Tracking of Coastal Swell Fields in SAR Images for Sea Depth Retrieval: Application to ALOS L-Band Data

Swell propagation in shallow water sets specific relationships between water depth and swell parameters like swell wavelength and period. These relationships allow coastal bathymetry to be retrieved if swell parameters are measured. Synthetic aperture radar (SAR) is able to image swell waves, and spectral analysis of SAR images is a well-known approach for measuring swell parameters. However, owing to nonlinearities in SAR imaging, speckle and noise, spectral analysis can result in significant bathymetric errors. The paper individuates the conditions in which linear imaging is achieved and presents an algorithm able to preserve the accuracy of the calculated bathymetry against image speckle and noise. The proposed approach includes: 1) image resizing and filtering before spectral analysis; 2) limitations to the domain of the spectral analysis; and 3) spatial smoothing of the estimated parameters. The algorithm is tested on L-band ALOS PALSAR data collected over coastal regions in the Gulf of Naples, Italy, showing that swell can be properly tracked from open sea to shoreline. Dense coverage and submetric accuracy is thus achieved.