# A Review of Oil Spill Remote Sensing
## 3 Use of Active Sensors for Oil Spill Detection and Mapping
### Radar  
Sea capillary waves reflect microwave (radar) signals, yielding an illuminated image called \"sea clutter\". Oil on the sea attenuates capillary waves, and the sea clutter from radar imagery. Oil is then shown as a \"dark\" spot or a region of sea clutter absence [34]. Many substances also attenuate capillary waves. There are many slick look-alikes, such as fresh water slicks, calm areas, wave shadows behind structures or topographical features, shallow seaweed beds, biogenic oils, and sea-life sperm [3]. In certain areas, slick look-alikes could number in the hundreds. [PERSON] et al. revealed that slick look-alikes in the studied synthetic aperture radar (SAR) imagery were as high as 20% even after extensive processing [35]. Even with these many look-alikes, radar is essential for oil spill remote sensing, as radar is uniquely capable of operating at night and with clouds or fog. There are two configurations of radar: SAR and side-looking airborne radar (SLAR). SLAR is cheaper and uses a horizontal antenna to produce imagery along the flight path. Synthetic aperture  
Figure 6: Operator’s monitor of the SLEAF system, showing sensor parameters and output map display (Environment Canada). radar uses the aircraft forward motion of the aircraft to attain spatial resolution. SAR resolution is not dependent on range, but uses extensive electronic processing to produce high resolution images. SAR has larger range and resolution than SLAR. Comparative testing indicates that SAR has several advantages and is used in all radar satellites [4]. SLAR is mostly used for airborne oil spill remote sensing, as it is cheaper. Studies on oil spill remote sensing indicate that X-band radar provides better data than L- or C-band radar [34]. The lengthy availability of oil during the Deepwater Horizon spill provided several researchers with the opportunity to study radar remote sensing as well as band relationships [36]. Overall, X-band is superior to other bands, but C-band radar and even L-band radar, to a degree, can provide useful oil spill data [34]. The contrast between oil and water is highest in X-band, moderate in C-band, and lowest in L-band [34]. Signal polarizations using vertical (V) and horizontal (H) electromagnetic wave propagation can be used to provide further information [37]. Transmission and reception using the same polarization is denoted by the repetition of the letters, i.e., VV or HH (transmission polarity then reception polarization). Four poles are available: HH, VV, HV, and VH and if all four poles are used, the polarization is called quadrupole. Vertical antenna transmission and reception polarizations (VV) produces better imagery using airborne radar. A new form of polarimetry is compact SAR, in which the system uses a mixed polarization where the transmitter polarization is either circular or orientated at 45 degrees and the receivers are aligned horizontally and vertically [38]. Compact SAR a form of coherent dual-polarity has advantages over single-pole work and has been used on oil spills. Polarimetric SAR yields information to aid in the discrimination between slicks and look-alikes [39; 40]. Sea state limits radar ability to image oil. Low sea states do not create sufficient sea clutter to contrast with the oil that has damped sea clutter. High seas scatter radar to the extent that detection is not possible inside the wave troughs. Minimum wind speeds of 1.5 m/s (\(\sim\)3 knots) are required for detectability, and a maximum wind speed of 6-10 m/s (up to 20 knots) will again hamper radar detection [4]. Figure 7: Radarsat imagery of the Deepwater Horizon spill. The square shows an expanded view of the center of the spill area and the bright dots are ships working to control the spill. Radar also detects dampening agents that appear to be like oil. New means of data processing has attempted to eliminate these look-alikes from imagery and automate the process of slick detection [41; 42]. Overall, radar is a very good sensor for large-area, night-time, and foul weather detection work.