Pressure Ridge Shapes and Slope Angles from Multi-beam Sonar Data

Abstract

Abstract We describe our work undertaken to analyse sea ice pressure ridge morphology in the Arctic ocean from Autonomous Underwater Vehicles (AUVs) and submarines. Details of cruises, from ship-based launches of large (7m) AUVs to hand deployed AUVs are given, as well as the first submarine voyage to incorporate an upward looking multibeam sonar. We discuss our techniques of recovering meaningful data from the record, and introduce our method to calculate pressure ridge slope angles, orientation and frequency. This work is done in the context of the loads that both level and deformed sea ice exert on off-shore structures. Introduction There are important reasons to know the three-dimensional topography of the under-ice surface. The polar sea ice cover consists of two distinct components: thermodynamic ice, which has reached its current thickness by natural growth; and deformed ice, which can attain a much greater thickness through rafting and ridging. Pressure ridges, formed by the crushing of refrozen leads, can reach a draft of more than 50 m. From the viewpoint of modeling the role of sea ice in climate, this composite nature of the ice cover must be considered in order to deal with dynamics, thermodynamics and mechanics correctly. For the offshore industry, ridging is important, as the deepest multiyear pressure ridge represents the design load on an offshore structure, while the deepest ridges also control scouring frequencies in the nearshore zone. The topography of sea ice is important for such diverse applications as calculating its containment potential for oil blowouts, its role as a substrate for a sea ice ecosystem, its impact on icebreaker design, and its scattering potential for under-ice acoustic propagation. The first measurements of under-ice topography were simple linear profiles generated by narrow-beam upward-looking sonar. These were first obtained from naval submarines, and it was the voyage of USS "Nautilus" in 1958 which first revealed, via an upward sonar profile, the rugged topography of the ice underside with its landscape of steep pressure ridges separated by smoother undeformed floe sections (Lyon, 1961). From then on, frequent submarine transects of the Arctic Ocean in different years and seasons have enabled the regional distribution of ice topography to be determined (Bourke and Garrett, 1987) and, more recently, the rapid decline in mean ice draft throughout the Arctic to be documented (Rothrock et al., 1999, 2003, 2008; Wadhams, 1990; Wadhams and Davis, 2000, 2001; Yu et al., 2004).