CGRG Bibliography of Canadian Geomorphology
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Author : Blasco, S.; Clarke, J.H.; Bennett, R.; Bartlett, J.; Beaudoin, J.; and Rainey, W.
Date : 2004.
Title : Seafloor features and geological processes as revealed by new multibeam mapping technology, Northwest Passage.
Publication : Arctic Net. First Annual Scientific Meeting, Palace Royal, Quebec City. December 5-8, 2004. Program.
Issue :
Page(s) : 7-8.
Abstract
Two geophysical systems are used in combination to map seabed geology onboard the CCGS Amundsen. The Kongsberg-Simrad EM300 multibeam echo sounder emits 135 individual beams per pulse which generates a continuous cross-track swath of digital bathymetric and bottom classification data with 100% bottom coverage. The multibeam sonar operates at a nominal frequency of 30 kHz allowing it to sound to 5000 metres water depth. This system is unique in that it is made up of 3 to 9 frequency-coded sectors which allows for pitch, roll and yaw. This motion compensation removes acoustic distortion of the data. The Knudsen 320R subbottom chirp profiler sweeps through a frequency band from 2 – 7 kHz with a nominal frequency of 3.5 kHz. This system is capable of imaging at full ocean depth and can penetrate up to 70 metres below seafloor depending on sediment type. Multibeam and subbottom data processing is primarily conducted using the UNB Ocean Mapping Group’s in-house software “Swathed”. This software includes the full suite of data cleaning and visualization tools as well as state-of-the-art functionality for the post-processing of various parameters. To groundtruth bathymetric, backscatter and subbottom data a 1200 kg GSCA style piston corer is used on the Amundsen to obtain core samples up to 9m in length. A box core system is used in conjunction with the piston corer to obtain undisturbed samples of seabed sediments. The digital multibeam bathymetry, backscatter and subbottom profile data are contributing significantly to the knowledge and understanding of seabed geological processes in the Northwest Passage. These acoustic data are essential to identifying localized pondings of thick Holocene sediment accumulation for selective box and piston coring sites. These high sedimentation rate, high resolution sample sites are being used to resolve the paleoceanographic history, evolution of sea ice regime, and ultimately the climate record of the last 10,000 years. To ensure a continuous sediment record over this time frame, undisturbed sample sites free of slumping, turbidity flows, current and ice scouring must be selected. Beaufort upper slope instability is documented by a massive submarine creep failure. Active shelf edge dynamics are demonstrated by the presence of only a thin recent sediment cover. Actively venting mud volcanoes mapped on the shelf have implications to benthic ecosystems, greenhouse gas emissions, the potential presence of hydrocarbon deposits at depth and shallow gas drilling hazards. Present day sea-ice pressure ridge keels scour to water depths of 55m on the Beaufort Shelf. However, paleo-ice scours have been observed to water depths of 400m. The relationship of these deep water events to lower sea level and changing ice regimes over time is not clearly understood. Drumlin-like features and associated grooves excavated into the seabed in Amundsen Gulf relate to the dynamics of the Late Pleistocene Laurentide ice sheet. Benthic ecosystem mapping is accelerated by integrating multibeam backscatter bottom classification maps with focused benthic sampling programs.
Bibliography of Canadian Geomorphology