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Seafloor analysis based on multibeam bathymetry and backscatter data = Meeresbodenanalyse auf der Basis von Bathymetrie und akustischer Rückstreuung

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Beyer, A. (2006): Seafloor analysis based on multibeam bathymetry and backscatter data = Meeresbodenanalyse auf der Basis von Bathymetrie und akustischer Rückstreuung , Berichte zur Polar- und Meeresforschung (Reports on Polar and Marine Research), Bremerhaven, Alfred Wegener Institute for Polar and Marine Research, 540 , 100 p. .
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Large areas of the worlds oceans are still unexplored. Previously unknown structures and ecosystems are discovered due to the increasing exploration of the oceans, in particular at the continental margin and the continental shelf. Research is essential to understand these structures and to analyze their potential and importance for human society and the ecosystem. This PhD thesis is one part of multidisciplinary research at the European continental margin. Hydroacustic methods were applied in order to identify and distinguish different seabed structures and facies.Hydroacustic techniques are an established approach to analyze the seafloor. Echo intensity was used in this study for interpretation in addition to the travel time of the acoustic signal. The ship-borne multibeam system Hydrosweep DS-2 and the sub-bottom profiler Parasound were used for data recording.The investigation areas of this study comprise the Håkon Mosby mud volcano situated at the Norwegian-Barents-Svalbard continental margin and the carbonate mounds in the Belgica mound province together with adjacent channels situated in the Porcupine Seabight at the Irish continental margin. Mud volcanoes and carbonate mounds are characterized by active sediment and transport processes and are related to the existence of subsurface hydrocarbon, mainly methane. Seafloor channels indicate sediment transport in lateral and vertical direction. These structures are focus of recent research due to the contribution of methane to the carbon cycle.A terrain model of the Håkon Mosby mud volcano was created providing the basis for detailed studies which require precise positioning for sampling and observation devices. The fine structure of the mud volcano was mapped by dense survey lines and overlapping swathes. The Håkon Mosby mud volcano is located at a water depth of about 1270 m and can be separated into three morphological segments: weak, crater-like center (950 m diameter, 12 m height), circular embankment (1350 m diameter, 100 m breadth, 2 m height) and the area influenced by mud volcanism which is characterized by a reduced surface slope compared to the general margin (2500 m diameter).Investigations of the Belgica mound province show that the carbonate mounds occur in a depth range between 1000 m and 700 m. They are aligned along the margin and show heights of about 50 m up to 100 m maximum. Their morphology resembles ellipses with axes ranging from 0.5 km to 1.0 km and from 1.0 km to 1.5 km, respectively, ridge-like mounds or terrace-like structures. Other morphological properties comprise steep surface slopes of about 20° and depressions at the foot of some mounds having variable depths up to 50 m. This morphology is different from other mound provinces in the Porcupine Seabight. Carbonate mounds proposed south of the Belgica province in the Gollum Channel System were not evidenced by this study.In addition to the morphology of the seabed structures, acoustic backscatter data were analyzed in the study areas. The Håkon Mosby mud volcano shows strong backscatter at the central area in contrast to reduced backscatter at the surrounding, circular depression. This might be due to gas occurrence in the sediment and high surface roughness at the central crater. Soft and homogeneous sediments with low surface roughness are expected at the surrounding moat. The eastern part of the mud volcano, however, shows spatially different variability of morphology and backscatter data.Angular backscatter data of the individual depth measurements were analyzed in the Porcupine Seabight area. Segments of the seafloor of different acoustical properties were separated based on morphology and angular backscatter data. These segments were studied with respect to the parameters mean angular response, slope and variation of the angular backscatter data. Seabed facies like surface mounds, buried mounds, channels and inter-channel areas were characterized and distinguished. First information on the seabed variability is now available to select specific sample locations.The maps showing the spatial distribution of the backscatter data indicate sediment transport at the seabed by lineated structures. These structures occur in the vicinity of the carbonate mounds in the Porcupine Seabight and point to seabed currents with an oblique direction to the continental slope. Backscatter data at the Håkon Mosby mud volcano show that the outflowing material is transported down slope, probably by bottom currents or gravity.

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"Berichte zur Polar- und Meeresforschung"
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