed’ pockmarks revealed by their acoustic backscatter response. Our data suggest that in muddy sediments morphological lows combined with a strong multibeam backscatter signal can be indicative of free shallow gas and the subsequent advection of freshwater.

D1042 |
EGU2020-9901
Calvin Campbell, Alexandre Normandeau, Paul Fraser, and Adam MacDonald

Cold seeps occur where fluids, such as hydrocarbons, migrate from depth and escape at the seabed. They are relatively common features in petroleum basins around the world. Cold seeps often host unique biological communities and are a potential geological hazard as they can indicate excess pore fluid pressures in shallow sediments. In addition, they can provide critical information about fluid migration pathways and fluid source. This study presents the detailed geomorphology and seismic stratigraphy of recently discovered cold seeps in 2700 metres water depth offshore Nova Scotia, Canada. 

Petroleum industry 3D seismic reflection data, high-resolution single channel G.I. gun and sparker seismic reflection data, Autonomous Underwater Vehicle (AUV) sidescan, swath bathymetry, and sub-bottom profiler data were used to investigate the geomorphology of the cold seep and surrounding seabed. Piston core samples and seabed photography were also acquired in the study area.

The geomorphology in the study area is dominated by the seafloor expression of a salt diapir (L. Triassic to E. Jurassic). Despite being buried by ~1700 m of Cretaceous to Holocene sediment, the diapir forms an oblong mound, 10 km long by 5 km wide that rises 200 m above the surrounding seabed. Two major orthogonal faults are apparent on the seabed that cut the mound along its major and minor axes. Several crestal faults are imaged in the 3D seismic data but do not have a seabed expression. AUV data acquired over the crest of the diapir reveal a 500 m by 200 m fissure on the western flank of the diapir. The fissure is composed of a blocky central zone along its axis, and radiating “cracks” that show backscatter variation, possibly indicating recent fluid expulsion. Integration of the AUV data with the 3D seismic data show that the fissure is fed by a vertical chimney that intersects a bottom simulating reflection above the diapir. Remarkably, the chimney does not appear to be related to any of the sub-vertical crestal faults. Another seep occurs on the eastern flank of the diapir crest and, in contrast, coincides with a crestal fault. There is also evidence for mass wasting down-dip from the fault. Core samples recovered from the second seep contained gas hydrate. In both cases, the cold seeps present as very subtle features on the 3D seismic reflection data and are only positively identified in the AUV datasets. This study shows that conventional surface-acquired acoustic data are potentially insufficient for detecting cold seep morphologies in deep-water settings.

D1043 |
EGU2020-17601
Joana Gafeira, Dave McCarthy, Tom Dodd, and Gayle Plenderleith

The North Falkland Basin, a Mesozoic-aged sedimentary basin, located 40 km north of the Falkland Islands, is a rift system comprising a series of offset depocentres. The largest of the depocentres, the Eastern Graben, has a proven petroleum system hosting stratigraphic and combined structural-stratigraphic traps. In the shallow section overlying this significant sedimentary basin, there is a selection of unusual geomorphological features observable on 3D seismic data.

These features are observed at time-depths around 20–150 ms two-way-travel-time below the seabed, and are concentrated in three main geographical areas, covering in total more than 600 km2. These oval to polygonal depressions are typically 350–650 m across and have c. 5–10 ms of relief. The depressions are delineated by an interconnected network of V-shaped cracks, which do not appear to have a preferred orientation, and seem to be limited stratigraphically within two reflectors. The features were initially attributed to polygonal faulting but, after further investigation, they appear to be very similar to honeycomb structures observed in the Great South Basin of New Zealand, that were attributed to diagenetic processes. Immediately above the honeycomb structure, there is a series of pockmarks that may be related to fluid expulsion from below. The shallow depths at which they are found and the evidence of fluid expulsion suggests these features could be due to the opal-A/CT transition.

Other intriguing features include a sequence of mounds that are typically 150–250 m wide and display 2–5 ms of height. In seismic profiles, the first few horizons directly below the mounds show small centres of disturbance of the reflection. The amplitude map of this reflector shows a strong amplitude contrast between the mounds and the surrounding areas. The vast majority of the mounds present acoustic shadow towards NW that can extend for a few hundred meters. The geometry and dimensions of these mounds are consistent with deep-water coral mounds and the observed acoustic shadow could result from the preferential accumulation of coral rubble NW of the mounds, which could be indicative of the predominant currents during that period.

This contribution will provide a detailed discussion of the morphology of these geomorphological features and their relationship to the overlying fluid expulsion structures, sedimentary setting as well as suggesting possible mechanisms for their formation.

D1044 |
EGU2020-8258
Yin-Hsuan Liao, Ho-Han Hsu, Jyun-Nai Wu, Tzu-Ting Chen, Eason Yi-Cheng Yang, Arif Mirza, and Char-Shine Liu

        Submarine sand waves are known to be induced by tidal currents and their migration has become an important issue since it may affect seafloor installations. In Taiwan Strait, widely spreading sand waves have been recognized on the Changyun Ridge, a tide-dominated giant sand ridge offshore western Taiwan. However, due to lacking of high-resolution and repeated geophysical surveys before, detailed characteristics and migrating features of the sand waves in Taiwan Strait were poorly understood. As new multibeam bathymetric and seismic data were collected repeatedly during 2016 - 2018 for offshore wind farm projects, we can now advance the understanding of sand wave characteristics and migration patterns in the study area. We apply a geostatistical analysis method on bathymetry data to reveal distribution and spatial characteristics of the sand waves, and estimate its migration pattern by using an updated spatial cross-correlation method. Then, sedimentary features, internal structures and thicknesses of sand waves are observed and estimated on high-resolution seismic profiles. Our results show that the study area is mostly superimposed by multi-scaled sandy rhythmic bed forms. However, the geomorphological and migrating characteristics of the sand waves are complicated. Their wavelengths range from 80 to 200 m, heights range from 1.5 to 8 m, and crests are generally oriented in the WNW-ESE direction. Obvious sand wave migration was detected from repeated high-resolution multi-beam data between 2016 and 2018, and migration distances can be up to ~150 m in 15 months. The average elevation change of the seafloor over the whole survey area is ~3.0 m, with a maximum value of 6.9 m. Moreover, the sand waves can migrate over 30 m with ~2.5 m elevation change in 2 months and migrate over 5 m with ~1 m elevation change in 15 days. The results also show that the orientation of wave movement can be reversed even within a small distance. By identifying the base of sand wave on seismic profiles, the thicknesses of sand waves are found ranging from 1 to 10 meters. The base of wave structure become slightly deeper from nearshore to offshore. Our results indicate that the thickness of sand waves increases with degree of asymmetry and migration rate. By bathymetric and reflection seismic data analyses, systematic spatial information of sand waves in the study area are established, and we suggest that not only tidal currents can affect sand wave migration patterns, but also wave structures and thicknesses play important roles in sand wave migrating processes and related geomorphological changes.

D1045 |
EGU2020-11973
Yu Gan, Xiaochuan Ma, Zhendong Luan, and Jun Yan

Many seamounts in the deep sea have been found and surveyed in detail in recent decades of years. However, these seamounts are mostly described qualitatively or with little quantitative analysis, which counts against deep understanding of the dynamic processes of the seafloor. Here, a recently-surveyed guyot on the Caroline ridge in West Pacific is reported and its geomorphology is documented in detail based on the high-resolution Digital Elevation Models (DEMs). Multifractal Detrended Fluctuation Analysis (MFDFA) is firstly applied on the bathymetric data to investigate the multifractal features, and the cause of multifractality is also verified by analyzing shuffled and surrogate data. The shape of the multifractal spectrum is depicted by the width of the spectrum (W), the maximum singularity strength (α0) and the degree of asymmetry (B). To examine distinctions between submarine seamounts and subaerial volcanic structures, the same method and statistical comparison have also been applied on DEMs of other seamounts adjoining the guyot, the SRTM 90m DEMs of 50 subaerial stratovolcanoes and the Mars MGS MOLA-MEX HRSC Blended 200m DEMs of 5 Martian volcanoes. In the guyot area, geomorphological units of the guyot can be recognized and classified into large-scale volcanic structures and small-scale erosive-depositional landforms. The result shows that the topography of the guyot has multifractal features and the multifractal strength (Δh) differs spatially. Multifractality of the seafloor with the flat guyot top is mostly caused by the broad probability density function of the values of bathymetric data, while multifractality of the seafloor with highly-correlated small-scale landscapes (gullies and faults) by different long-range correlations of the small and large fluctuations. The guyot and other landforms with flat tops around are featured by higher maximum singularity strength (α0). Areas with widely-distributed small-scale landforms and intense fluctuations in curvature values tend to have negative degrees of asymmetry (B). Moreover, two-sample unequal-variance t-test results show that Hurst exponents (H) and the multifractal strength (Δh) of seamounts are generally lower than those of earth and Martian subaerial volcanoes, which implies that seamounts may have distinct fractal behaviors and multifractal features compared to their subaerial counterparts. The study presents a case of quantifying geomorphological characters and multiscale behaviors of seamounts in the deep-sea area, which could encourage more explorations for the morphologies and processes of the analogous structures in submarine, terrestrial or even planetary environments. Nevertheless, more detailed and comparative works are still needed to be done.

