5 Direct Surface Current Field Imaging from Space
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5.4.3 Bathymetric Mapping and Monitoring
The mapping and monitoring of underwater bathymetry in coastal waters with
strong tidal currents (for example, the Wadden Sea off the coast of the Netherlands,
Germany, and Denmark) on the basis of conventional SAR intensity imagery was
demonstrated by Calkoen et al. (2001) and has been available as an operational commercial service for several years. This technique exploits that bathymetric features
become visible in SAR images due to a modulation of the tidal flow by the spatially varying water depth and a corresponding surface roughness modulation via
wave-current interaction.
Since the imaging mechanism is quite indirect and includes several nonlinearities and dependencies on parameters that are not well known, the inversion is done
through an iterative optimisation scheme for water depths and model parameters,
which are modified until best possible agreement between observed and simulated
radar image intensity variations is obtained with correct depths at known calibration points. This method is sufficient for the identification of major bathymetric
changes, which can then be examined in more detail by conventional echosoundings. This way, the use of SAR data improves the cost efficiency of ship operations
of the responsible monitoring agencies.
Even more improvement can be expected from the use of ATI data instead of conventional SAR images, since the relation between water depths and surface currents
is clearly more direct than the one between water depths and SAR image intensities. Romeiser et al. (2002) demonstrated a bathymetric mapping on the basis of
airborne ATI data using a very simple approach, exploiting just linearised surface
current – water depth relations derived from reference data at a few locations in
the test area by a regression analysis. An example result from an experiment at the
German island of Sylt is shown in Fig. 5.9. In combination with a full physics-based
flow model, such as the one used by Calkoen et al. (2001), bathymetry retrievals
from ATI data should be more accurate and reliable than the conventional SARbased approach, and the method should be applicable to more complex scenarios
since the data interpretation is less ambiguous.
5.4.4 Coastal and Offshore Engineering
Coastal and offshore engineers are interested in effects of currents, waves, and
winds on shore and water based structures, as well as effects of modifications of
such structures. For example, British and German engineers have recently formulated requirements for the site selection of electric power generators in tidal waters
and for further investigations on variations in the 3-D flow around such generators (European Commission, 2005). While it is obvious that the turbines should
be placed in areas with long periods of strong and uniform currents, data from a
British prototype system indicate that currents acting on the rotor exhibit unexpected
strong variations on short time scales, which affect efficiency and wear of the device
quite strongly. To study and optimise the relevant dynamic processes systematically,
85
5.4.3 Bathymetric Mapping and Monitoring
The mapping and monitoring of underwater bathymetry in coastal waters with
strong tidal currents (for example, the Wadden Sea off the coast of the Netherlands,
Germany, and Denmark) on the basis of conventional SAR intensity imagery was
demonstrated by Calkoen et al. (2001) and has been available as an operational commercial service for several years. This technique exploits that bathymetric features
become visible in SAR images due to a modulation of the tidal flow by the spatially varying water depth and a corresponding surface roughness modulation via
wave-current interaction.
Since the imaging mechanism is quite indirect and includes several nonlinearities and dependencies on parameters that are not well known, the inversion is done
through an iterative optimisation scheme for water depths and model parameters,
which are modified until best possible agreement between observed and simulated
radar image intensity variations is obtained with correct depths at known calibration points. This method is sufficient for the identification of major bathymetric
changes, which can then be examined in more detail by conventional echosoundings. This way, the use of SAR data improves the cost efficiency of ship operations
of the responsible monitoring agencies.
Even more improvement can be expected from the use of ATI data instead of conventional SAR images, since the relation between water depths and surface currents
is clearly more direct than the one between water depths and SAR image intensities. Romeiser et al. (2002) demonstrated a bathymetric mapping on the basis of
airborne ATI data using a very simple approach, exploiting just linearised surface
current – water depth relations derived from reference data at a few locations in
the test area by a regression analysis. An example result from an experiment at the
German island of Sylt is shown in Fig. 5.9. In combination with a full physics-based
flow model, such as the one used by Calkoen et al. (2001), bathymetry retrievals
from ATI data should be more accurate and reliable than the conventional SARbased approach, and the method should be applicable to more complex scenarios
since the data interpretation is less ambiguous.
5.4.4 Coastal and Offshore Engineering
Coastal and offshore engineers are interested in effects of currents, waves, and
winds on shore and water based structures, as well as effects of modifications of
such structures. For example, British and German engineers have recently formulated requirements for the site selection of electric power generators in tidal waters
and for further investigations on variations in the 3-D flow around such generators (European Commission, 2005). While it is obvious that the turbines should
be placed in areas with long periods of strong and uniform currents, data from a
British prototype system indicate that currents acting on the rotor exhibit unexpected
strong variations on short time scales, which affect efficiency and wear of the device
quite strongly. To study and optimise the relevant dynamic processes systematically,