A study of the morphology of a discontinuous section of a first year arctic pressure ridge

The upper and lower surfaces of a 300 m long section of a first year pressure ridge containing both linear and corner regions have been surveyed. The upper surface survey employed conventional ground based techniques combined with aerial stereophotography, whilst the lower surface was surveyed with a narrow beam echo sounder mounted on a tethered submersible. The ridge was sited on the southern edge of a re-frozen lead, and estimates based on the block thicknesses and on ice drift velocity data records, place the time of formation between January 25 and 30, 1991, about 3 months prior to the survey. The southern edge of the keel consisted of large blocks and rafted sections of the original sheet, whilst the northern edge was composed of smaller scale consolidated rubble from the re-frozen ice of the lead. The section of ridge surveyed contained about 3 times as much ice volume as would be estimated by applying the triangular geometry of the classical 2-dimensional ridge, most of the excess volume being associated with the corner regions. Profiles of the draft across the structure had a fractal dimension of 1.5, similar to values reported by other authors for unweathered ice structures. Considerations of the equilibrium of the complete structure suggest that when formed it was capable of sustaining significant bending moments on scales of 20m and that up to 3 thicknesses of rafted ice of the original sheet must have been present.