D1046 |
EGU2020-8451
Katja Kuhwald, Philipp Held, Florian Gausepohl, Jens Schneider von Deimling, and Natascha Oppelt

Seagrass meadows cover large benthic areas of the Baltic Sea, but eutrophication and climate change imply declining seagrass coverage. Apart from acoustic methods and traditional diver mappings, optical remote sensing techniques allow for mapping seagrass. Optical satellite analyses of seagrass mapping may supplement acoustic methods in shallow coastal waters with observations that are more frequent and have a larger spatial coverage.

In the clear Greek Mediterranean Sea, Sentinel-2 was already applied successfully to detect bathymetry and seagrass meadows. We are now testing whether Sentinel-2 data are also suitable for analysing the sublittoral in the turbid waters of the Baltic Sea. We focus on an extensive shallow water area near Kiel/Germany. Based on Sentinel-2 data, we analyse water depth and differentiate between seagrass covered and bare sandy ground. We derive these parameters using empirical and process-based models. First results show that Sentinel-2 allows to determine water depths up to 4 m (RMSE ~ 0.2 m). Comparisons with LiDAR water depths show that inaccuracies increase in overgrown areas. Our study also shows that the atmospheric correction algorithm influences sublittoral ground mappings with Sentinel-2 data. For instance, the absolute water depths of the process-based modelling differ up to 2.5 m on average depending on the atmospheric correction algorithm (ACOLITE, Sen2Cor, iCOR).

Comparing Sentinel-2 seagrass classifications with diver mappings and aerial imagery emphasises that empiric approaches provide plausible sublittoral ground classifications up to approximately 4 m water depth. Combining these results with seagrass mappings based on acoustic measurements (deeper than 4 m water) provides a synthesised sublittoral classification map of the study area up to the present growth limit of seagrass (~ 7 m in the study area).

The Baltic Sea is considered as a very turbid environment, nevertheless we show that satellite-based remote sensing has a great potential for shedding light into the  "white ribbon". The spatial coverage and temporal resolution of the analysed Sentinel-2 data increases the knowledge about the occurrence of seagrass and its spatio-temporal dynamics. Nevertheless, the influence of the selected atmospheric correction approach on the results shows that further research in remote sensing is necessary to assess seagrass meadows reliably.

D1047 |
EGU2020-10024
Svenja Papenmeier, Alexander Darr, Agata Feldens, Peter Feldens, and Jennifer Valerius

The increasing demand by national legislation and European marine policy amplifies the need for high resolution and area-wide habitat maps of the seafloor. The basis for a consistent and objective delineation of habitats is a consistent derivation of sediment classes. For this reason, the German Federal Maritime and Hydrographic Agency (BSH) has published a guideline for high-resolution, hydroacoustic sediment mapping, with the advice of scientific experts. Large areas within the German exclusive economic zone, in particular in the special areas of conservation (SAC) have already been mapped in agreement with this guideline.

We will introduce the mapping guideline and its sediment classification system, present the mapping progress and demonstrate the successful use of sediment maps for the creation of habitat maps. The approach by the BSH works very well for the large-scale mapping of the German North and Baltic Sea. However, more specific tasks like environmental investigations for approval procedures or scientific questions need adjustments to the mapping criteria. We will show this exemplary for hard substrate habitats. According to the guideline of the BSH (update is in progress), hard substrates presence is given as raster information with cell size of at least 100 x 100 m. For e.g. approval procedures, this raster information is not sufficient to map the protected natural habitat “reefs” in a large scale. Therefore, the German Federal Agency for Nature Conservation (BfN) has developed its own criteria where individual objects have to be detected. The object detection is still to be done manually which might be acceptable for small study sites, but not practicable for large-scale mapping. Consequently, current work is concerned with the automation of object detection to speed up the interpretation and make it objective / reproducible.

D1048 |
EGU2020-6699
Rune Michaelis, Lasse Sander, Finn Mielck, Svenja Papenmeier, and H. Christian Hass

The North Sea is a shallow marine environment. The sediment distribution of the seabed is dominated by sand-sized material. Hard-substrate areas are a relatively rare, but important habitat for sessile and mobile species. This habitat type forms island-like geomorphic features owing to the presence of glacial deposits in the shallow subsurface. While their ecological importance is widely acknowledged, hard-substrate areas are characterized by a large degree of spatial heterogeneity and an unaccounted high local diversity in physical surface properties, sediment composition and temporal change.

The aim of this study is the detailed investigation into the spatial characteristics and temporal variability of an exemplary hard-substrate complex located 10 km offshore the island of Sylt (N-Germany). The area has a size of c. 3 km2and was investigated between 2008 and 2019 using a range of hydroacoustic and optical sensors (multibeam echosounder, sidescan sonar, sub-bottom profiler, acoustic ground discrimination system, underwater videos) and machine learning algorithms (haar-like features) to track the changes in the number and local distribution of exposed stones.

The maximum water depth in the area is 16 m and a linear arrangement of hard substrates emerges up to 4 m from the seabed. A layer of fine sand with a thickness of 0.5 m overlays the more planarly deposited coarse sediments in the proximity of the stony outcrop. This layer of fine sand is relatively mobile and leads to a frequent temporal change of the distribution of sediment on the seabed, whilst the stony outcrop is only marginally affected by the spatial dislocation of sediments. The spatial extent of hard substrates is variable due to the presence of a mobile sand cover on the seabed.

This study emphasizes the need for quick and automated object classification routines to be integrated in monitoring approaches in the highly dynamic coastal zone. It has shown that the geomorphological diversity and interannual variability of hard-substrate areas can be captured using the presented approach. Detailed studies and monitoring tools are important to better understand the interrelation of geomorphological and sedimentary processes at the seabed with the ecology of epibenthic organisms.

Keywords: North Sea; hard-substrate habitats; mobile sediments; hydroacoustic; haar-like features

D1049 |
EGU2020-4175
Guillaume Michel, Sophie Le Bot, Sandric Lesourd, and Robert Lafite

Estuarine benthic habitat quality health is integrated within the framework of the EU Water Directive and Marine Strategy Framework Directive. The long-term monitoring of small and medium-scale estuarine benthic habitat is based on recurrent observation of several factors, mainly bathymetry and seabed nature. Numerous studies have already addressed the performance and limitations of acoustic remote sensing and mapping techniques. However, most of these studies are limited to the marine and coastal domains and do not include the estuarine domain. Estuaries are considered as transitional domains, with various seabed morphologies (from rocky reefs to hydraulic dunes with anthropic modification overlap) and subtle granulometric variations of the seabed nature.

The objectives of our study are to explore the mapping performance of several acoustic remote sensing techniques and to determine which physical factors are the most representative of morphological and sedimentological characteristics of subtidal estuarine environment and of its evolution. The exploration of these cartographic variables has been performed for three small and medium-scale French estuaries: the Orne estuary, the Baie de Somme and the Belon estuary. These estuaries have been chosen to cover different morphological and sedimentological estuarine contexts.

Firstly, we evaluate the capacity of the main variables derived from bathymetry (slope, curvature, ruggedness) to map seabed morphology. We extend the variable exploration to the “Terrain Variable” GIS category and BTM (Benthic Terrain Modeler Toolbox) as well. Secondly, we explore the capacity of several cartographic variables, extracted from bathymetric, seabed acoustic backscatter and acoustic ground discrimination system (i.e. RoxAnn©), to map seabed sediment characteristics and variations. The seabed nature mapping is validated with ground truthing data, namely grab samples and seabed video profiles. Moreover, quantitative (D90, roughness, sorting) and qualitive information (apparent roughness of the seabed, benthic habitat) are extracted from the grab samples and seabed video profiles, respectively. The capacity of these variables to produce seabed nature maps is also explored.

