Detection of marine slicks with SAR: Scientific and experimental legacy of werner alpers, his students and colleagues

It has long been known from the synthetic aperture radar (SAR) data record that marine slicks can be identified as zones of reduced backscatter separately from the surrounding wind-roughened waters. The composition and properties of marine slicks dampens the shortwave field by both suppression of wave growth and increase in wave dissipation, through an increase in surface tension and a reduction in wind friction. There are, however, clear complications related to SAR marine slick detection. The first problem is that marine slicks may be confused on SAR imagery with areas of low wind and other low-backscatter ocean features such as air-sea temperature differences, rain, and freshwater plumes. The second problem is that there are two primary forms of marine slicks, one composed of biogenic material and the other related to mineral oils from natural seeps or oil spills largely from anthropogenic-related hazards. Biogenic oils, often called surface active agents or surfactants and/or natural films, occur in a thin monolayer and are highly viscoelastic oils that are a byproduct of ocean plant and animal growth. Surfactants readily accumulate in convergent zones by internal waves and current/eddy fields, but are mixed into the upper ocean and rapidly disperse and disappear under windy conditions. The conundrum of detecting and isolating spilled mineral oil from biogenic films and other ocean forms of low backscatter areas has long been recognized by Werner Alpers, his students, and colleagues, and continues to this day. This rich experimental and theoretical legacy, largely focused on the use of radar and SAR, took on steam in the 1980s and continues to this day. This includes laboratory experiments, detailed examination of the properties of both mineral and biogenic slicks and the dampening effects on radar scattering, collection of radar and SAR imagery from ocean platforms, aircraft, the space shuttle, and satellites. The complications and difficulties of this seemingly relative simple notion of separating biogenic films and mineral oil, including as these materials interact with wind, waves, and currents and the use of all sorts of radar frequencies and polarizations collecting data from all over the world, continues to prove challenging in identifying a reliable methodology. In this study, we will review the past efforts of Professor Alpers and his continued influence on current efforts to clearly identify mineral oil and its properties within the vagaries of the ocean environment.