TOPOGRAPHY OF THE UPPER AND LOWER SURFACES OF 10 HECTARES OF DEFORMEDSEA-ICE

An ama of free-floating deformed first-year sea ice nearly 10 hectares in extent has been surveyed. On the upper surface, conventional surve y methods were used, whereas on the lower surface the survey was accom plished using an echo sounder mounted on a tethered submersible. The d eformation 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 elud ed description using conventional ridge models. Because the displaceme nts associated with rafting were large, the axes of ridges were genera lly offset laterally from those of keels. Keels and sails were compose d of blocks of horizontal dimension typically 2-3 times their thicknes s. Profiles of draft across ridge keels had fractal dimensions near 1. 5. This is an expected consequence of the heavily fractured, unweather ed condition of the submerged ice at this site. On the largest scale a ccessible 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 struct ural inhomogeneity and isostatic imbalance were significant.