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Long-term microwave radar monitoring of ocean slicks at
low grazing angles
Christine P Gommenginger and Simon R Boxall
Ocean Circulation and Climate Division, National Oceanography Centre,
Southampton, United Kingdom
Abstract.
This paper proposes the use of a digital microwave marine ra-
dar system for the detection and monitoring of surfactant slicks at sea. The
spatial, temporal and radiometric resolutions of this low grazing angle in-
strument are compared to those of satellite and airborne radars traditionally
used for oil pollution monitoring. Experimental evidence is presented in
two artificial surfactant releases in the coastal zone which were success-
fully detected and tracked with a shore-based marine radar. The in-
formation contained in a long-term time-series of NRCS images is ex-
plored and its synergistic use with traditional satellite- and air-borne radar
data discussed.
1 Introduction
The detection of ocean surface slicks by microwave radars at intermediate
incidence angles is well established. The all-weather day-and-night capabi-
lities of microwave remote-sensing has resulted in its wide operational use
by harbour authorities for the control of oil pollution in the coastal zone
(Attema and Hoogeboom 1978, Sherman 1992). In research, microwave
radars can also help further our understanding of air-sea interactions and
hydrodynamic processes by providing extensive synoptic two-dimensional
maps of the slicks' spatial distribution (Espedal et al. 1996, da Silva et al.
1998).
The reduction of ocean backscatter power by slicks has been attributed
to the damping of the small scale waves predominantly responsible for the
radar returns from the sea surface (Alpers and Hühnerfuss 1988). Success-
ful slick imaging relies on good spatial and radiometric resolution to en-
sure sufficient contrast with the backscatter from the surrounding “clean”
sea. At low grazing angles, the level of backscatter becomes very low as
different scattering mechanisms such as shadowing become dominant
