Abstract
Optical remote sensing (ORS) of reflected sun light has been used to assess oil spills in the ocean for several decades. While most applications are toward simple presence/absence detections based on the spatial contrast between oiled water and oil-free water, recent advances indicate the possibility of classifying oil types and quantifying oil volumes based on their spectral contrasts with oil-free water. However, a review of the current literature suggests that there is still confusion on whether this is possible and, if so, how. Here, based on the recent findings from numerical models, laboratory measurements, and applications to satellite or airborne imagery, we attempt to clarify this situation by summarizing (1) the optics behind oil spill remote sensing, and in turn, (2) how to interpret optical remote sensing imagery based on optical principles. In the end, we discuss the existing limitations and challenges as well as pathways forward to advance ORS of oil spills.
Highlights
Remote sensing through airborne or satellite platforms provides rapid and synoptic measurements of the targets, has been used for decades to assess oil spills in the ocean, from near real-time mapping to postspill assessment
Summary and Conclusion In Optical remote sensing (ORS) imagery, oil-containing pixels can be detected and characterized for two reasons, both due to oil’s optical properties: oil has a higher refraction index than water, can change surface Fresnel reflectance from the oil-modulated surface roughness, and oil has much higher absorption coefficient than water and higher scattering coefficient than water when oil is mixed with water to form emulsions
While the detection of presence/absence is not unique to ORS images as synthetic aperture radar (SAR) can do an good or even better job, characterizing oil type and quantifying oil concentration or thickness make ORS advantageous over other technique on oil spill assessment. This is especially true when considering that many multiband ORS sensors at various spatial resolutions and revisit frequencies are currently in orbit with data freely available
Summary
Remote sensing through airborne or satellite platforms provides rapid and synoptic measurements of the targets, has been used for decades to assess oil spills in the ocean, from near real-time mapping to postspill assessment (see reviews by [1,2,3,4,5]). Unless some a priori knowledge is available, this often makes it difficult to distinguish surface oil from other features that can make similar spatial contrasts in SAR imagery (e.g., phytoplankton surfactant, freshwater slicks, current shears, floating algae, and among others), let alone classifying oil types or quantifying oil volume despite some preliminary attempts [6, 7] Another popular technique is optical remote sensing (ORS) of reflected sun light in the visible-near infraredshortwave infrared (vis-NIR-SWIR) wavelengths, which is the subject of this paper. Passive remote sensing using ultraviolet sun light and active remote sensing using laser-induced fluorescence have been shown useful, because most satellite sensors are not equipped with such a capacity, they are not discussed here
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