Mapping results on the three areas are compared, in terms of mapping precision and reproducibility, and transposed into recommendations for small and medium-scale estuaries monitoring. The next step of the AUPASED project is he exploration of image analysis and machine learning classifications and their comparison to manual morphological and sedimentological maps produced.

The AUPASED project is funded by the AFB (French Agency for Biodiversity) as part of a convention between the AFB and the CNRS (UMR 6143, M2C).

D1050 |
EGU2020-18339
Didier Charlot, Philippe Alain, Geraldine Duffait, Olivier Lerda, and Guillaume Matte

MULTISWATH MULTIBEAM ECHOSOUNDER FOR EFFICIENT SEABED BACKSCATTER IMAGING AND CLASSIFICATION

  1. Didier Charlot(1),Philippe  Alain(1), Géraldine Duffait(2) ,Olivier Lerda(2), Guillaume Matte(2)

 (1) iXBlue Sonar System Division, 256 rue Rivoalon, 29200 Brest,  France

(2) iXBlue Sonar System Division, 46 Quai F. Mitterrand, 13600 La Ciotat, France.

 

               Managing marine resources and habitats require a classification system to identify and characterized seabed properties. Acoustic systems are recognized to be remote sensing tools that measure efficiently sediment properties and seabed morphology [1].Single beam, multibeam echosounder and sidescan sonar systems are commonly used to characterize seabed type by respectively analyzing echo strength returns, backscatter (BS) angular response, and texture analysis. Multibeam  (and interferometric sidescan ) systems  have the great advantage to measure the bottom bathymetry hence the true grazing angle at least in the across track direction. But there are still some challenges to face to get a robust calibrated BS value. 

First, standard multibeam systems  do not measure directly the full BS backscatter angular response on each soundings. This can be accomplished by using a dual axis multibeam to record the BS in the along track direction[2]. The BS angular response is  a powerful metric to characterize the sediment type. 

Second, the BS response is sensitive to the insonification direction (azimuth) and this dependency should also be considered to improve calibration procedure.  Recently, a full 3D steerable high resolution multibeam system has been developed [3]. First investigation ([3],[4]) have shown the high potential of multiswath multibeam system. With the 3D steerable swath capability, the bidirectional BS angular response can be recorded on each insonified soundings. This presentation will emphasize recent advances in processing using the  full multiswath multibeam capabilities.

 

References:

[1] John T. Anderson, Editor,”Acoustic Seabed Classification of Marine Physical And biological Landscapes”, ICES Report N° 286, August 2007

[2]M.  Gutberlet and H. W. Schenke ,“HYDROSWEEP : New Era in High precision bathymetric Surveying in Deep and Shallow water” , Marine Geodesy,1989, Vol13,pp1-23

[3] F. Mosca & al., “Scientific potential of a new 3D multibeam echosounder in fisheries and ecosystem research”, Fisheries Research 178 pg. 130-141, 2016.

[4] Nguyen, Trung Kiên , Charlot D. , Boucher  J.-M , Le Chenadec G.,  Fablet R., “Seabed classification using a steerable multibeam echo sounder”. Oceans 2016 MTS/IEEE 2016,Monterey

[5] Nguyen, Trung Kiên, ”Seafloor classification with a multi-swath multi-beam echo sounder”, PhD thesis 2017, IMT Atlantique; http://www.theses.fr/2017IMTA0035

D1051 |
EGU2020-22399
Robert Mzungu Runya, Chris McGonigle, and Rory Quinn

Acoustic methods are frequently used to provide broad-scale information on the spatial extent, range and distribution of marine habitats and sedimentary environments. Although single frequency multibeam echosounders have dominated seabed mapping for decades, multi-frequency approaches are starting to present in the scientific literature. Multibeam survey strategies are generally optimized for the acquisition of bathymetry data, often overlooking the ecological and geological value of backscatter data. This study examines the benefits of combining multi-frequency backscatter responses to discriminate seabed properties in areas with strong geomorphological gradients and associated ecological variability. The frequency-dependence element of backscatter strength is linked to: (i) the dominant scattering regime, (ii) seabed roughness, and (iii) the input of volume scattering related to signal penetration. In 2019, we collected and analyzed multifrequency (200, 95 and 30-kHz) backscatter data from Hempton’s Turbot Bank, a marine protected area off the north coast of Ireland. We compare these data with legacy 300 kHz backscatter data from 2013 to explore the backscatter variability in the context of geomorphological change. We assess the explanatory power of multi-frequency vis-à-vis single-frequency backscatter data in terms of bathymetry, sediment granulometry and infaunal community structure. Results improve our understanding of the link between backscatter properties and geomorphology, with specific recommendations towards minimizing information loss and establishing minimum data requirements for frequency-based benthic habitat discrimination. Improved discrimination of geomorphology and benthic habitat characteristics enhances the reliability of backscatter data as a monitoring technique for area-based protection of marine resources.

D1052 |
EGU2020-20349
Shauna Creane, Dr Mark Coughlan, Dr Jimmy Murphy, Dr Julie Clarke, and Dr Paul Doherty

To date, Ireland’s only operational offshore wind farm project is located on Arklow Bank; a sediment bank situated nearshore in shallow (up to 2mbsl) waters. Such bank structures are preferable for wind farm development due to their competent sediment composition, shallow waters and proximity to shore for cable routing. However, as proven at Arklow Bank, issues of scour and sediment mobility around fixed foundations and cabling can compromise infrastructure stability. These issues highlight the need for adequate ground model assessments of such banks to aid planning, design, construction and maintenance phases of wind farm development.

The southern Irish Sea is characterised by a series of such NNE-SSW trending sediment banks, many of which are earmarked for further wind farm developments. These coast-parallel bedforms are located approximately 10km off the south-east coast of Ireland, in circa 20-40m water depth and rise to only a couple of metres below sea level. They exert a strong control on the tidal flow pathways along the coast and offer coastal protection [1]. The banks themselves are quasi-stable in their own environment, influencing local hydro- and morphodynamics in terms of sediment waves. For offshore sediment banks to develop two fundamental conditions must be present: (1) an adequate source of sediment and; (2) a hydrodynamic regime capable of moving sediment [2]. European continental shelf sediment bank origins generally fall into two broad categories [2]; a) those formed under present day hydrodynamic and sediment conditions, or b) relict features created during post LGM transgression during periods of rapid sea level rise and stronger tidal current velocities. Paleo-tidal models have been used to reconstruct post-glacial hydrodynamic conditions in support of the proposed view of a glaciomarine environment genesis of these large bedforms in the Irish Sea [3].

This study will use previously collected high resolution multi-beam echo sounder (MBES) data from the Integrated Mapping for the Sustainable Development of Ireland’s Marine Resource (INFOMAR) project in conjunction with newly obtained MBES, sub-bottom profiler, grab sample and vibro-core data to characterise; a) the stratigraphy of this highly dynamic and geologically complex region and b) sediment mobility on and around the sediment banks including their stability in the current hydrodynamic regime. Furthermore, this study will use this data to attempt to elucidate the origin of these sediment banks and their evolution through geological time. Data processed and analysed from this study will be used as an input and as validation for a coupled hydrodynamic, spectral wave and sediment transport 2D numerical model developed using MIKE 21 software.

 

References

  1. Williams, J.J., MacDonald, N.J., O’Connor, B.A., Pan, S., 2000. Offshore sand bank dynamics. Journal of Marine Systems, 24, 153-173.
  2. Dyer, K.R., Huntley, D.A., 1999. The origin, classification and modelling of sandbanks and ridges. Continental Shelf Research, 19, 1285–1330.
  3. Uehara., K., Scourse, J.D., Horsburgh, K.J, Lambeck, K., Purcell, A.P., 2006. Tidal evolution of the northwest European shelf seas from the Last Glacial Maximum to the present. Journal of Geophysical Research, 111(9).
D1053 |
EGU2020-1743
Vladimir Anokhin, Dina Dudakova, and Mikhael Dudakov

In 2019, the Institute of Limnology of the Russian Academy of Sciences (IL RAS) carried out geological and geomorphological studies of the bottom and shores of Lake Ladoga within the framework of the State project of the IL RAS No. 0154-2018-0003 / 5. The research included the study of the bottom landscapes of Lake Ladoga with help of a series of underwater vehicles Limnoscout, designed and assembled at the IL RAS.

Underwater photo and video of the bottom in the coastal zone was carried out by the Limnoscout-230 vehicle from a boat. Each video filming polygon  included 2 continuous video profiles of 1-2 km normal to the shore, and 1 connecting profile parallel to the shore of 200-400 m, in the deep part.

