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Shallow Subsurface Detection of Buried, Weathered Hydrocarbons Using Integrated Geophysical Techniques


Weathered hydrocarbons, commonly emulsified or in the form of tar balls, wash ashore along beaches due to natural oil seepages or offshore oil spills. They remain buried in the sand until a hurricane or storm exposes them; therefore it is important to understand the progression and extent of these hydrocarbons. Elmer's Island, Louisiana, a site known for having large amounts of oil washed ashore from the 2010 Deepwater Horizon oil spill, was selected for a geophysical survey to detect the presence of buried, weathered hydrocarbons. Ground penetrating radar (GPR) surveys, using 200 MHz and 400 MHz antennae, were conducted at the site along several traverses. The 400 MHz data show two distinct anomalous zones with strong positive amplitudes. An electromagnetic (EM) profiler, operated along the same lines, displays rows of oval shaped anomalies in conductivity contour maps. The two anomalous zones observed in the GPR data correspond with anomalies displayed on these maps. A large region of low conductivity is persistent throughout all EM contour maps. This region also depicts a higher density of anomalies in 400 MHz radargrams. Because oil sands have low conductivities, a mixture comprised of weathered hydrocarbons, sand and brackish water would result in a lower conductive region than areas with non-contaminated sand and brackish water. Therefore, this region obviously contains a contaminant in the sand. Field observations confirmed the existence of contaminated sand buried at the survey site in distinct layers. This contaminated sand enclosed small aggregates of weathered hydrocarbons, which may be associated to the oval shape of anomalies. To test whether these shallow subsurface features can be mapped by airborne remote sensing, NASA's Uninhabited Aerial Vehicle Synthetic Aperture Radar imagery will be processed to detect shallow subsurface features. These images and the data collected at the survey site will be correlated to map the extent of buried hydrocarbons along shorelines and to understand the movement of weathered hydrocarbons in these environments. By correlating this data with aerial imagery, maps can be created which will aid clean-up efforts for future oil spills.