Anormal Doppler shifts of radar signals backscattered from marine slicks

Results of field experiments on remote sensing of organic slicks on the sea surface using Ka-band radar are presented. Slicks of different surfactants with the film elasticity values ranging from some mN/m to some tens of mN/m were studied. It is shown that the radar Doppler shifts can change strongly in slicks, depending on the film elasticity, while the depression of radar backscatter in slicks is nearly the same for the different substances. A physical explanation of the effect is given assuming different contributions of free capillary-gravity waves and "bound waves" - nonlinear harmonics of longer surface waves in the spectrum of cm- to mm-scale wind waves. A simple theory of wave damping in slicks is used to explain qualitatively the dependence of the Doppler shifts on film elasticity.