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 30ø). 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. 1988, and references therein]. Since these waves are responsible for the backscattering of microwaves of comparable wavelengths (Bragg scattering, see Valenzuela [1978]), oil spills are visible as dark patches on radar images of the ocean surface. Further advantages of the use of radar techniques are that the microwaves are mostly not influenced by clouds and that active microwave techniques are independent of daylight. For these reasons the 5.3 GHz (C-band) synthetic aperture ra-Paper number 97JC01915. 0148-0227/98/97JC-01915509.00 dar (SAR) aboard the first and second European Remote Sensing Satellites (ERS-1/2) is already used for operational oil surveillance (reported, e.g., by Wahl et al. [1994] and by Pellemans et al. [1995]). However, not only anthropogenic surface films consisting of mineral oil dampen the surface waves and can be delineated on SAR images. Also, biogenic surface slicks, which a...

SAR raw signal simulation of oil slicks in ocean environments

“…For clean water, these latter coefficients can be written as [8] (4.6a) and (4.6b) respectively, wherein is the water density; [N s m] is the water dynamic viscosity [8], [32], [38]; and is a dimensionless coefficient equal to about unity, its exact value being unavailable. Simulation results (see Section VI) suggest a value equal to 1.15.…”

“…By using a simple energy balance equation, in equilibrium conditions we can write [8], [32], [38], [39] (4.1a) (4.1b) wherein subscripts and refer to clean water and to oil slick, respectively. In (4.1), and are the wind input terms; are the nonlinear interaction input terms; and and are the dissipation terms due to viscosity and to breaking waves, respectively.…”

Wind‐wave tank measurements of bound and freely propagating short gravity‐capillary waves

“…wave damping, e.g., the reduction of the wind friction velocity [Gade, 1996;Gade et al, 1998a, c; Wismann er ai., i998], the reduction of wave breaking, and the modification of nonlinear wave-wave interactions in the presence of surface films [Alpers and Hiihnerfuss, 1989;Gade et al, 1998b].…”

Dark-spot detection from SAR intensity imagery with spatial density thresholding for oil-spill monitoring

“…Several researchers reported that the oil-water backscatter contrast decreases as the radar wavelength increases (Neville et al, 1984;Wismann et al, 1998;Gade et al, 1998). Though some studies show that there is no real difference between horizontal (HH) and vertical (VV) polarization for oil spill detection (Wismann et al, 1998), there is agreement in the literature that VV polarized images provide a better oil-water contrast than either HH or HV polarized images for the C-band (Neville et al, 1984;Singh, 1987;Masuko, 1995;Wismann et al, 1998).…”