Underwater video filming of the bottom in the open water area of ​​the lake was carried out by the Limnoscout-50 vehicle from the board of the r/v “Poseidon”, by point diving, in which the bottom was shot within a radius of 2-4 m from the dive point.

Maximal deep of studies was 117 m.

All underwater surveys were accompanied by echo-sounding surveys and GPS tracking.

In total, 24 underwater video filming  polygons in the coastal zone and 23 underwater video filming points in the open water area of ​​the northern part of Lake Ladoga were worked out.

The collected extensive photo and video materials made it possible to make preliminary typology of the bottom landscapes of Lake Ladoga and evaluate their condition.

Several new important facts of the structure of the bottom of Lake Ladoga and biota distribution were discovered, in particular:

- For the first time on the bottom of Lake Ladoga, an invasive species of mollusk Dreissena polymorpha was discovered, which in other large lakes has a significant impact on ecosystems.

- For the first time at the bottom in the northern part of the lake, outlets of presumably Riphean sandstones were discovered, which significantly complements the geological picture of the area.

- For the first time at the bottom in the northeastern part of the lake an abnormally deep occurrence of coarse deposits was discovered, which is likely to be associated with the intense activity of glaciers.

The use of underwater photo and video in combination with traditional methods for studying the bottom landscapes of Lake Ladoga has shown the high efficiency of these methods. The studies will be continued.

D1054 |
EGU2020-1769
Ziyin Wu, Dineng Zhao, Jieqiong Zhou, Xiaowen Luo, and Jihong Shang

Pockmarks are relict seafloor geomorphological features formed by seepage of gas or fluid from the seabed. While seafloor pockmarks are widely distributed around the world, mega pockmarks with diameters > 1 km are rare, and their formation and maintenance mechanisms remain enigmatic. Using high-resolution multi-beam bathymetric data, this paper systematically identified mega pockmark groups in the southern depression of the Reed Basin in the South China Sea. Mega pockmarks of various shapes occur in groups in the Reed Basin, primarily along the sides of submarine canyons. Observed geomorphologic characteristics differ significantly from features reported in the published literature. Based on the collected data, the average ratio of pockmark depth to pockmark radius (d/r) is evaluated as ~0.3, which is consistent with analyses of additional pockmarks in 21 regions around the world. Our observations also agree with the previously reported generalization that small pockmarks are developed in shallow water and large pockmarks in deep water. We propose that pockmarks in the Reed Basin are formed by seafloor gas explosions. Widely developed carbonates store buried gas that continuously seeps along NE-SW trending faults. Cap layers are undercut by submarine canyons forming lines of mechanical weakness. During periods of rapid sea level fall, depressurization causes buried gas to be rapidly ejected along these lines, forming large pockmark groups. Because these results correlate easily observed bathymetric features to the presence of buried gas deposits, they have important implications for the exploration and research of deep-sea gas resources.

D1055 |
EGU2020-20792
Yoe Perez, Julia Fonseca, Helenice Vital, Andre Silva, and David Castro

The Brazilian Continental Margin (BEM) deep-water regions contain important geological features that need advance in their characterization. Mass-transport deposits (MTD) are important not only by their significance in the sedimentary but also because of their negative impact economically. A slump is a coherent mass of sediment that moves on a concave-up glide plane and undergoes rotational movements causing internal deformation and one of the basic types of MTD. The study area comprises part of the offshore Potiguar Basin in NE Brazil, on the distal eastern portion of the Touros High and Fernando de Noronha Ridge. This portion of the Potiguar Basin comprises a transform rift system that has evolved into a continental passive margin. This basin represents an important location related to the breakup between South America and Africa. The database used in this work included 2D post-stack time-migrated seismic profiles from the Brazilian Agency of Petroleum, Natural Gas, and Biofuels (ANP). The slumps reflectors are identified on the continental shelf profiles in form of present clinoform configuration, medium to high continuity, high amplitudes, and medium to high frequencies, representing a sigmoidal oblique complex prograding reflector. The slump scars at the continental slope indicate that this is a gravitationally unstable area that will eventually collapse, resulting in erosional features on the continental slope and deposition on the continental rise. Our results provide some insights regarding MDT slumps sedimentary evolution in the BEM deep water area as well as their interrelation with other sedimentary deposits.

D1056 |
EGU2020-3123
Hugo Seiti Yamassaki and Fernando Farias Vesely

Seismic geomorphology has shown to be a powerful tool to assess deep-water systems, allowing to characterize the geometry and composition of depositional elements and to reconstruct erosion, transport and deposition. However, this approach has been applied mainly to describe a relatively short period of the depositional time, preventing the interpretation of long-term changes in geomorphology and the resulting depositional architecture of individual systems. In this research, we analyze the geomorphological evolution of a deeply buried submarine fan in the Upper Cretaceous of northern Santos Basin, SE Brazil. The submarine fan is 3100 m below seafloor, it has an area of ~700 km2, 200 m of maximum thickness, and the external geometry is influenced by the topography of an underlying mass transport deposit (MTD) and salt domes. By using 3D seismic data, we mapped 5 horizons related to the submarine fan (Hz1 – fan base; Hz2 – lower fan; Hz3 – middle fan; Hz4 – upper fan; Hz5 – fan top). We generated isochron maps to define the overall geometry and to examine spatial changes in deposition during different stages of growth. We produced a coherence map of Hz1 to highlight slope and substrate irregularities. Spectral decomposition attribute was extracted from internal fan horizons (Hz2 to 4), which clearly revealed channel networks radiating from a feeder canyon. A total of 109 channel segments were measured to calculate sinuosity indexes (SI) considering three classes (SI<1.1 = straight; 1.1<SI<1.5 sinuous; SI>1.5 = meandering. The results show important aspects of fan development and changes in channel style with time. The isochron maps reveal lobe avulsion caused by compensation cycles (Hz2 to Hz3) and fan progradation towards the basin in an elongated shape. The submarine fan was first build up with straight channels and we observe an upward increase in channel sinuosity. Hz2 has 100% of straight channels, Hz3 shows 65% of straight, 30% of sinuous and 5% of meandering channels, and the Hz4 presents 51% of sinuous and 49% of straight channels. We interpret this overall increase in sinuosity as a result of a decrease in fan surface gradient caused by a progressive aggradation. The lack of MTDs within the fan and the few terminal depositional lobes observed in seismic attribute maps imply that superimposed overbank and channel-fill elements dominate the submarine fan architecture. Furthermore, the application of seismic attributes based on amplitude of acoustic impedance contrasts shows that channel-fill elements concentrate most of the sandy deposits.

Chat time: Wednesday, 6 May 2020, 16:15–18:00

Chairperson: Christian Hass
D1057 |
EGU2020-12212
Andressa Lima Ferreira, Helenice Vital, Moab Gomes, Andre Aquino da Silva, and Yoe Perez

The Galinhos-tidal-channel system is located in the Brazilian equatorial margin, northeastern coast of Rio Grande do Norte State. The economic importance of the region began around 1600s and continues until today with salt exports, later shrimp farms, handmade fishing, oil and gas industry, ecotourism and wind energy. A spit, behind which an intricate system of tidal channels has developed, with practically absent riverine influence, characterizes the area. The integration of interferometric sonographic data (total of 4.7 km2), calibrated with sediment samples, and radar images were used to map geomorphological features on the area.  The ALOS PALSAR image, allowed to integrate the altitude information of the emerged and submerged portions, resulting in efficient method for coastal flooding areas and substrate mapping. The sonographic study allowed recognizing bedforms, which are important morphological elements that influence water and sediment discharge. Four main types of submerged geomorphic units were identified: a) 2D sandy dunes, b) 3D sandy dunes c) muddy flatbeds and d) irregular beds. Dunes were classified according to their size into small, medium and large. Bathymetric data revealed that depths from 2 to 8 m along the area. The main tidal channel Galinhos has a width of 900m, 12km long, irregular bottom, and asymmetrical margins. The Pisa Sal tidal channel has an average width of 150m and 3km long, U shaped cross-section, slight asymmetric margins and slightly irregular bottom. Deepest parts occurs close to its mouth (between 6,5m and 8m), gradually decreasing until they reach 5m on its inner portion. The Tomaz tidal channel, until to central portion has an asymmetrical bed with the highest depths on its right side reaching 7m. Its left side range from 5.5m to 6m. In the south portion, this channel becomes shallower (5m) and its asymmetry is reversed. After splitting the channel width is reduced from 260m to 140m and the bottom becomes less irregular and flat sometimes. In this portion, the highest depths reach 7m. The data made it possible to identify the regions of higher and lower altitudes using as reference the mean sea level. Altitudes throughout the region range from 0 (sea level) to 20m and come from local topographic elevation. The south portion concentrates altitudes above 10m and the lower regions are located in the central portion of the area. The central portion is the flattest and this behaviour extends over 5km to the dunes located in the Galinhos spit, when the altitudes exceed values above 10m. The Galinhos spit integrates an area with average altitude ranging from two to seven m. Flooded or wet regions were well delimited due to non-penetration or absorption of electromagnetic energy (low frequency) when it interacts with the water dynamics; however, results are better where the depth is higher than 3m.

