Ice cover

When the sea surface is covered by an ice canopy, as in the polar regions (see Figures 2.11 and 2.12), acoustic interaction with the surface is further complicated by an irregular under-ice surface. The Arctic environment can be segregated into three distinct regions according to the type of ice cover: (1) pack ice; (2) MIZ; and (3) open ocean. Field measurements have shown that forward scatter from a rough anisotropic ice canopy is a function

Figure 2.H Average boundaries of sea ice (coverage at least five- to eight-tenths) in autumn and spring in the Arctic. Arrows indicate the general drift pattern. The width of the stippled area indicates the range of ice limits between autumn and spring (Untersteiner, 1966).

Figure 2.H Average boundaries of sea ice (coverage at least five- to eight-tenths) in autumn and spring in the Arctic. Arrows indicate the general drift pattern. The width of the stippled area indicates the range of ice limits between autumn and spring (Untersteiner, 1966).

of acoustic frequency, geometry and the statistical (spatial correlation) properties of the under-ice surface.

2.4.3 Sea floor

The sea floor is a reflecting and scattering boundary having a number of characteristics similar in nature to those of the sea surface. Its effects, however, are more complicated than those of the sea surface because of its diverse and multilayered composition. Specifically, the sea floor is often layered, with a density and sound speed that may change gradually or abruptly with depth or even over short ranges. Furthermore, the sea floor is more variable in its acoustic properties since its composition may vary from hard rock to soft mud. One feature that is distinct from the sea surface is that the bottom characteristics can be considered to be constant over time, whereas the configuration of the sea surface is statistically in a state of change as the wind velocity changes.

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