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  • 1
    Call number: ZSP-166(235)
    Type of Medium: Series available for loan
    Pages: 92 S. , Ill., graph. Darst.
    Series Statement: Berichte aus dem Fachbereich Geowissenschaften der Universität Bremen 235
    Classification: D.3.
    Location: Lower compact magazine
    Branch Library: GFZ Library
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  • 2
    Publication Date: 2017-10-23
    Description: Kelp forests represent a major habitat type in coastal waters worldwide and their structure and distribution is predicted to change due to global warming. Despite their ecological and economical importance, there is still a lack of reliable spatial information on their abundance and distribution. In recent years, various hydroacoustic mapping techniques for sublittoral environments evolved. However, in turbid coastal waters, such as off the island of Helgoland (Germany, North Sea), the kelp vegetation is present in shallow water depths normally excluded from hydroacoustic surveys. In this study, single beam survey data consisting of the two seafloor parameters roughness and hardness were obtained with RoxAnn from water depth between 2 and 18 m. Our primary aim was to reliably detect the kelp forest habitat with different densities and distinguish it from other vegetated zones. Five habitat classes were identified using underwater-video and were applied for classification of acoustic signatures. Subsequently, spatial prediction maps were produced via two classification approaches: Linear discriminant analysis (LDA) and manual classification routine (MC). LDA was able to distinguish dense kelp forest from other habitats (i.e. mixed seaweed vegetation, sand, and barren bedrock), but no variances in kelp density. In contrast, MC also provided information on medium dense kelp distribution which is characterized by intermediate roughness and hardness values evoked by reduced kelp abundances. The prediction maps reach accordance levels of 62% (LDA) and 68% (MC). The presence of vegetation (kelp and mixed seaweed vegetation) was determined with higher prediction abilities of 75% (LDA) and 76% (MC). Since the different habitat classes reveal acoustic signatures that strongly overlap, the manual classification method was more appropriate for separating different kelp forest densities and low-lying vegetation. It became evident that the occurrence of kelp in this area is not simply linked to water depth. Moreover, this study shows that the two seafloor parameters collected with RoxAnn are suitable indicators for the discrimination of different densely vegetated seafloor habitats in shallow environments.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
    Format: application/pdf
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  • 3
    Publication Date: 2018-02-15
    Description: Sorted bedforms are ubiquitous on the inner continental shelves worldwide. They are described as spatially-grain-size-sorted features consisting of small rippled medium-to-coarse sand and can remain stable for decades. However, the knowledge about their genesis and development is still fragmentary. For this study, a representative investigation area (water depth 〈15 m) located on the shelf west of the island of Sylt (SE North Sea, Germany) was periodically surveyed with hydroacoustic means (i.e. sidescan sonar, multibeam echo sounder, and sub-bottom profiler) during 2010-2014. Since this area is influenced by tidal and wind-driven currents, the aim was to detect and examine interannual variabilities in the characteristics of the prevailing sorted bedforms. Our measurements reveal sinuous stripes of rippled medium sand which are embedded in shallow symmetrical depressions. These domains are surrounded by relatively smooth fine-sand areas. These sorted bedforms were identified as flow-transverse features that are maintained by ebb and flood currents of almost equal strengths that flow in opposite directions. This bidirectional flow field generates sharp boundaries between the medium- and fine-sand domains in both current directions. Further to the north, where flood currents are dominant, asymmetric sorted bedforms were detected which show sharp boundaries only in flood-current direction. Comparisons between the measurements of the different years show no significant variations in morphology and distribution of the sorted bedforms. However, variations of the boundaries between the medium and the fine-sand domains were observed. Additionally, new minor sorted bedforms and rippled excavation marks as well as new fine-sand areas developed and disappeared occasionally. It can be supposed that such sediment winnowing and focusing processes take place during periodically recurring storm surges, which change the shapes of the features. Moreover, variations in alignments and sizes of the small ripple formations were detected. They seem to indicate the directions and intensities of previous storm events.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 4
    Publication Date: 2017-08-03
    Description: Several gas seeps and near-surface gas hydrate deposits have been identified in 850–900-m water depth on the continental slope offshore Batumi, Georgia (eastern Black Sea) using deep-towed high-resolution sidescan sonar data. The seeps are located on a ridge named Kobuleti Ridge separating two canyons: the Supsa canyon north of the ridge and the deeply incised central canyon south of it. The southern wall of this canyon shows signs for additional gas seeps. Gas seeps are shown by acoustic anomalies in the water column on raw sonar records and as high backscatter intensity areas on processed data. The seeps on Kobuleti Ridge are characterised by carbonate deposits at the centre and a much wider area where finely disseminated gas hydrates are present. Fractures of a NW–SE direction are present at the seep sites and are probably related to the formation and decomposition of gas. Individual sites of gas emission apparently exert their influence for a circular area of up to 40 m in diameter. Gas geochemistry from gravity cores shows high gas content and a mixture of biogenic and thermogenic gases together with the presence of gas hydrates. The seeps offshore Georgia are different from other known cold seeps in the Black Sea such as shallow water seeps of biogenic gas and deep water mud volcanoes. They are located in deep water within the zone of gas hydrate stability, lack significant relief and are characterised by active gas emission and the absence of mud volcanism.
    Type: Article , PeerReviewed
    Format: text
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  • 5
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    In:  Eos, Transactions American Geophysical Union, 86 (38). pp. 341-346.