D1058 |
EGU2020-19360
Alex Hughes, Javier Escartín, Jean-Arthur Olive, Jeremy Billant, Christine Deplus, Nathalie Feuillet, Frédérique Leclerc, and Luca Malatesta

At the scale of individual faults, few studies have investigated fundamental interactions between active faulting, erosion, and deposition in submarine landscapes dominated by magmatic and volcaniclastic deposits with thin sedimentary cover. Such landscapes comprise a high percentage of the global seafloor. Therefore, there is a significant gap in our understanding of first-order processes of erosion and deposition for a large portion of the Earth’s surface. The paucity of studies derives mainly from challenges involved in the acquisition of high-resolution bathymetry and seafloor data in a deep-marine environment. In this study, we use bathymetry data obtained with autonomous deep-sea vehicles and processed to obtain a 1-m resolution digital elevation model along the active Roseau normal fault, in the Lesser Antilles volcanic arc. The Roseau fault was the source of the 2004 Mw6.3 Les Saintes earthquake, and Mw 5-6 events are thought to occur on the Roseau fault every few thousand years. Building on the work of Vilaseca (MSc Thesis, 2015), we quantify the height, slope, and volume eroded from a well-defined fault scarp created by the Roseau fault and calculate volumes for a series of erosional footwall catchments developed in the scarp. We also quantify the volume and morphology of a series of dejection cones in the hanging wall of the Roseau fault to facilitate mass-balancing between the hanging wall and footwall of the scarp.

 

Mass-balancing indicates that in isolated basins, where the primary supply of sediment is from the adjacent footwall scarp, dejection cone volumes are around half of the total volume of material eroded from the individual footwall catchments. Geomorphological analyses show that dejection cones have surface slopes as high as 30°and form as radial depositional features adjacent to catchment outlets. The results of the mass-balancing, the high slope values for the cone surface, and the identification of >1 m sized blocks of eroded material present on the cone surfaces indicate that dejection cones form through episodic, coseismic and/or post-seismic, gravitationally driven mass-wasting of the uplifting footwall scarp. Preliminary morphometric analysis of the Roseau fault scarp potentially indicates that erosion of normal fault scarps in volcaniclastic and magmatic deposits may primarily occur beyond a threshold in fault scarp height between ~40­­–70 m. Above ~40–70 m height, erosional catchments may begin to develop on the footwall scarp and average scarp slope decreases with increasing scarp height until average slope values reach an equilibrium of ~35°. The quantitative survey of the Roseau fault scarp in this study demonstrates that episodic earthquake-related mass-wasting is a key erosional process for volcanic and sedimentary deposits in submarine landscapes. Furthermore, the results presented here will be used as first-order inputs to develop models of seafloor erosion and apply them to understand submarine landscape evolution of the oceanic lithosphere.

D1059 |
EGU2020-13201
Ting-Yi Liu, Kenn-Ming Yang, Liang-Fu Lin, and Char-Shine Liu

Sediment-routing system in a developing marine accretionary prism is highly related to the thrusting and the accompanied fold structures. The main purpose of this study is to investigate how the thrust fault growth had spatially and temporally affected sediment transport paths in the frontal part of the developing accretionary prism in offshore Southwest Taiwan. This study attempts to reveal the lateral change in dipping angle of and displacement along thrust fault and the accompanied variation in trend of the submarine channels by interpreting a grid of seismic section.

 

The Frontal Ridge is the outmost topographic high in the frontal part of the accretionary prism in offshore Southwest Taiwan and has been regarded as the resultant anticlinal fold caused by westward thrusting. The major structural trend of the ridge is NW-SE and the fold tightness with the displacement along the fold-forming thrust decreases southeastward. Beneath the backlimb of the fold, basal boundary of the growth strata can be defined by a major unconformity and the overlying strata wedging westward to the ridge. The backlimb dipping angle is smaller than that of the thrust ramp. While narrow channels appear in the growth strata and parallel to the strike of fault, a wide submarine fan can be observed in the pre-growth strata. In the lower part of the growth strata, upstream of the channels is characterized by several narrow, concentrated and deep-cutting channels younging toward the east. On the other hand, downstream of the narrow channels appears as the distributed channels, which are overlain by the younger narrow and concentrated channels that are running along a synclinal axis in the upper part of the growth strata.

 

We propose that the Frontal Ridge is a manifestation of a shear fault-bend fold in the subsurface and the ridge uplifted first from its northwestern end and propagated toward the southeast. The southeastward propagating fold had strongly affected sediment-routing path and deposition. At each stage, the active thrusting and accompanied piggy-back structure offered the loci for narrow and deep-cutting channels in the upstream areas. On the other hand, in the downstream areas, the channels ran far from pre-existing thrust front and spread out into distributed channels. When the frontal thrust continued to propagate to the southeastern end, the younger channels developed and migrated gradually to be restricted in the synclinal axial area. Meantime, the distributed channels in the original downstream areas would become a part of the folded pre-growth strata underneath the ridge.

 

Key words: Sediment-routing system, accretionary prism, syn-tectonic deposition, Southwest Taiwan

D1060 |
EGU2020-4069
Tzu-Ting Chen, Ho-Han Hsu, Chih-Chieh Su, Char-Shine Liu, Song-Chuen Chen, and Yu-Huang Chen

There is a high potential of hydrothermal mineral deposits in the Southern Okinawa Trough offshore northeastern Taiwan. This study aims to integrate bathymetry data, water column images, seabed image, sub-bottom profiles and ROV observations to better understand a hydrothermal field, submarine Mienhua Volcano (MHV). A repeated mapping survey equipped with EM-712 was conducted to see if significant bathymetric changes took place since 50 days in MHV. The volcano of 2 km width and 240-m relief height is present at about 1300-m water depth near Mienhua Canyon. The volcano combines with several small and rugged mounds. From ROV observations, two of the mounds show active hydrothermal vents covered by white tubeworms and a white smoker with some mussels, respectively. In addition, active water column gas flares are observed on the southeastern part of MHV. Based on the identification of sub-bottom profiles, the special distribution of acoustic blanking zone shows the southeastern part is more narrow than the northwestern area. However, comparing the bathymetry mapped in two times, no obvious overall morphological changes are detected, except on the volcano rim. Seabed image also reveals similar backscatter intensity within the rugged mounds, suggested they may be at similar stages of morphological development. We consider that the hydrothermal vents are variable. In addition, the preliminary results indicate the fluid migration is the important process to influence the evolution of the hydrothermal field in MHV. Therefore, we also applied more seafloor images, geochemical and geophysical data which have been reported in MHV. Our results could evaluate the relationship between the flare activities, geomorphological features and the location of the blanking zone in the sedimentary strata.

D1061 |
EGU2020-3247
Shu-Yunn Lo and Gwo-shyh Song

In recent years, the Government of Taiwan has put considerable effort into the environment impact assessment for offshore wind potential area in Taiwan Strait, especially the research and regulation related to Underwater Cultural Heritage (UCH). From prehistorical evidence and historical record, it can be seen that Taiwan Strait is rich in archaeological character, which involves the traces of people living in 10,000 years ago (The Paleolithic Age), and the remains of human activities on the water before 100 years. Since the cultural and historical value of the material preserved under the sea, the Underwater Cultural Heritage Preservation Act, which follows the Convention on the Protection of the Underwater Cultural Heritage announced by UNESCO in 2001, was published by the Ministry of Culture to set an official standard for development and management in 2015. The guideline for UCH survey was soon be announced in the next year. All these acts lead Taiwan into the forefront of UCH survey in the world.
The techniques or methods used in UCH survey, as the rule in the UCH Preservation Act, must be non-destructive to reduce harm as much as possible. Marine geophysical techniques therefore become the major tools used in the UCH surveys in Taiwan. These tools include side-scan sonar for wide range seafloor mapping, multi-beam sonar for getting accurate position and 3-D image, magnetometer for finding ferromagnetic material, and sub-bottom profiler for searching buried objects. The requirements of using these techniques within UCH surveys are written in the official guideline.
After data analysis, the records from different instruments will be compared with others to identify the targets. However, the results of recognition involve objects unwanted, such as acoustic noise, rocks, or other nature features. Errors produced during the survey may also increase the difficulty of recognition. With the experience from previous research, this study will introduce some cases which were done before, and take a review of the official guideline to provide the suggestion that can help to improve the results of UCH survey in Taiwan.


