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Comparison of data and model predictions of current, wave and radar cross-section modulation by seabed sand waves

Cees de Valk(1)

(1) ARGOSS, P.O. Box 61, 8325ZH, Netherlands

Abstract

A comprehensive set of ERS1 and ERS2 SAR images with high-resolution bathymetry, hydrodynamic and weather data collected offshore of Hoek van Holland was analysed to estimate the relationship between the seabed sand waves, with wave lengths in the range of 100-1000 m, and the modulation of the normalised radar cross-section (NRCS) in C-band. From a selected subset of 17 images, empirical Modulation Transfer Functions (MTF's) were estimated and compared with MTF's computed using a two-scale wave-current interaction model. The model performed well in reproducing the measured phase shifts between depth and normalised radar cross-section in the wavenumber range where the S/N is at least 1. The predicted magnitude of MTF is overall somewhat lower than the empirical magnitude. For the six SAR images with the strongest bathymetry-related signal, the model predicts phase shifts close to +/– 90º, while the two-scale mechanism (through tilting of Bragg waves by longer waves interacting with the current) is dominant. This means that under the conditions prevailing during acquisition of these images, the tilting waves relax fast enough to remain near equilibrium with straining and refraction by the local current gradient. The application of this model to reconstruct the seabed from SAR images gave less satisfactory results. Noise/loss of effective resolution appears as the main problem in the reconstruction of the seabed. In a previous study with airborne SAR data, the noise was attributed to sea surface waves longer than the Bragg waves. In the model, an ensemble average over the longer waves is taken, but a satellite or airborne SAR image is a snapshot. We see evidence of this effect in the finding of this study that the average of the seabed estimates from several images gives a much better result than any of the reconstructions from individual images.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry