Chemistry Reference
In-Depth Information
5.6 cm, is capable of observing sea surface phenomena, including surface
films through cloud cover, rain, and in the absence of daylight. The ability
of SAR to detect films is due to the damping of small-scale roughness on
the sea surface in areas covered by a film. These relatively smooth areas
are characterized by decreased levels of backscattered signals (normalized
radar cross section - NRCS - Vq), appearing darker on a radar image.
Shipping traffic and legal and illegal ballast water discharges are constant
sources of sea oil pollution. In order to study the effects of surface films,
their relations with oceanic phenomena and to estimate the potential of dif-
ferent remote sensing instruments, including SAR, for their detection and
monitoring special experiments were carried out (Alpers and Hühnerfuss
1989, Croswell et al. 1983, Espedal et al. 1998, Hollinger and Mennella
1973) and many ERS-1/2 SAR images of the Mediterranean, Baltic and
others seas were analysed (Espedal et al. 1998, Fiscella et al. 2000, Gade
and Alpers 1999, Girard-Ardhuin et al. 2003, Ivanov et al. 2004, Kost-
yanoy et al. 2004). Several automatic techniques have been developed to
detect oil spills, ships and ship wakes in SAR images (Brekke and Solberg
2005, Del Frate et al. 2000, Eldhuset 1996, Girard-Ardhuin et al. 2004, Lin
and Khoo 1997, Solberg et al. 1999, Wahl et al. 1993).
The radar contrasts of oil films and of ship wakes are a product of the
environmental conditions at the time of imaging. The distribution of wind
speed and direction is of decisive importance and determines the state of
the ocean's upper layer: the roughness parameters, the presence, amount
and distribution of wave breaking, the presence of organized structures in
the surface roughness distribution. In turn, the state of the ocean surface
determines the brightness of the background, the presence of structures in
the Vq distribution and the level of speckle noise. For low winds
(W d 23 ms
1
), no roughness over the unaffected sea surface is observed.
Films look dark on a SAR image and their contrast to the surroundings is
absent. On the other hand, during high winds (W > 810 ms
1
), the radar
contrast of oil-polluted waters declines due to the increase in the brightness
of the background and the level of speckle noise as well as the decrease in
small-scale roughness damping by films, which are torn to small patches.
This implies that the radar contrast of an oil film is a complicated function
of wind characteristics and time (Alpers and Hühnerfuss 1989, Hamre and
Espedal 1997, Espedal and Wahl 1999, Hühnerfuss et al. 1994, Brekke and
Solberg 2005).
In this paper we will consider two simpler cases where radar contrast of
a spill resulting from the discharge of ballast waters from a moving ship
was high and the spill was clearly visible on a SAR image, without appli-
cation of special transforms. The primary emphasis will focus on a detailed
