suppression (damping) by surfactants results in visible changes in surface
roughness by producing a slick that is smoother than the surrounding windroughened ocean (Hu et al. 2009). SAR instruments are very sensitive to this
roughness change, which can occur in the presence of even very small quantities of
oil or other surfactants. The radar signal is forward scattered off the smooth ocean
with decreased backscatter to the SAR instrument, resulting in a dark region in the
image. The oil-mapping ability of SAR instruments is well known, and amply
demonstrated during responses to both major and minor oil spills (Clemente-Colón
and Pichel 2006; Clemente-Colón et al. 1997; Gade and Alpers 1999). Oil spills
can be the result of accidents, intentional dumping (e.g., bilge pumping), pipeline
leaks, oil platform releases, releases from sunken vessels, or coastal effluents
(Clemente-Colón and Pichel 2006). Unfortunately, a dark region in a SAR image
is not always indicative of an oil spill. There are many other low-backscatter
features that are not oil spills (i.e., false positives) such as: areas of low wind
speed; upwelling; natural oil seeps; biogenic slicks such as algal blooms and coral
spawn; fish oils released during fishing and fish processing activities; and organic
surfactant runoff directly from land and/or carried by rivers. Nonetheless, despite
these potential confusion areas, criteria have been developed for successful remote
sensing of oil slicks.
Oils can be detected, in general, when wind speeds are within 3–15 m/s (Espedal et al. 1998; Johannessen et al. 1994; Wahl et al. 1996; Wismann et al. 1998),
while biogenic surfactants can usually be detected when the wind speed is between
2 and 8 m/s (DiGiacomo and Holt 2001). When winds are strong (i.e., exceeding
10 m/s for extensive periods during a period of 24 h), the oil becomes mixed into
the upper ocean and slicks do not form (Simecek-Beatty and Pichel 2006; Simecek-Beatty and Clemente-Colón 2004). Differentiating between oil spills and other
naturally occurring slicks requires a trained analyst and extensive knowledge of
local ocean and atmospheric conditions. Fully automated detection algorithms
using neural networks, fuzzy logic, wavelet transforms, and other trained analysis
techniques have also achieved some success (Garcia-Pineda et al. 2009; Liu et al.
2000, 2010); however, no single-channel, single-polarization algorithm can detect
oil without some false positives. Multi-frequency and multi-polarization analysis
techniques, which exploit changes in polarization of the backscattered signal as a
result of different scattering mechanisms for oil-dampened ocean and normal
wind-roughened ocean, may help to reduce these false alarms (Gade et al. 1998;
Trivero et al. 1998; Mugliaccio et al. 2009; Zhang et al. 2011).
Ocean features Since SAR instruments are very sensitive to any ocean phenomenon that alters capillary and small gravity waves at the scale of the SAR
wavelength (i.e., about 5 cm for C-band), many ocean features appear on SAR
imagery. These include:
• Currents. Since wave propagation and height are affected by horizontal shears in
ocean currents, phenomena like strong currents, eddies, and river plumes show
up clearly in SAR imagery, often as narrow bright features marking their
boundaries (Zheng et al. 2004). Natural surfactants can also be swept along by
13 Radar Applications
353
roughness by producing a slick that is smoother than the surrounding windroughened ocean (Hu et al. 2009). SAR instruments are very sensitive to this
roughness change, which can occur in the presence of even very small quantities of
oil or other surfactants. The radar signal is forward scattered off the smooth ocean
with decreased backscatter to the SAR instrument, resulting in a dark region in the
image. The oil-mapping ability of SAR instruments is well known, and amply
demonstrated during responses to both major and minor oil spills (Clemente-Colón
and Pichel 2006; Clemente-Colón et al. 1997; Gade and Alpers 1999). Oil spills
can be the result of accidents, intentional dumping (e.g., bilge pumping), pipeline
leaks, oil platform releases, releases from sunken vessels, or coastal effluents
(Clemente-Colón and Pichel 2006). Unfortunately, a dark region in a SAR image
is not always indicative of an oil spill. There are many other low-backscatter
features that are not oil spills (i.e., false positives) such as: areas of low wind
speed; upwelling; natural oil seeps; biogenic slicks such as algal blooms and coral
spawn; fish oils released during fishing and fish processing activities; and organic
surfactant runoff directly from land and/or carried by rivers. Nonetheless, despite
these potential confusion areas, criteria have been developed for successful remote
sensing of oil slicks.
Oils can be detected, in general, when wind speeds are within 3–15 m/s (Espedal et al. 1998; Johannessen et al. 1994; Wahl et al. 1996; Wismann et al. 1998),
while biogenic surfactants can usually be detected when the wind speed is between
2 and 8 m/s (DiGiacomo and Holt 2001). When winds are strong (i.e., exceeding
10 m/s for extensive periods during a period of 24 h), the oil becomes mixed into
the upper ocean and slicks do not form (Simecek-Beatty and Pichel 2006; Simecek-Beatty and Clemente-Colón 2004). Differentiating between oil spills and other
naturally occurring slicks requires a trained analyst and extensive knowledge of
local ocean and atmospheric conditions. Fully automated detection algorithms
using neural networks, fuzzy logic, wavelet transforms, and other trained analysis
techniques have also achieved some success (Garcia-Pineda et al. 2009; Liu et al.
2000, 2010); however, no single-channel, single-polarization algorithm can detect
oil without some false positives. Multi-frequency and multi-polarization analysis
techniques, which exploit changes in polarization of the backscattered signal as a
result of different scattering mechanisms for oil-dampened ocean and normal
wind-roughened ocean, may help to reduce these false alarms (Gade et al. 1998;
Trivero et al. 1998; Mugliaccio et al. 2009; Zhang et al. 2011).
Ocean features Since SAR instruments are very sensitive to any ocean phenomenon that alters capillary and small gravity waves at the scale of the SAR
wavelength (i.e., about 5 cm for C-band), many ocean features appear on SAR
imagery. These include:
• Currents. Since wave propagation and height are affected by horizontal shears in
ocean currents, phenomena like strong currents, eddies, and river plumes show
up clearly in SAR imagery, often as narrow bright features marking their
boundaries (Zheng et al. 2004). Natural surfactants can also be swept along by
13 Radar Applications
353