Chemistry Reference
In-Depth Information
1 Setting the scene
1.1 The use of satellite imagery to detect oil pollution at sea
Oil spills originated from land-based or sea-based sources are causing
damage to the marine environment. Sea-based sources are discharges com-
ing from ships or offshore platforms. Among the different tools for moni-
toring such oil spills are the use of airplanes and also images from satel-
lites equipped with Synthetic Aperture Radar (SAR).
In the context of monitoring of marine oil pollution, satellite images are
currently used for the following applications:
1. Support response operations in case of maritime accidents (see as an ex-
ample the Prestige case);
2. Providing an early warning of possible threats to coastal areas in time to
take counter-measures;
3. Detecting and deterring illegal discharges from sea-based sources;
4. Measuring the state of European seas - means, variations, trends, to in-
form policymaking and assess the success or otherwise of action taken.
It must be noted that the first three applications are complementary to ae-
rial surveillance traditionally used by national Coast Guards and require a
near real time service (i.e., “dynamic” use of the images), while for the
fourth one, the images can be analysed in a “static” way.
1.2 Identification of Oil Spills in SAR Images
The possibility of detecting an oil spill in a SAR image relies on the fact
that the oil film decreases the backscattering of the sea surface (Alpers and
Hühnerfuss 1989) resulting in a dark feature that contrasts the brightness
of the surrounding spill-free sea. The analysis of this basic fact needs to
start from a description of the different mechanisms responsible for the sea
surface radar backscattering, which strongly depends on the incidence an-
gle of the radar platform. In a quite large range of angles, approximately
from 20 to 85 deg, the angular span of particular interest for space-borne
observations, the main agent of radar backscattering are the wind-gene-
rated short gravity-capillary waves. The oil film has a dampening effect on
these waves locally decreasing the backscattering and finally the bright-
ness in the image. It is implicitly assumed that a light wind field exists in
order to activate short gravity-capillary waves. The minimum wind speed
is in fact depending on the frequency of observation and the incidence an-
gle. For operational satellites operating in C-band, a minimum wind field
of 2-3 m s
-1
creates sufficient brightness in the image and makes the oil
film visible (Donelan and Pierson 1987). On the other end, when the wind
speed is too high, it causes the spill to disappear. First, because the short