Keywords: Underwater Cultural Heritage, marine geophysics, side-scan sonar

D1062 |
EGU2020-4063
Ho-Han Hsu, Liang-Fu Lin, Tzu-Ting Chen, Char-Shine Liu, Jih-Hsin Chang, Chih-Chieh Su, and Song-Chuen Chen

    Multi-scale geophysical survey including pseudo-3D seismic, sub-bottom profiling, side-scan sonar, multi-beam and single-beam bathymetry, heat flow investigations as well as bottom-water, core and dredge sampling works have been conducted in a newly discovered hydrothermal field named as Geolin Mounds at about 1,510 depth in the Okinawa Trough. Ship-mounted bathymetric data cannot detect specific morphological features on the seafloor in this field; however, “rock grove” morphological characteristics are observed by using deep-towed side-scan sonar. Moreover, vigorous flare features in water column are detected by multi-beam and single-beam echo sounder, and widely distributed high heat flow anomalies (> 10,000 mW/m2) also exist in the survey area. Due to strong Kuroshio Current during our multi-channel seismic survey, 5-40°streamer feathering effect occurred. To take advantage of swath distributed seismic reflection points caused by streamer feathering, we developed a pseudo-3D technique and produced a 3D seismic cube from this uneven seismic dataset. The 3D seismic imaging and sub-bottom profiling results indicate widely-distributed anomalies such as blanking zone and high-amplitude reflectors around the Geolin Mounds hydrothermal field and could link the specific features above seafloor. The 3D seismic cube also provides better estimation of the areas of blanking zone on selected time slice and better characterizes fault structures in the hydrothermal field. The geochemical analysis results present high Ag, Au, As, Bi, Cd, Cu, Fe, Pb, Sb, and Zn concentrations have been found in our coring and dredging samples. Relatively high concentration of methane, rare earth elements and 3He/4He ratio in near bottom seawater samples are also shown. Based on the geophysical and geochemical works, we propose that the Geolin Mounds hydrothermal field is without underlying submarine volcanos and hydrothermal fluid migration could be related to fault development. This hydrothermal field is in its embryo stage of evolution and constantly supported by active hydrothermal circulation. As a consequence, seafloor massive sulfides deposits and related geomorphological features can keep developing with hydrothermal fluid circulation along migration conduits. The fault structures and volcanic activities due to back-arc spreading process in the Southern Okinawa Trough should dominated creatures of those fluid migrating conduits.  We suggest that the Geolin Mounds hydrothermal field could sustainably grow and have high potential of massive sulfides resources in the Southern Okinawa Trough. Furthermore, this hydrothermal field can serve as a good observatory for get better understanding of seafloor edifice development and ore mineralization associated with hydrothermal circulation activities in a back-arc spreading tectonics.

D1063 |
EGU2020-12701
Wei-Chung Hsiao, Yi-Ching Yeh, Yen-Yu Cho, and Shu-Kun Hsu

The Kaoping submarine canyon (KPSC) originates from Kaoping River, southwestern Taiwan that extends about 250 kilometers long from the Kaoping River mouth down to the Manila Trench. It can be divided into three major sections: upper reach (meandering), middle reach (NW-SE trending and V-shaped canyon) and lower reach (meandering). Based on recent a swath bathymetric data in the uppermost KPSC, an obvious seafloor depression can be observed in the eastern bank of the canyon. The eastern bank of the canyon reveals about 30-50 meters in average lower than western bank. The mechanism is blurred. In this study, to investigate fine sedimentary structures in 3D point of view, we used marine sparker seismic method. The seismic source frequency varies from 100 to 1200 Hz which can provide about 0.6 meters vertical resolution (i.e. central frequency 600 Hz and 1,600 m/s Vp). We have collected 75 in-lines across the canyon and 3 cross-lines perpendicular to the in-line. The data went through conventional marine seismic data processing procedures such as bad trace kill, band-pass filter, 2D geometry settings, NMO stacking, swell correction, match filter and predictive deconvolution. The 2D dataset was reformatted by applying 3D geometry settings to create a 3D seismic cube. The result shows that a wide incision channel was first found in the north of Xiaoliuchiu islet. Through depth, this channel becomes two narrower channels divided by a mud diapir. This down cutting can be traced down to transgressive sequence in prior to LGM (Last Glacial Maximum). In addition, a deep-towed sub-bottom profiler shows an obvious down-lapping structures heading off canyon that indicates over banking flow may be a key role to cause this erosional event.

D1064 |
EGU2020-7538
Carsten Lehmann, Wilfried Jokat, and Tabea Altenbernd

Ongoing research aims to constrain the extent of grounded ice shelves around the Arctic Ocean during the last glacial periods. Here, the Chukchi region is of special interest because of its broad, shallow shelf. In general, little is known about possible sources and the areal extent of ice sheets if any existed on the Chukchi shelf.

Bathymetric and sub bottom profiler data from the Chukchi Sea margins as well as from the Arlis Plateau to the west show complex patterns of glaciogenic erosion like Mega Scale Glacial Lineations (MSGL) at water depths of more than 500 m. The different directions of those MSGL indicate the presence of ice shelves and streams and point to an East Siberian Ice Sheet of unknown size. On the Chukchi Shelf, no evidences for the existence of an ice shelf for water depths shallower than 350 m have been described yet.

We re-processed 2D multi-channel seismic data acquired in 2011 from R/V Marcus G. Langseth to investigate glaciogenic features on the shallow shelf. These data reveal new insights into the formation of the northern Chukchi Shelf. The first up to 500 ms TWT of the seismic data show strongly eroded reflectors and reworked sediments. Additionally, small-scale channels with a few tens of meters width and a depth of 20 ms TWT disrupt the horizons below the reworked layers. Furthermore, the seismic data show channels with widths up to few km and depths up to 150 ms TWT as well as truncated well stratified reflectors. All above described features can be observed on parts of the Chukchi shelf with water depths shallower than 900 m.

 

D1065 |
EGU2020-7539
Ursula Schlager, Wilfried Jokat, and Estella Weigelt

The Lomonosov Ridge is an 1800 km long continental sliver in the center of the Arctic Ocean. Beside its tectonic relevance it hosts glaciogenic features caused either by deep reaching icebergs or grounded ice sheets as well as indications for mass wastings. Systematic swath bathymetry acquired in 2014 provided an almost complete image of these shallow disturbances from almost 84˚ N to the foot of the Laptev margin.

Several arcuate transverse features are present on both sides of the crest of the eastern part of Lomonosov Ridge between 81˚ and 84˚ N. Eight of them are 2.1-10.2 km wide, 1.7-8.2 km long, 125-851 m deep, with height of headwall between 58-207 m and a slid mass volume of 0.09-7.58 km3. Due to the absence of scientific drill holes only a tentative seismic stratigraphy can be used for a rough age estimate of the mass wasting. All but one show a glide plane on top of a pronounced stratigraphic seismic horizon with strong seismic amplitudes. We will introduce the different geometries and statistics of these mass wasting features.

D1066 |
EGU2020-8539
Dominik Palgan, Karol Tylmann, Colin Devey, Davíð Óðinsson, and Morgane Le Saout

It is accepted that during the Last Glacial Maximum (LGM), about 21,000 years ago, the head of the Icelandic ice sheet was extending on a shelf area of the Iceland plateau, beyond the present-day coastline of the island. Attempts at locating the ice sheet edge were made many times, mainly on the basis of the recognition of the end moraines and other marginal glacial landforms on submerged parts of the Iceland plateau. There is, as yet, no full agreement on the exact reconstruction of the extent of the Icelandic ice sheet during the LGM. Both the thermodynamic models of the ice sheet and the glacial landforms discovered around Iceland indicate that the ice sheet has slipped onto (perhaps beyond) the insular shelf; however, determining the exact extent of the ice edge within its individual sectors can be problematic, mainly due to insufficient recognition of underwater glacial depositional or erosional landforms.

