Radar signatures of oil films floating on the sea surface and the Marangoni effect

Airborne radar backscattering experiments carried out recently by Singh et al. (1986) over sea surfaces covered with mineral oil films show that the radar cross section depression has a maximum as a function of incidence angle. In this paper we show that Singh et al.'s measurements can be explained by the Marangoni effect if the assumption is made that the mineral oil spill contained surface active material as “impurities.” The Marangoni effect causes a resonance-type wave damping in the short gravity wave region when the sea surface is covered with a viscoelastic film. Maximum wave damping was observed by Singh et al. (1986) at frequencies around 8 Hz. Marangoni theory predicts such a maximum for surface active compounds of medium to low wave damping ability with a dilational modulus of the order of 0.01 Nm−1 Furthermore, it is demonstrated that the Ku band and C band radar backscattering depression curves do not contradict each other. It is pointed out that when converting the radar data into information on depression of spectral energy density of short surface waves, one has to take into account that the radar backscattering at low incidence angles is dominated by specular reflection, while at intermediate incidence angles it is dominated by Bragg scattering.