INEARSHORE GEOHAZARDS IN THE SOUTHERN BEAUFORT SEA, CANADA

摘要

Proposed development of a gas pipeline southward from the Mackenzie Delta and the presence of known accumulations of gas and oil in the southern Beaufort Sea suggest that construction of pipelines and associated infrastructure in the nearshore are likely to be proposed in the future. Recent surveys undertaken by Natural Resources Canada and its partners have focused on the shallow, poorly mapped nearshore region of the Mackenzie Delta (<6 m water depth) that extends ~50 km offshore and lies largely within the landfast ice zone. Ice-keel scouring, strudel scour and nearsurface ice-bonding are being investigated. High resolution sidescan sonar and multibeam bathymetry systems were used to map the seabed over three consecutive years and show that ice keel scouring of the seabed is extensive. The maximum scour depth measured was 0.6 m in 6 m water depth with an average scour depth of 0.2 m. The same scours were visible in repeat surveys indicating that sedimentation was sufficiently low during the study, so that the scours were not infilled. Strudel drainage and associated seabed scour occurs when spring-melt river water overflows onto the surface of the landfast and bottomfast ice once discharge exceeds under-ice channel capacity, then drains back through the floating landfast ice via cracks and holes. Although common offshore of small deltas on the Alaska and Yukon coast, these features were first documented in the Mackenzie Delta area during field surveys in 2006 and 2007 that revealed strudel drainage (radial drainage patterns) features on the ice surface. A total of three strudel scours were later identified using swath-survey equipment in 1.2 m of water. The largest scour was 20 m wide with a maximum depth of 0.8 m below the surrounding seabed. Extensive surveys in Alaska have identified strudel scours exceeding 3 m below the seabed. Nearsurface ice-bonding and permafrost are known to occur in shallow water where sea ice freezes to the seabed. A combination of Synthetic Aperture Radar (SAR), ground penetrating radar (GPR) and multi-year ground temperature measurements have been used to map the horizontal and vertical extent of nearsurface ice bonding in extensive shoals found off the front of the Mackenzie Delta. In the shallowest water depths permafrost extends to 22 m below the seabed with an active layer of less than 1.2 m. In deeper water, permafrost disappears but seasonal frost can form in the upper 2-3 m of the seabed.