We present the results of the scientific expeditions A200608 from 2006 carried by former Marine Research Institute in Reykjavik on board R/V Árni Friðriksson and MSM75 from 2018 carried by GEOMAR on board the R/V Maria S. Merian. The aim of the study was a detailed geological characterization of the axial and near-axial part of the northern Reykjanes Ridge. The Kongsberg EM 300 30 kHz and Kongsberg EM712 75kHz multibeam echosounders were used (on A200608 and MSM75 cruises, respectively) to investigate the topography and surface morphology of the seafloor. In addition, acoustic backscatter was used to determine relative hardness of the substrate.

The bathymetry of the axial (neovolcanic) part of the Reykjanes Ridge, north of 63°N, indicates a rough bottom typical of the mid-oceanic ridge, made up of single hummocky volcanoes, hummocky ridges, shallow faults, volcanic cones and flat-top volcanoes. The last two types are characterized by steep, rough slopes and nearly circular shape. In both axial and off-axis areas, some volcanoes exhibit a dome-like structure with very smooth summits and slopes. Such volcanoes, in off-axis setting, most likely formed in the neovolcanic zone and migrated off-axis as the seafloor spreading progressed.

We suggest that observed domed-like volcanoes are the result of glacial erosion associated with the transgression and recession of the Icelandic ice sheet. High backscatter intensities indicate presence of a hard substrate (i.e. lava) on smooth summits and low intensities around dome-like volcanoes demonstrating potential direction of deposition of eroded material (or re-deposited by modern bottom currents). The research area has very slow sedimentation rate and strong bottom currents system; hence, there are no other geological processes (other than sedimentation) on the seafloor that could lead to such smoothening of these features and their evolution into dome-like volcanoes, especially in the neovolcanic zone where new seafloor is formed and rough terrain is observed.

D1067 |
EGU2020-9042
Pedro Terrinha, Carlos Ribeiro, João Noiva, Marcos Rosa, Pedro Brito, Vitor Magalhães, Marta Neres, Pedro Nogueira, Sandra Velez, Ângela Pacheco, Mário Mil-Homens, Mariana Luis, Laura Andrade, André Carvalho, Paula Afonso, and Mariana Silva

The MINEPLAT project (Assessment of the mineral resources potential in the continental shelf of Alentejo and of the environmental conditions caused by the tectonic uplift in the Pliocene-Quaternary) allowed acquisition of 1700 km of ultra- high resolution seismic profiles, and full coverage of multibeam bathymetry  and acoustic  backscatter of 1450 km2 and 1940km of magnetic data of the Alentejo continental shelf, SW Portugal. 270 sediment samples were collected (Smyth-Macyntire dredges and multicores) and processed for sediment, geochemical and mineralogical analyses (granulometry, Xray diffractometry, major and trace metals analysis).

The wealth of data is meeting its full processing phase. Preliminary interpretation of the large dataset has already allowed to understand various novel contributions: i) identification of various sea level stand stills in Pliocene-Quaternary times; ii) drainage network during low stand sea levels; iii) grainsize dependency on submarine relief and inherited morphology from low stand periods; iv) eustatic,  oceanographic, fluvial and depth dependency of the post-alpine orogeny deposits of Pliocene-Quaternary age; v) location of deposits with high-quality sand for beach nourishment; and vi) identification of submarine harbor waste disposal sites and their environmental impact and dispersal; vii) high resolution mapping of magnetic anomalies related with magmatic events that can be source of heavy minerals.

The authors would like to acknowledge the FCT financial support through project UIDB/50019/2020 – IDL and MINEPLAT project ALT20-03-0145-FEDER-000013

D1068 |
EGU2020-6755
Markus Diesing

The deep-sea floor accounts for >90% of seafloor area and >70% of the Earth’s surface. It acts as a receptor of the particle flux from the surface layers of the global ocean, is a place of biogeochemical cycling, records environmental and climate conditions through time and provides habitat for benthic organisms. Maps of the spatial patterns of deep-sea sediments are therefore a major prerequisite for many studies addressing aspects of deep-sea sedimentation, biogeochemistry, ecology and related fields.

A new digital map of deep-sea sediments of the global ocean is presented. The map was derived by applying the Random Forest machine-learning algorithm to published sample data of seafloor lithologies and environmental predictor variables. The selection of environmental predictors was initially based on the current understanding of the controls on the distribution of deep-sea sediments and the availability of data. A predictor variable selection process ensured that only important and uncorrelated variables were employed in the model. The three most important predictor variables were sea-surface maximum salinity, sea-floor maximum temperature and bathymetry. The occurrence probabilities of seven seafloor lithologies (Calcareous sediment, Clay, Diatom ooze, Lithogenous sediment, Mixed calcareous-siliceous ooze, Radiolarian ooze and Siliceous mud) were spatially predicted. The final map shows the most probable seafloor lithology and an associated probability value, which may be viewed as a spatially explicit measure of map confidence. An assessment of the accuracy of the map was based on a test set of observations not used for model training. Overall map accuracy was 69.5% (95% confidence interval: 67.9% - 71.1%). The sea-floor lithology map bears some resemblance with previously published hand-drawn maps in that the distribution of Calcareous sediment, Clay and Diatom ooze are very similar. Clear differences were however also noted: Most strikingly, the map presented here does not display a band of Radiolarian ooze in the equatorial Pacific.

The probability surfaces of individual seafloor lithologies, the categorical map of the seven mapped lithologies and the associated map confidence will be made freely available. It is hoped that they form a useful basis for research pertaining to deep-sea sediments.

D1069 |
EGU2020-7510
Felix Gross, Kilian Etter, Philipp Held, and Jens Schneider von Deimling

Seagrass meadows are crucial habitats since they serve as fish nurseries and food sources for many marine species. They prevent nearshore erosion and are an important CO2 sink. As the plants are bound to the photic zone, seagrass meadows normally populate the shallow coastal zones. Unmanned aerial vehicles (UAV) are gaining popularity within the earth sciences community. Most surveys are of terrestrial nature and carried out by using the camera of the UAV to obtain orthophotos and three-dimensional surface models of a survey area. In comparison to space-borne systems, UAVs are capable of higher resolution image quality and time independent measurements, which enables an event-based surveying approach. We here present a submarine habitat mapping study, obtained by using an UAV flying 75 m above the water surface. Within the frame of the BONUS ECOMAP project, we aim to conduct repeated UAV surveys over the seasonal cycle to observe changes within coastal seagrass bed habitats. The key study area is located in the Baltic Sea offshore Heidkate (near Kiel, Germany). For data acquisition, we are using a commercial DJI Inspire 2 UAV with a gimbal mounted 20.8 megapixel Zenmuse X5S camera with a 15 mm/ 1.7 ASPH lens. For less reflection and distortion at the air-water interface, we are using a B&W circular polarized filter. Ground control points are measured and leveled with a Leica RTK system, which has a lateral resolution of ~2 cm. We process the data with the commercial software Pix4D™ and Agisoft PhotoScan™ to compute orthomosaic images and digital elevation/surface models. Since February 2018, we were able to conduct repeated surveys offshore Heidkate and Wendtorf (Germany). The average resolution of the orthomosaic data is better than 5 cm/px. First results show that we can obtain high-resolution images of habitats within water depths less than ~4 m in the Baltic Sea. Penetration is limited to factors like wave action, suspended sediment load and angle of the solar radiation. We perform supervised classification and pattern detection for habitat identification and discrimination. The data show the presence of seagrass, algae but also rocks, which are exposed at the seafloor. All scenes show a seasonal variability of the extent of seagrass meadows which are affected by migrating sand bars and major storm events. These data are the basis for a long-term monitoring framework, we are currently establishing in the working area.

D1070 |
EGU2020-10390
Seamus Coveney, Xavier Monteys, Brian Kelleher, and John D Hedley

Bathymetric measurement using remotely sensed data acquired in shallow-water marine contexts generally incurs challenges regardless of acquisition method. Multibeam sonar can be challenged with respect to survey vessel access and diminishing swath width, Airborne Laser Bathymetry is often affected by nearshore wave action and turbidity, and Satellite Derived Bathymetry can be complicated by local variations in water-column backscatter and bottom reflectance.

