Evaluation de la mesure de la direction des vagues à partir des données d'une bouée instrumentée

The pitch and roll buoy developped at CNEXO-COB to measure directional spectra of waves (following the method employed by Cartwright, Longuet-Higgins and Smith) uses recently manufactured sensors for pitch, roll, heave and direction which give it a large autonomy. This article presents an evaluation of the validity of measurements obtained with this instrument during the Marsen expriment in the North Sea. To do this, we have sought to determine the errors arising at different stages of the acquisition and analysis of data : sensor calibrations, influence of numerical analysis (filtering) and Doppler effects due to tidal currents. The redundancy in the simultaneous measurements of the vertical displacements and the two components of slope of the sea surface has been used to check, in a number of frequency bands, the satisfactory behaviour of these measurements and to make sure that mooring solicitations did not affect the bahaviour of the buoy. Moreover, the evolution in time of the propagation directions of different components of the spectrum is an indication of the validity of the data ; for the high frequency components, these directions are closely linked to the wind direction. Finally, a Doppler effect due to tidal currents has been observed on the buoy data and evaluated by a simple model. [NOT CONTROLLED OCR]