Imaging of biogenic and anthropogenic ocean surface films by the multifrequency/multipolarization SIR‐C/X‐SAR

Abstract

Results from the analyses of several spaceborne imaging radar‐C/X‐band synthetic aperture radar (SIR‐C/X‐SAR) images are presented, which were acquired during the two SIR‐C/X‐SAR missions in April and October 1994 by the L‐, C‐, and X‐band multipolarization SAR aboard the space shuttle Endeavour. The images showing natural (biogenic) surface slicks as well as man‐made (anthropogenic) mineral oil spills were analyzed with the aim to study whether or not active radar techniques can be applied to discriminating between these two kinds of surface films. Controlled slick experiments were carried out during both shuttle missions in the German Bight of the North Sea as well as in the northern part of the Sea of Japan and the Kuroshio Stream region, where surface films of different viscoelastic properties were deployed within the swath of the shuttle radars. The results show that the damping behavior of the same substance is strongly dependent on wind speed. At high wind speed (8–12 m/s) the ratio of the radar backscatter from a slick‐free and a slick‐covered water surface (damping ratio) is smaller than at low to moderate wind speeds (4–7 m/s). At 12 m/s, only slight differences in the damping behavior of different substances were measured by SIR‐C/X‐SAR. Furthermore, several SAR scenes from various parts of the world's oceans showing radar signatures of biogenic as well as anthropogenic surface films at low to moderate wind speeds are analyzed. The damping behavior of these different kinds of oceanic surface films varies particularly at Lband where the biogenic surface films exhibit larger damping characteristics. Results of polarimetric studies from multipolarization SAR images showing various surface films are presented. It can be delineated from these results that Bragg scattering as well as specular reflection contribute to the backscattered radar signal at low incidence angles (up to 30°). It is concluded that at low to moderate wind speeds, multifrequency radar techniques seem to be capable of discriminating between the different surface films, whereas at high wind conditions a discrimination seems to be difficult.