Author(s): A. L. Forrest; A. K. Hamilton; V. Schmidt; B. E. Laval; D. Mueller; A. Crawford; S. Brucker; T. Hamilton
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Abstract: Understanding subsurface shape, topography and roughness of icebergs and ice islands (large tabular icebergs) in the Canadian High Arctic is limited by a paucity of underwater ice terrain data. This information is critical for calibrating operational iceberg drift and deterioration models that aim to mitigate hazards to shipping and the offshore petroleum industry. Observations beneath icebergs and ice islands have been limited in part by the associated risks operating in close proximity to unstable targets and an inability to map the underside of the ice. In this work, we use a combination of three-dimensional digital terrain mapping sonar systems, including an interferometric sonar mounted on a small Autonomous Underwater Vehicle (AUV), to map portions of the sidewalls and the underside of Petermann Ice Island (PII) fragments in Lancaster Sound and Baffin Bay. The complete perimeter of a small free-drifting ice island fragment named Berghaus (0.13 km2; up to 131 m thick), was mapped with surface vessel multibeam sonar and volume and mass estimated. Portions of the sidewalls of the free-drifting PII-Ba (15 km2; up to 106 m thick) and the grounded fragment PII-B (90 km2; up to 102 m thick) were also mapped with surface vessel sonar. Subsurface roughness is compared between the three ice islands and shows relatively smooth surfaces at the scales analyzed (20 cm to 5 m). PII-Ba was surveyed with a remotely operated vehicle (ROV) that revealed widespread 'scallop' concavities on the order of 5–10 cm down the full depth of the sidewall. A small AUV, UBC-Gavia, was used to map a roughly 700 m × 500 m portion of the underside of PII-B and combined with a surface vessel sidewall survey produced a 3D terrain map of the underwater portion of the ice island. A highly eroded ice wall is revealed, showing a pronounced ice skirt indicative of intense surface wave erosion. The draft of the ice island increased from 30 m at the edge to 50 m at the furthest extent of the survey beneath the ice island with a relative uniform basal topography. While only a partial survey could be completed due to the vast size of the ice island, this work demonstrates a unique application of AUVs to contribute to improved understanding of the roughly 9/10ths of the ice that lies below the ocean surface. The ultimate goal of this project, to understand ice island drift and in situ melt and deterioration processes will require knowledge of the original glacier topography (pre-calving) and a re-survey of the ice islands over time, along with measurements of the external forcing on the fragments. Here we focus on the digital terrain mapping data which will provide the baseline for further studies and help improve operational iceberg drift and deterioration model parameterization of underwater shape, topography and surface roughness.
Year: 2012