Chemistry Reference
In-Depth Information
waves receive enough energy to counterbalance the dumping effect of the
oil film. Then, when the sea-state is fully developed, the turbulence of the
upper sea layer may break and/or sink the spill or a part of it.
As a consequence of the above brief discussion the identification of an
oil spill in a SAR image includes always as first and basic step the detec-
tion of dark features. Typically, a SAR image may show some dark fea-
tures that are not oil spills (i.e., in most cases due to both meteorological
and/or oceanographic effects). These look-alike features pose a fundamen-
tal problem to the identification of oil spills and require the intervention of
an experienced radar image analyst. In general terms the problem can be
approached in different ways, through an automatic system, completely re-
lying on dedicated image analysis software tools, or in a semi-automatic
mode, where the most critical decisions are taken by an experienced opera-
tor, able to understand and correctly interpret the images.
The basic functions of a system for automatic identification of oil spills
can be described as follows:
1. Isolation and contouring of all dark signatures, through appropriate
threshold and segmentation processing of the image.
2. Extraction of key parameters for each candidate signature, which usu-
ally are related to its shape, internal structure and radar backscattering
contrast.
3. Test of the extracted parameters against predefined values, which char-
acterise man-made oil spills, usually determined through phe-
nomenological considerations and statistical assessments.
4. Computation of probabilities for each candidate signature.
The approach can be also more sophisticated taking into account relevant
environmental parameters having an impact on the spill shape, such as the
time history of wind fields and currents (Espedal and Wahl 1999). While
the two first steps do not require too much assistance from the image ana-
lyst, the two last steps, basically concerning the discrimination of oil spills
from other look-alike, strongly rely on the experience and ability of the
operator.
1.3 Legal aspects concerning sea-based oil pollution
Not all oil spills detected by using satellite images are necessarily illegal.
In fact, we have to distinguish between spills outside special areas and
spills inside special areas. Moreover, different rules apply to discharges
coming from ships or offshore platforms.
Special areas are defined according to the International Convention for
the Prevention of Pollution from Ships (MARPOL 73/78), which in its
Annex 1 deals specifically with prevention of pollution by oil. Almost all
