# A Review of Oil Spill Remote Sensing
## 2 Detection and Mapping of Oil on a Water Surface
### Use of the Visible Spectrum  
The visible portion of the electromagnetic spectrum ranges from 400 to 700 nm. Overall, oil displays a moderately larger reflectance than water, but does not show specific absorption or reflection tendencies. Thin oil layers or sheen appear silvery to the human eye and reflect light over a wide spectral range--as far as the blue. Thick oil layers appear to be the same color as bulk oil, typically brown or black. Overall, oil has no specific spectral information that differ from the water upon which the oil floats [4]. As a consequence, processes that inspect particular spectral regions do not enhance discrimination [4]. One method of using visible spectra is to use a push-broom scanner that uses a CCD detector and an optical system to direct ground elements to different parts of the CCD detector [5]. The signal resulting from such a CCD scanner can then be processed or enhanced to yield the desired data.  
Oil on water has a polarizing effect on light, so viewing oiled water with polarized lenses can improve contrast and thus oil detection [6; 7]. Light reflected from a water surface reflects at 53 degrees (Brewster angle), so one can improve contrast by setting the detector at this Brewster angle (53 degrees from the vertical) [8]. This light directly reflected from the surface contains the most information on surface oil. This technique may increase contrast as much as 100% [8].  
Three major obstacles interfere with the use of visible light, darkness, cloud cover, and sun glitter. Sun glitter, which is often confused for oil sheens, is problematic in visible remote sensing. Sun glitter can be reduced through signal processing techniques [9; 10].  
Video cameras are used extensively for oil spill monitoring. Modifications to standard cameras have improved images somewhat, however have not vastly improved oil detection capability. Light-enhanced video cameras function in complete darkness. Night-vision camera tests showed that light-enhancement can provide good imagery in total darkness [11].  
Hyperspectral imaging is a technique of using a spectrometer to collect many images of the same target at different wavelengths [3]. Hyperspectral images involve extensive data and require highly advanced processing to produce an image, so they are not useful for real-time monitoring. A technique called spectral un-mixing is commonly used for characterizing individual pixels. This technique becomes important because the spatial resolution of hyperspectral sensors may be as much as several meters and different types of target materials may be recorded in each measured pixel. Hyperspectralsensing is being increasingly employed for general environmental purposes, but application to oil spills remains low [3].  
In summary, the visible spectrum remains a research area as well as an economical method for monitoring oil spills, particularly on countermeasures support operations. Figures 1 and 2 show examples of a visible image from the Deepwater Horizon oil spill (USA Gulf of Mexico).  
Figure 1: An example image from MODIS during the DWH (Deepwater Horizon) spill in the U.S. Gulf of Mexico. The MODIS sensor is on the NASA Aqua satellite. The oil slick is shown by the white rectangle. Note the many clouds in the image. (Photo from NASA website, www.nasa.gov).  
Figure 2: A visible image of the DWH spill. The image is from the NASA Terra satellite and was taken on 24 May 2010. Clouds do not obscure the main part of this image but are present around the periphery (photo from [[https://en.wikipedia.org/wiki/File:Deepwater_Horizon_oil_spill](https://en.wikipedia.org/wiki/File:Deepwater_Horizon_oil_spill)]([https://en.wikipedia.org/wiki/File:Deepwater_Horizon_oil_spill](https://en.wikipedia.org/wiki/File:Deepwater_Horizon_oil_spill))).