Classification of sea slicks by multifrequency radar techniques: New chemical insights and their geophysical implications

Abstract

Sea slick experiments with an airborne five‐frequency radar scatterometer were performed in the presence of surface active substances that represent different fractions of biogenic slicks (fats, amines, sugar derivatives, and fatty acids). Measurements at water temperatures of 282.2 K (9.0°C) and 290.6 K (17.4°C) showed that temperature effects appear to play a secondary role for slick‐induced water wave damping, at least in the temperature range encountered during the present experiments. Different procedures of slick generation, with and without application of a spreading solvent, indicated that the wave‐damping effect in the short‐gravity/capillary wave range, and thus the modification of backscattered radar signals, is not only dependent on the chemical structure but also on the arrangement and distribution (morphology; formation of domains) of the surface‐active compounds. Thus far this aspect, which appears to be of particular importance for biogenic sea slicks, has been completely ignored. External infrared reflection‐absorption spectroscopy laboratory measurements with infrared radiation in the wavelength range between 3.3 μm and 7 μm enabled us allowed to form a link between some important elements of the morphological structure of the monolayers and their viscoelastic characteristics, which are closely related to the wave‐damping effect of surface active compounds and to the compounds influence on remote sensing signals. Furthermore, the IRAS measurements supplied detailed insight into the relaxation process that occurs during the generation of a sea slick and on a slick‐covered undulating water surface. In particular, strong hydration/dehydration effects appear to play an important role.