Context and objectives
Till now bathymetrical maps are created by means of digital depth measurements with echo sounders and multibeam equipment on board of oceanographic vessels. The data obtained have to be processed to nautical and bathymetrical maps. The amount of data, money and time needed to create new up to date bathymetrical maps is enormous. Therefore new tools and new methods, which will help to retrieve with less money. time and data up to date bathymetrical maps are very welcome. This project is concentrated on the research to obtain up to date bathymetrical images from ERS-1 radar satellite images.
It is important to know that the radar waves don't penetrate the sea water for more than 5 cm, sand banks can not been seen by means of radar satellite images. The whole research is therefore based upon the understanding of the relationships between the sea surface characteristics (roughness, reflections, patterns), that may be derived from the radar satellite ERS-1 images, due to the characteristics of the radar waves (sensitive for surface roughness by analysing the influence differences, strong reflectors, and patterns) and the sea floor morphology (banks and gullies), of the combination of the parameters: superficial currents, tides. morphology of the subsurface banks and the meteo-marine conditions, on the sea surface.
In spite of the fact that our project is mainly focused on the use of radar ERS-1 satellite images. other methods, which can yield solutions to the purpose "the creation of actual bathymetrical maps" are also taken into consideration.
Parameters such as sea water temperature, sea water salinity and chlorophyll-a have no influence on the radar waves, but can be detected by optical satellites such as SPOT, LANDSAT, Coastal Zone Color Scanner (CZCS). Only when the circumstances are ideal. very clear water and no suspension. the sea bottom can be seen up to a few meters, but this is not the case for the North Sea. So the bottom topography is invisible just as on ERS-1 images. One of the objectives of the project is therefore the analysis of the sea water salinity. the chlorophyll-a, the sea water temperature in relation to the bathymetry, during different meteo-marine conditions. If correlations between these parameters and the bathymetry can be proved, a new tool to deduce bathymetrical maps from optical images can be checked.
Expected scientific results
The bottom morphology and the meteo-marine parameters influence the sea surface roughness and the location of the peaks of extreme backscattering values on the radar images. Till now, we can say that there exist 3 great hydrodynamic processes according to the morphology of the sandbanks and the current direction. Most of the peaks and in situ observed lines correspond with the hypotheses. But in the future, it would be interesting to take into account more accurately, the influence of the combined parameters, especially the wind velocity and direction in combination with the currents direction and speed and the morphology. The wind influences the sea surface roughness; for example, the radar images for which the wind velocity is greater than 4 - 5 BEF, the contrast is low. Till now, it is difficult to estimate the influence of the wind on the location of front m relation to the crestlines of the banks.
A real up-to-date bathymetrical map is not yet performed, but a first important step, which can be added in a model, is carried out:
This is the distances of the extreme backscattering values according to the underlying crestlines of the sandbanks, for all the different meteo-marine conditions.
In this way, the localisation of the crestline of the banks can now be deduced from ERS-1 SAR.PRI images