Abstract
Artificial oil slick observations with a real-aperture Side-Looking Airborne Radar (SLAR) were conducted on Nov. 10, 1991 and Nov. 1, 1992 at 100 km off the coast of Japan in the Pacific Ocean. The SLAR obtained X-band VV-polarized radar images of an oil slick in synchronization with the synthetic aperture radar of European Remote Sensing Satellite 1 (ERS-1 SAR). The oil slicks were created by spilling oleyl alcohol from a small ship. Sea truth data were collected by a research vessel and the small ship. The SLAR has a 40-degree fanbeam in the cross-track direction. The sea surface backscatter depression, called "damping", caused by the oil slick was measured at various incidence angles from 20 degrees to 58 degrees by SLAR. The sea surface wind speed was 11.3 m/s in the experiment of 1991 and 6.6 m/s in the experiment of 1992. The magnitude of damping was plotted as a function of the incidence angle. There are two notable features in the plots. The first one is that the damping in 1991 is smaller than that in 1992 at all incidence angles, because of the difference in wind speed. This tendency is consistent with the results of other oil slick experiments in which lower wind speeds cause larger damping. The second feature is that there is no clear peak in the plots. The value of damping increases with incidence angles up to 40 degrees, but it is not clear whether it further increases with incidence angles beyond 40 degrees.
While other researchers often found a peak in the damping at a certain incidence angle around 50 degrees, the existence of such a peak is not clear from the data obtained in our experiments. The SLAR data are compared with ERS-1 SAR data taken simultaneously. The incidence angle of ERS-1 SAR is fixed at 23 degrees. The damping values obtained from these measurements are in good agreement on both days. In addition, the amount of damping obtained is well coincident with those estimated from the airborne scatterometer measurement.
