Fig. 6

> Figure 6. Sidescan sonar image of two typical multi-year floes, profiled with the HMS Sovereign submarine running at a transducer depth of 75 m. The floes are of dimensions 880 m x 620 m and 430 m x 50 m, and are bounded and separated by keels of pressure ridges. Dark areas on the image represent strong sound returns, generated by rough surfaces or by planes inclined toward the fan-shaped beam emitted by the sidescan (mainly the nearside flanks of pressure-ridge keels). White areas are shadow zones, typically on the far sides of keels (P.Wadhams, see NATURE, v.333, No 6160, pp.161-164).