Mapping Oil-Water Emulsions from the Deepwater Horizon Oil Spill Using Imaging Spectroscopy
Abstract
The oil spill resulting from the April 20, 2010 Deepwater Horizon accident provided an opportunity to test the effectiveness of using UV-NIR imaging spectroscopy for measuring the oil-to-water ratio, sub-pixel areal fraction, thickness, and volume of widespread emulsion slicks (Clark et al., 2010). The NASA/JPL Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) was used to measure the reflectance of the ocean over hundreds of km2 where it was covered by thin oil sheens (~0.2 to 6 microns) and thicker water-in-oil emulsions (~1 to 20 mm). Water-in-oil emulsions (hereafter referred to as "emulsions") have a strong UV absorption that imparts concentration-dependent colors in the visible, but are surprisingly bright (up to 60%) in reflectance between 1 and 1.3 microns, with diagnostic, sometimes overlapping C-H and H2O absorptions at 0.9, 1.2, 1.4, 1.75, 2, and 2.3 microns. An 80 km transect of the spill by boat from the Mississippi delta out to the incident site provided emulsion samples that were then used to create a series of emulsions that spanned nearly the entire oil:water ratio range by addition of seawater or evaporation of contained water. The diagnostic spectral features of this series, measured over a range of thicknesses, were used to map emulsion signatures in AVIRIS data collected during a relatively calm, nearly cloud-free day on May 17, 2010. This procedure allowed calculation of oil in emulsions on a per pixel basis of 19,000 to 34,000 barrels of oil in the AVIRIS scenes. Based on laboratory measurements, NIR photons only penetrate a few mm into water-in-oil emulsions, thus the volume of oil derived using this method is a minimum estimate. Sheens were too thin to exhibit diagnostic vibrational absorptions, and methods for estimating their volume with AVIRIS data are under development. AVIRIS only covered about 30% of the core spill area composed of emulsions and sheens. Thus, extrapolation of AVIRIS-derived emulsion coverage to the entire core spill area defined on a MODIS image collected the same day indicates a minimum of 66,000 to 120, 000 barrels of thick oil floating on the ocean surface that day. This estimate does not include oil in sheens, oil under the surface, oil washed onto land, or oil burned, evaporated, or biodegraded as of May 17. Also, because of the limited penetration of light into emulsions, and based on field observations that emulsions sometimes exceed 20 mm in thickness, we estimate that the oil volume, including oil thicker than can be probed with AVIRIS imagery, is possibly as high as 150,000 barrels in the AVIRIS scenes. Extrapolation of this value to the entire core spill gives a possible volume of 500,000 barrels for thick oil remaining on the ocean surface as of May 17. Clark et al., 2010, A method for quantitative mapping of thick oil spills using imaging spectroscopy, U.S. Geological Survey Open File Report 2010-1167, 51p. http://pubs.usgs.gov/of/2010/1167/
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMOS41D..08S
- Keywords:
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- 4251 OCEANOGRAPHY: GENERAL / Marine pollution;
- 4275 OCEANOGRAPHY: GENERAL / Remote sensing and electromagnetic processes