Natural and man-made sea slicks in the North Sea investigated by a helicopter-borne 5-frequency radar scatterometer

Abstract

In April 1994 natural sea slicks and five man-made slicks, which were spread as a pure substance and from two different spreading solvents, n-hexane and ethanol, respectively, were overflown by a helicopter carrying a five-frequency multi-polarization radar scatterometer, the so-called Heliscat. For the first time, natural sea slicks were successfully simulated by spreading hexadecanoic acid methyl ester (palmitic acid methyl ester (PME)), representing the fatty acid fraction of biogenic slicks, from the spreading solvent ethanol. It is shown that the different slick generation procedures (pure substance, spreading solvents n-hexane or ethanol) give rise to different influences on backscattered radar signals: a PME slick spread from ethanol induces slightly stronger suppressions of the radar backscattering than a PME slick spread from n-hexane; the wave damping maximum of oleyl alcohol (OLA) spread as a pure substance is shifted to higher wave numbers as compared to OLA slicks spread from solvents and the backscattered radar power measured over both PME and OLA slicks spread from solvents show a dependence on the incidence angle, while no angular dependence was observed over OLA slicks spread without a spreading solvent. These phenomena are explained by the different morphology of the slicks induced by the respective spreading solvent. Furthermore, previous assumptions that the physicochemical characteristics of OLA slicks may sufficiently well represent those of natural sea slicks were verified. With regard to the effects summarized above no differences were discernable between the HH and VV radar signals. Simulation of the very first stage of the development of sea slicks revealed that wave damping is continuously increasing during the first time after slick generation. This is in accordance with the assumption that relaxation effects give rise to increasing order within the surface film and thus to increasing hydrophobic interaction both between the film-forming substances and the adjacent water layer.