Underwater Bottom Topography
See examples of Underwater Bottom Topography:
Underwater bottom topographic features become visible on radar images of the sea surface when there is a current (usually tidal current) which flows over these features. This causes local perturbations to the current which in turn modulates the sea surface roughness. Locating underwater sand banks by roughness variations of the sea surface has been used for several hundred years by mariners to avoid running aground. Since SAR is a very sensitive roughness sensor, it is an ideal instrument for mapping the roughness pattern induced by (tidal) flow over underwater bottom topography.
Theories describing the radar imaging of underwater bottom topography have to account for (1) the modulation of the current by the underwater features, (2) the modulation of the sea surface waves by the variable surface current and (3) the interaction of the microwaves with the surface waves. The last two parts of the SAR imaging theory of underwater bottom topography are the same as for the SAR imaging theory of internal waves. Papers dealing with the SAR imaging of underwater bottom topography are given in the reference list below, but see also the reference list of the general section on "Oceanic phenomena". These theories are the basis of commercial services which generate bathymetry maps by inverting ERS SAR images at a significantly lower cost than conventional survey techniques.
Fig.: Schematic plot of the relationship between an asymmetric sand wave profile and associated variations in tidal current velocity, short-scale surface roughness, and radar image intensity. The steep slopes of the sand waves face the flow direction and are associated with strongly reduced image intensity (dark streaks).
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