A Wavelet Technique to Extract the Backscatter Signatures from SAR Images of the Sea

SAR images of the sea often show backscatter patterns linked to the horizontal structure of the Marine Atmospheric Boundary Layer (MABL) at the interface with the sea surface. In general, their dimensions are spread over a wide range of length scales, presenting spatial periodicity as well as intermittence. With the aim to isolate such backscatter structures, the two-dimensional Continuous Wavelet Transform (CWT2) analysis has been applied to SAR images of the sea. The CWT2 analysis permits to highlight the backscatter cells associated to the structure of MABL, as well as to evidence the structure of the atmospheric gravity waves occurring at the lee side of islands and coast. The cells detected in the range 0.3 km ÷ 4 km are directly associated to the wind spatial structure deriving, in turns, from the turbulent characteristics of the wind flow. They have an elliptic shape, with the major axis along the (aliased) wind direction. Those with size falling inside the spatial range 4 km ÷ 20 km describe, instead, the atmospheric gravity waves structure (if present) and the structures linked to the wind shading. The technique developed is the background for several applications: it has been used to compute the wind fields without any a priori information, as well as to study the inner structure of the Langmuir atmospheric circulation. Other applications could be on the detection of sea surface oil slicks.