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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B7. Istanbul 2004
al. 1996) and the conviction in an oil-spill case in Singapore
(Lu et al., 1999).
For strengthening ties of friendship between Japan and UAE,
the Japan Oil Development Co., Ltd. (JODCO) with the
collaboration of Japanese Information Centre for Petroleum
Exploration and Production (ICEP) created and supported
Satellite Image Processing project (SIPP). ICEP supports
financially the SIPP as an environment monitoring component
in their project "Environmental Protection System in Oil
Producing Offshore Areas of Abu Dhabi." JODOCO supports
SIPP in its logistic, techniques and scientific aspects. SIPP aims
to contribute to development of the basic infrastructure,
management and operation of satellite image processing system
in the United Arab Emirates University using Japanese
technology and assist the UAEU with the development of the
practical use of remote sensing data for environmental
protection.
2. UTILITY OF SAR IMAGES FOR OIL SPILL
DETECTION
The problem of pollution of the oceanic waters by crude oil and
oil materials is presently considered as one of the most sharp.
As first step for solution of this problem is offered a
development and using airborne and spaceborne remote sensing
systems for surveillance of the sea surface, detection the
pollution and spying for their spreading in space. The
requirements to such system were formulated by Wirte (1986).
Such system must provide the all-weather observation, be
independent from of illumination conditions and cloud cover,
define the position, type and volume of oil spill and work in
real scale of time. The sensors of the system must distinguish
anthropogenic films from natural biogenic films produced in the
sea. Synthetic aperture radar (SAR) is an active remote sensing
tool in which an antenna on a satellite transmits microwave
signals toward to the ocean surface. SAR signal after interaction
with the sea surface returns to the antenna (Sabins, 1997). The
interaction between the sea surface and microwaves is very
sensitive to variations in sea surface roughness. Rough surfaces
scatter large amount of energy back to the antenna and have
bright signatures while smooth surfaces reflect the energy away
from the antenna and have dark signatures. Since short surface
waves (ripples and capillary waves) are usually present on the
water surface, it effectively scatters microwaves via the Brag
scattering mechanism (Valenzuela, 1978) and gives radar
signatures. It is well-known, that crude oil and other oil
substances form films of various thickness on the sea surface.
Oil films locally damp sea surface roughness and give dark
signatures, so-called slicks, on the SAR images (Huhnerfuss et
al, 1981; Alpers and Huhnerfuss, 1988). They look on the SAR
images as dark patches among brighter surrounding surface.
This fact gives a physical basis for application of spaceborne
radars for oil spill detection and monitoring in the ocean
(Huhnerfuss et al, 1981).
During the lifetime of oil spill in the sea it will be expose to a
number of processes, which dramatically influence physical-
chemical properties. Called by term weathering, these processes
are as follows: spreading, drift, evaporation, dispersion,
emulsification, bacterial degradation and photo oxidation
(Kotova et al, 1996). With time the physical-chemical
properties of oil spills are changed due to effect of these
processes. These processes play important role in oil spill
detection by using space borne SAR. But relative importance of
659
each process is not well understood. It is also reported that the
thick part of oil film usually covers only 10% of spill area,
while remaining oil covers up to 90% of an area as blue shine
Sabins (1997).
SAR is independent of weather and sun illumination conditions
and allow to acquire SAR images day and night under cloud
cover that is an advantage over other remote sensing sensors.
The detectability of oil slicks/spills in SAR images strongly
depends on the wind speed at the sea surface. Under low wind
speeds, typically between 0 and 2-3 m/s, the sea surface looks
dark on SAR images. In this case the wind-generated waves are
not already developed and oil films looks dark on a dark
background, - detection in this case is impossible. Wind speed
between 3 and 6 m/s is ideal for oil slick detection, the sea
surface roughness is developed and oil slicks appear as dark
patches on a bright background. However, when wind speed
reaches 10-12 m/s, detectability is impossible again or
obstructed due to the redistribution of oil spills/slicks by the
surface waves and wind-induced mixing in the upper ocean
layer (Scott, 1986). As the result slick disappears from the sea
surface and SAR imagery. The upper wind speed threshold for
spill detection with SAR is suggested to be between 10 and 14
m/s (Gade and Ufermann, 1998; Ivanov, 2000).
Other detection problem is discrimination between man-made
and natural organic oil slicks (Huhnerfuss et al., 1986). Natural
biogenic films of a very small thickness resulting from life-
cycle of plankton and other marine organisms can form surface
slicks on the sea surface and, in turn, produce similar dark
signatures on the SAR images (sce Gade et al, 1998, and
references herein). Experiments conducted with multi-
frequency/multi-polarization SIR-C/X aboard the space shuttle
(Masuko et al., 1995; Gade et al., 1998), as expected could
provide more reliable information for oil slick discrimination,
but an important progress has not been achieved. Gade ef al.
(1998) concluded that multi-SAR only capable of
discriminating between different kinds of oil slicks under low
winds, while discrimination at high winds is impossible.
3. STUDY AREAS AND OIL POLLUTION
3.1 General description
The study area lies between longitude 52°E and 56°45’E,
latitudes 24°15’N and 26°N and covers two offshore areas of
the UAE; one from Abu Dhabi to the northeast up to Ras al
Khaimah Emirate in the Arabian Gulf and the other offshore
Fujairah in the Gulf of Oman (Figure 1). The Arabian Gulf is
about 990 km long: the maximum width is about 338 km. He
surface area and volume of the Gulf are estimated as 239,000
km2 and 8,630 km2, respectively. It is a shallow sea oriented in
NW-SE direction with an opening connected to the Gulf of
Oman through the Strait of Hormuz. The average water depth is
about 36m. evaporation and wind are major driving forces of
water circulation in the Arabian Gulf. Evaporation is stronger in
winter due to high wind speed, than summer when the water
surface temperature is higher.