The NASA ICESat-2 (Ice, Cloud and Land Elevation Satellite-2) acquires elevation data for the global monitoring of temporal elevation change in ice caps, glaciers, sea ice and forests. The ICESat-2 Advanced Topographic Laser Altimeter System (ATLAS) issues three along-track parallel pairs of laser beams set 3.3km apart at a Pulse Repetition Frequency of 10,000 points per second (equivalent to 0.7m ground resolution). Returning photons (approximately 10 only are required from the trillions of photons within each laser pulse) are captured as returns from 1387 orbit tracks, which repeat every 31 days. While the ATLAS instrument is not intended for bathymetric measurement, ATLAS Global Geolocated Photon data (ATL-03) acquired over water may include laser returns from the seabed. Evaluation by the ICESat-2 research team indicates the potential for bathymetric measurement to depths of 30m in sub-tropical waters. ICESat-2 bathymetric measurement errors of approximately 0.5m RMSE were highlighted using ALB reference data, with a sub-decimetre error component attributable to laser refraction beyond 30m.

The EO-Intertide project, based at Dublin City University in Ireland and funded by the Irish Geological Survey research programme, is evaluating the potential for bathymetric inference using ICESat-2 data acquired across a wide range of seabed conditions around the entire Irish coast. Initial results demonstrate the potential for bathymetric measurement from ICESAT-2 to depths of between 5 and 20 metres, depending upon seabed sediment type and exposure to ocean swell conditions. A repeatable method is applied within the EO-Intertide project to extract bathymetric ATL-03 photon returns, using ATL-03 photon confidence scores in conjunction with a 3D spatial-data selection approach and a custom local elevation filter.

The elevation accuracy of the extracted ICESat-2 bathymetric profiles are validated using  combination of published Multibeam Echosounder (MBES) and airborne Laser Bathymetry (ALB) data acquired under the Integrated Mapping For the Sustainable Development of Ireland's Marine Resource (INFOMAR) project. INFOMAR is the seabed mapping programme jointly operated by Geological Survey Ireland and the Marine Institute. It is anticipated that extracted ICESAT-2 bathymetric profiles will provide a valuable input to EO-Intertide nearshore / intertidal bathymetric model generation from Sentinel-2 tidal-shoreline extractions and Satellite Derived Bathymetry (SDB). Validated bathymetric models issuing from this process will subsequently be applied as an input to the analysis of nearshore sediment dynamics within the Dublin City University PREDICT research project, to which the EO-Intertide project is allied.

D1071 |
EGU2020-18153
Irene Díez-García, María Gómez-Ballesteros, Francisco Sánchez-Delgado, and José Luis Granja-Bruña

El Cachucho, also known as Le Danois bank, is the first and only marine zone declared as Marine Protected Area (MPA) in Spain since 2008. This bank consists on a 72 km-long E-W trending marginal platform located at Spanish Cantabrian margin (southern Bay of Biscay) and interpreted as horst block separated from the Spanish continental shelf by an interior basin. The bank seafloor has an almost flat-topped morphology with minimum water depth of 424 m, having only local structural and erosive features. During last decades researchers have highlighted the importance of the bathyal ecosystem developed in this geological formation. As a result, significant efforts are being carried out to asses and monitor the evolution of this MPA in order to ensure the conservation of its biodiversity, applying new techniques as 3D scanning bathymetry.

Since 2013 the Spanish Institute of Oceanography (IEO) is leading the ESMAREC project (founded by the Spanish Government) for the monitoring of El Cachucho in order to guarantee the continuity as MPA based European Union regulations. This monitoring mainly consists on repeated multibeam seafloor bathymetries to assess the geomorphological evolution and reflectivity mosaics in order to map and classify the seafloor that can be related to the different types of marine habitats. The survey plan of the ECOMARG-2019 oceanographic cruise included four different locations that were chosen along the El Cachucho for sampling stations with the remotely operated towed vehicle (ROTV) POLITOLANA in order to identify various species of gorgonians and sponges with video images. Furthermore, in those locations, a multi-parametric platform system (lander) was anchored to study the oceanographic dynamics of the Benthic Boundary Layer (BBL). Both ROTV operations and lander anchorages require a detailed knowledge of the seafloor morphology for instrumental safety and optimize efforts. Existing multibeam bathymetry along El Cachucho before the ECOMARG-2019 cruise was only 75 meter and then inadequate to carry out those seafloor operations.

With the aim to improve the existing bathymetry, during the ECOMARG-2019 cruise was used the Kongsberg EM710 multibeam echo-sounder using the 3D Scanning technique. In this technique the vessel navigates to 0.5 knots and 250 beams sweep the bottom with an 45º opening angle and 10º horizontal movement. Higher point density was achieved, so it was possible to increase the average resolution of bathymetry and reflectivity up to 5 meters. New high resolution data provided a precise image of the geomorphology and allowed a more detailed seafloor classification. In this way, potential risks were reduced during ROTV operations and anchorages. In addition, the locations for ROTV operations were optimized based on the reflectivity mosaics that allowed to identify hard seafloor zones, preferred typology of seabed for gorgonians and sponges. Using the 3D Scanning in El Cachucho has resulted in an essential tool for safety and to optimize the seafloor operations. This technique allows to achieve a detailed knowledge of the seafloor in order to better assess and monitor MPA.

D1072 |
EGU2020-15270
Manaf Alkhuzaei, Matthew Brolly, Niall Burnside, Chris Carey, and Georgios Maniatis

The marine area of Bahrain comprises 91% of the total area of the country, the management of which is crucial for decision-makers, as it contains the country’s most valuable resources. It is also ecologically important supporting such fauna as, sea dugong, dolphins, green turtles, and 70+ species of fish, and such flora as seagrass beds and algae which provide essential ecosystem services. Providing current benthic habitat maps using remote methods is vital for efficient management and monitoring of these dynamic resources. In this threefold study, remotely sensed Landsat 8/OLI and Sentinel-2 imagery, combined with field survey (176 points), are used to investigate, classify, and map benthic habitats in light of varying spatial and spectral image resolutions while also assessing the role sunglint correction methods perform. Two widely applied methodologies proposed by Hedley et al. (2005) and Lyzenga (2006) for sunglint correction in the water column are examined to assess their role in creating accurate classification maps in this region. Sunglint is an issue in Bahrain due to its shallow waters, long summer and clear skies. The results using unsupervised classification indicate the effectiveness of both correction methods, demonstrating comparable results of high classification accuracy using either 3 (Blue, Green and Red) or 4 (Coastal Aerosol, Blue, Green and Red) spectral band combinations. Maximum accuracy using Hedley was 74% (4 bands) for Landsat 8 and 80% (3 bands) for Sentinel-2 while for Lyzenga 74% (4 bands) for Landsat 8 and 80% (3 bands) for Sentinel 2. The outputs generated were all >68%, with the introduction of more spectral bands associated with higher accuracy for Landsat 8 but inversely for Sentinel 2.

D1073 |
EGU2020-12294
Assia Edderouzi

One of the biggest ongoing trends related to oceans is the growth of the Blue Economy, that is described as the sustainable use of ocean resources for economic growth, improved livelihoods and jobs, and ocean ecosystem health. In the EU only, it represents roughly 5.4 million jobs and generates a gross added value of almost €500 billion a year. Unlocking the value of the blue economy requires mapping our oceans with both environmental and social dimensions. To achieve this, it is of vital importance to have support and participation from governments, the scientific community and the private sector. Global initiatives like the Decade of Ocean Science for Sustainable Development and SeaBed2030 are good platforms for these stakeholders to overcome internal institutional inertia or distrust of novel types of partnerships, to consolidate or share existing data and help map areas where no data exist. The private sector in particular is key to reach the goal of a comprehensively mapped seafloor. We cannot rely solely on current academic scientific research funding mechanisms. Government funding for academic research is limited, and competition for grants can be expected to remain high in the future. Business can provide much more than just simple funding of Seabed Mapping projects. R&D, Local Content and Participation, Data Stores, Capacity Building and PR are just few examples. The private sector can help affect policy change through lobbying efforts, train the next generation of Ocean Mappers and Scientists, and help to create sustainable practices within the oceans.  So why aren’t more companies investing in these important initiatives? Here we will discuss reasons behind this lack of active participation. We will also explore ways encourage the private sector to think beyond “business as usual” and take ambitious actions in advancing ocean science toward addressing societal needs. Furthermore, we will showcase studies where this collaboration has been effective. The ball is in the “business court”. It is imperative that this sector shifts mindset, allowing data to live beyond its own immediate needs and serve the maximum good. When this happens, together we will move ocean science forward and meet our shared goal of a healthy, sustainable ocean for generations to come.

D1074 |
EGU2020-22253
Integrated study on the stratified gas hydrate accumulations in the Asian Seas
Renat Shakirov, Anatoly Obzhirov, Ryo Matsumoto, and Mariya Shakirova