TOPOGRAPHY OF THE UPPER AND LOWER SURFACES OF 10 HECTARES OF DEFORMED SEA-ICE

An ama of free-floating deformed first-year sea ice nearly 10 hectares in extent has been surveyed. On the upper surface, conventional survey methods were used, whereas on the lower surface the survey was accomplished using an echo sounder mounted on a tethered submersible. The deformation at the site occurred in response to a uniaxial compressive stress, which produced extensive rafting and heavy ridging of the 1.5 m thick ice present at the time. The resulting complex topography eluded description using conventional ridge models. Because the displacements associated with rafting were large, the axes of ridges were generally offset laterally from those of keels. Keels and sails were composed of blocks of horizontal dimension typically 2-3 times their thickness. Profiles of draft across ridge keels had fractal dimensions near 1.5. This is an expected consequence of the heavily fractured, unweathered condition of the submerged ice at this site. On the largest scale accessible in the survey (200-300 m), the site was in isostatic balance, but characterized by considerable inhomogeneity in the consolidation of its subaerial and submerged portions. On a 10 m scale, both structural inhomogeneity and isostatic imbalance were significant.