IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 46, NO. 8, AUGUST 2008 2267 Determination of Bathymetric and Current Maps by the Method DiSC Based on the Analysis of Nautical X-Band Radar Image Sequences of the Sea Surface (November 2007) Christian M. Senet, Joerg Seemann, Stylianos Flampouris, and Friedwart Ziemer Abstract—Morphodynamic processes in coastal areas are af- fected by tidal currents and sea state. The continuous observation of near coastal areas is important in order to monitor dangerous current-regime and bathymetry changes. Therefore, there is an urgent need for remote sensing techniques delivering the impor- tant hydrographic parameters with a high spatial resolution. Dis- persive Surface Classificator (DiSC) is a newly developed method based on the analysis of nautical X-band radar image sequences of sea surface waves to determine spatial maps of hydrographic parameters, e.g., spatial maps of the bathymetry and the ocean current field. The method DiSC is described and is illustrated by the presentation of results based on a dataset acquired with a ground-based X-band radar installation mounted on the Island of Sylt in the German Bight. The calculated bathymetric maps are verified by multibeam echo sounder observations. Index Terms—Geophysical inverse problems, geophysical mea- surements, geophysical signal processing, marine radar, waves. I. I NTRODUCTION T HE MORPHODYNAMIC processes in coastal areas are affected by the tidal currents and the sea state, leading to a transport of sediment along the sandy sea floors. Vice versa, the tidal currents are influenced by the changes of ac- tual bathymetry. The continuous observation of areas of high morphodynamical activity is important in order to avoid or, at least, to warn of crucial impacts such as changes in flood stream situations or the loss of land. In the frame of coastal water monitoring strategies, numerous data sources need to be synthesized to enable the responsible coastal protection authorities to take targeted actions. Echo sounding or side-scan sonar techniques are well- established techniques to observe high-precision bathymetric maps. The disadvantage of these techniques is the high ex- penses due to long shipping times necessary to retrieve the required information. As a consequence, the temporal evolution of morphodynamically highly active areas is almost always Manuscript received May 9, 2007; revised September 5, 2007. This work was supported in part in the frame of the Project Val-DiSC by the Ministry for Economic Affairs of the Schleswig–Holstein, Germany, and in part in the frame of the EUFP5 Project OROMA EVK3-CT-2001-00053. C. M. Senet and J. Seemann are with the V2T VISION TO TECHNOLOGY GmbH, 21502 Geesthacht, Germany (e-mail: senet@v2t.de; seemann@v2t.de). S. Flampouris and F. Ziemer are with the GKSS Research Center, 21502 Geesthacht, Germany (e-mail: stylianos.flampouris@gkss.de; friedwart. ziemer@gkss.de). Digital Object Identifier 10.1109/TGRS.2008.916474 undersampled by echo sounding measurements; huge sand transports, i.e., during storm events, cannot be focused. To overcome the aforementioned problem, a new technique, based on the analysis of nautical X-band radar image sequences of the sea surface waves, has been developed. The propagation of sea surface waves is modified in shallow waters according to the influence of the water depth and the currents. The proposed technique utilizes these propagation changes, which are physically described by the dispersion relation of sea surface waves, to determine bathymetric and current maps. Previously from the acquired image sequences, hydrographic parameters, such as the near-surface current velocity vector [1], the water depth [2], [3], and the calibrated full-directional wave spectra [4], are determined. The method used herein, the “global method,” is based on the analysis of gray-level variance spectra calculated by the squared modulus of a 3-D fast Fourier transform (FFT) performed on the image sequences. The 3-D FFT in terms of image processing is a global operator. Therefore, stationarity and homogeneity of the wave field must be fulfilled. This paper is mainly concerned with the presentation of an alternative method [5], [6], which analyzes the image sequences of dynamic dispersive boundaries and is utilized to determine physical parameters on a local spatial scale. The local analysis method, which allows the analysis of inhomogeneous image sequences of a dynamic and dispersive surface, has been labeled Dispersive Surface Classificator (DiSC). With regard to comparable approaches utilizing image se- quences acquired from optical sensors (charge-coupled device (CCD) cameras) and nautical radars to determine hydrographic parameters, the application of DiSC overcomes the following limitations of recently presented techniques. The approach to analyze a single swell wave [7], [8] allows the determination of the bathymetry and one component of the current vector on a local scale. By analyzing image sequences containing the full wave field, for example, a set of long and short waves from a broadband of directions, DiSC delivers bathymetry and current vector maps. Based on the higher degree of information, which is utilized in DiSC, the results contain not only one current vector component but also the full current vector and have a higher statistical significance by analyzing not only one wave but also a set of waves. 0196-2892/$25.00 © 2008 IEEE