1,720,979 research outputs found
Acquisition and processing of backscatter data for habitat mapping - comparison of multibeam and sidescan systems
No full textOften marine habitat surveys use multibeam bathymetry systems to model the seafloor. This describes the morphology but not the terrain lithology or substrate. Backscatter imagery helps the interpreter to better classify the physical environment that may support a particular biological community. In this contribution, the acquisition performance of both multibeam and sidescan sonar backscatter imagery are contrasted and examples shown. The logistical factors affecting the two systems during surveying are discussed and data from both systems compared.Choice of systems for habitat mapping is discussed. A relative cost analysis of the various survey systems is presented with varying resolution and coverage. The size and shape of the footprint, and thus resolution, is variable dependant on many factors, including ship speed, data processing and sampling. The resolution and formation of imagery is important, but high sampling rates are shown not to be a complete solution as over-sampling can present a false impression of high resolution. However, it is suggested that backscatter imagery should have least 7 bit sensitivity to aid visual and digital inspection. Habitat mapping examples are shown using multibeam backscatter and sidescan sonar, where the processing has been optimised for backscatter imagery. A key question is how much of high resolution bathymetry data is essential for habitat mapping, and whether backscatter imagery can provide more of the information required at a higher resolution than a bathymetric morphology map
Seafloor sediments and sedimentary processes on the outer continental shelf, continental slope and basin floor
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Flank collapse and large-scale landsliding in the Cape Verde Islands, off West Africa
Full text linkLarge-scale landslides occur on the flanks of many volcanic oceanic islands worldwide. None have taken place in historical time, but their geohazard potential, especially their ability to generate tsunamis, is large. The Cape Verde Islands are a group of 10 large and several smaller volcanic islands off the coast of West Africa between 15 and 17°N. A single flank landslide has previously been described from the island of Fogo, but systematic analysis of the Cape Verde group has until now been lacking. This paper describes and interprets a multibeam bathymetry data set covering the slopes of the western Cape Verde Islands, including those of the islands with the most recent volcanic activity, Fogo in the southwest, and Santo Antao in the northwest. All of the larger islands show evidence of large flank landslides, although only Fogo and the southwest part of Santo Antao have failed in the last 400 ka. Tope de Coroa, the volcano at the southwest end of Santo Antao, has been inactive for the past 170 ka and is judged to have a low landslide potential unless volcanic activity resumes. In contrast, there would seem to be a high probability of a future east directed landslide on Fogo, from the area of the highly active Pico do Fogo volcano, although it is impossible to predict a timescale for such an event. A tsunami generated by such a landslide could have a catastrophic effect on the adjacent island of Santiago and possibly even farther afield on the West African coast. <br/
Effectiveness of a deep-sea cold-water coral Marine Protected Area, following eight years of fisheries closure
Full text linkPressure on deep-sea ecosystems continues to increase as anthropogenic activities move into ever deeper waters. To mitigate impacts on vulnerable habitats, various conservation measures exist, such as the designation of Marine Protected Areas (MPAs). So far, however, little evidence is available about their effectiveness. This paper presents a unique follow-up study assessing the status and recovery of a deep-sea fisheries closure and MPA at ~1000 m water depth in the NE Atlantic, eight years after designation. The Darwin Mounds cold-water coral ecosystem was discovered in 1998, and closed to all bottom contact fisheries, especially trawling, in 2003. Our repeat survey in 2011 used both high-resolution sidescan sonar data collected by Autonomous Underwater Vehicle (AUV) and video footage from a Remotely Operated Vehicle (ROV) to evaluate recovery. The results demonstrate that (1) protection was successful and fishing impact was largely avoided in the Western Darwin Mounds, which contained similar proportions of live cold-water coral occurrence in 2011 as observed in 1998-2000; however (2) the Eastern Darwin Mounds suffered severe damage pre-closure, and by 2011 showed no coral recolonisation and very little regrowth. These results are further evidence for the low resilience and slow recovery potential of deep-sea ecosystems, and underline once again the importance of the precautionary principle in deep-sea conservation
Segment-scale volcanic episodicity: evidence from the North Kolbeinsey Ridge, Atlantic
Full text linkThe upper oceanic crust is produced by magmatism at mid-ocean ridges, a process thought to be characterized by cyclic bouts of intense magmatic activity, separated by periods when faulting accommodates most or even all of the plate motion. It is not known whether there is a distinct periodicity to such magmatic–tectonic cycles. Here we present high-resolution sidescan sonar data from the neovolcanic zone of the North Kolbeinsey Ridge, a shallow slow-spreading ridge where high glacial and steady post-glacial sedimentation rates allow relative flow ages to be determined with a resolution of around 2 kyr using backscatter amplitude as a proxy for sediment thickness and hence age. We identify 18 lava flow fields covering 40% of the area surveyed. A group of 7 flow fields showing the highest (and similar) backscatter intensity are scattered along 75 km of axial valley surveyed, suggesting that at least this length of the segment was magmatically active within a 1.2 kyr time window. Based on conservative age estimates for all datable flows and estimated eruption volumes, the post-glacial volcanic activity imaged is insufficient to maintain crustal thickness, implying that episode(s) of enhanced activity must have preceded the volcanism we image
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