    Publication Date: 2017-02-17
    Description: Emissions of methane gas from cold seeps on the seafloor have a strong impact on a number of biogeochemical processes. These processes include the development of deepsea benthic ecosystems via the process of anaerobic oxidation of methane [Boetius et al., 2000] or the precipitation of carbonates [Ritger et al., 1987]. The fluxes of other chemical species associated with methane emissions may even influence the chemical composition of seawater [Aloisi et al., 2004]. Such gas emissions may have been much more intensive in the past with a strong impact on global climate [Dickens, 1999], as suggested by carbon isotope data.
    Type: Article , NonPeerReviewed
    Format: text
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  • 6
    Publication Date: 2012-09-25
    Description: Kongsberg (EM120, EM1002) and ELAC (SB3050) multibeam systems of low to medium frequencies and various subbottom profilers were used to analyze the seafloor of the Baltic Sea between twenty and one hundred meter water depth. The working areas are characterized by soft mud allowing for significant penetration by both subbottom and multibeam signals, especially if lower frequencies were used. Locally shallow gas was found transforming the low-reflectivity mud acoustically into a strong volume scatterer. Single beam subbottom profiles across these shallow gas areas show distinct blanking effects below one and four meters below the seafloor. We demonstrate that low frequency multibeam systems are ideally suited to map those shallow gas areas over the entire swath of 140°. First the depth of the working areas was successfully determined with the shallow to mid-water 95kHz multibeam system. No backscatter anomaly was found while crossing the transition zone between mud and gas-bearing mud. In contrast a 12kHz survey over the same location reveals several meters deeper soundings. The resulting bathymetric data mimics the subbottom morphology of a till structure rather than the seafloor. The reason is strong penetration into the mud up to ten meters, even though the system was manually optimized for correct bottom detection. This makes the 12kHz system prone to subsurface mapping of strong reflectors within very soft sediments. High scattering gas bubbles embedded in the mud could be mapped by backscatter anomalies and misinterpretation of the shallow gas front as bottom echoes occurred. Angular range backscattering strength analysis suggests distinct differences between gassy and non-gassy areas and demonstrates the sensitivity of the low frequency multibeam sounder on free gas even on the very outer beams of the swath. The data is groundtruthed by subbottom profiling and geochemical sampling both indicating free gas. Even small gas pockets of only a few meters extension can be resolved demonstrating the advantages of high resolution and large coverage multibeam mapping compared to single beam surveys. Similar results were gathered using a mobile 50kHz system. (a) Backscatter amplitude chart of EM120. The red rectangle focuses on a transition zone between blue color/no-shallow-gas and red color/shallow-gas area; the inlet shows amplitude data from the 95kHz system not showing any transition. (b) PARASOUND subbottom data. The transition zone (red arrow) between shallow gas and no shallow gas plots exactly at the same location as seen in the multibeam data (a).
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 7
    Publication Date: 2019-07-16
    Description: Hydroacoustic seafloor mapping is a reliable and cost-effective method to investigate and monitor the seafloor in high spatial and temporal resolution. The results are important for the evaluation of benthic habitats and help to identify vulnerable environments that require protection. Yet, how can we overcome the problems that occur when different gear produces different results, which are evaluated by people that have different points of view and different backgrounds? These aspects form an integer part of the project WIMO ("Scientific concepts for monitoring the German Bight, SE North Sea", Subproject 1.1: “Hydroacoustic Habitat Mapping”). It aims at comparing different hydroacoustic gear, methodologies and workflows in order to work out basic routines for universal use in marine benthic habitat mapping. The project investigates a number of target areas in the German Bight (North Sea) using different sidescan sonars (SSS), acoustic seafloor-classification systems (AGDS), multibeams, and different sampling and grain-size analytical methods as well as sea-floor imaging methods. We tested different gear on different ships, on the same ship but not synchronously, and as many instruments as possible measuring at the same time on the same ship. Our results suggest that guidelines and requirements for surveys can hardly be standardized as they depend largely on the water depth, the seabed, and on the vessel and the equipment available. All of these frame conditions usually differ from survey to survey. Taking this into account, we present a reasonable workflow for time and cost-effective benthic habitat mapping and monitoring. Transect-line distances as well as monitoring frequencies, number and positioning of ground-truth samples and seabed imaging are discussed. We recommend frequency combinations and appropriate swath widths and overlaps for SSS and show a way to ground-truth lower-frequency data using high-frequency data. Acoustic ground discrimination systems are usually single-beam systems that need suitable interpolation routines. We present a workflow for on-survey visualization of hydroacoustic data using color interpolation and present a method to combine the hydroacoustic and ground-truth data sets. Eventually, we suggest a way to interpret the data in a most objective manner. The results from the coastal zone of the North Sea reveal that for scientific purposes it is mostly sufficient to maintain transect-line distances of two or three times the SSS swath width. It is suggested to build gray-scale SSS mosaics during the survey. As a general rule a classification into 20% gray-scale classes should be carried out and 5 samples per class should be taken as a minimum requirement. We recommend to apply two SSS frequencies synchronously to enable the discrimination between backscatter due to grain size and backscatter due to small bedforms. This information is also most important for the interpretation of roughness and hardness data provided by the AGDS. The synopsis of both, SSS and AGDS in combination with multibeam and ground truth data reveals the most reliable results.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 8
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 10
    Publication Date: 2019-07-17
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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