Movement and Fate of Natural and Unnatural Oil Slicks in the Gulf of Mexico

Oil spills are a frequent occurrence in the Gulf of Mexico (GOM). They occur by two principle processes: natural oil seepage and accidental spills during petroleum extraction, transportation, and consumption. Marine oil spill can be highly dangerous because wind, waves, and currents can scatter an oil spill over a wide area in a few hours. Accurate detection and predicting the fate of oil not only from large spills, but also chronic small-scale emissions lead to a better investigation of the effect of oil on the environment. Remote sensing plays an important role in oil spill response and monitoring by providing the oil slick location and its spatial and temporal distribution. The aim of this dissertation was to use Synthetic Aperture Radar (SAR) images in the GoM as a means to, map the location of the anthropogenic oil slicks reported by National Response Center (NRCen) and quantify their volume, identify chronic oil spill locations, analyze oil slick extent and drift by wind and surface currents, study the fate of Deepwater Horizon (DWH) oil spill. This dissertation consists of studies that are compiled into three manuscripts that are published, accepted for publication or ready for submission. One of the objectives in this research was to examine the feasibility of SAR images in oil slick detection. We used SAR images to obtain more precise estimates of the magnitude of the hydrocarbon discharges reported by NRCen in the GoM. These reports depend largely on unverified reporting from responsible parties and third parties, have not been validated by an independent assessment in terms of location and magnitude, and associated with three categories of source: 1) pipelines, platforms, or other energy industry sources, 2) the former location of the Taylor offshore platform, and 3) undetermined sources. A total of 67 reports were visible in 66 archived SAR images from 2004 to 2012 describing transient events. Of those, oil slicks observed at the Taylor site were generally much larger than those corresponding to other NRCen reports, and indicated a chronic source at this location. These long wind-driven layers of floating oil released from the Taylor site were verified by field sampling, aerial photography, Landsat 7 Enhanced Thematic Mapper Plus (ETM+) 30-m resolution data, and Moderate Resolution Imaging Spectroradiometer (MODIS) Terra and Aqua medium resolution (250-500 m) data. A Texture Classifying Neural Network Algorithm (TCNNA) delineated oil slicks area from SAR images. Comparison of SAR-extracted and NRCen-reported oil slicks areas showed a consistent under reporting by NRCen. Our second objective was to estimate the surface residence-time of the oil slicks and to determine the importance of wind and surface currents on the trajectory and fate of the released oil. Oil slicks released from natural hydrocarbon seeps located in Green Canyon 600 (GC600) lease block in the GoM were analyzed in 41 SAR images. A relatively simple surface oil drift model deriving with wind and surface currents was used to obtain the closest resemblance between the simulated oil pathways and the length and shape of the oil slicks observed in SAR images. The average surface residence-time predicted from the hindcast modeling was 6.4 hr (± 5.7 hr). Respectively, the effect of winds and surface currents on disappearance and stretching of the oil slicks from sea surface were indicated. Results from the numerical experimentation were supported by in situ observations conducted by a wind-powered autonomous surface vehicle (SailDrone). Finally, our third objective was to discuss the fate of remaining oil after the DWH oil spill. The hypothesis of what happened to the surface discharge of DWH oil was tested by surface oil advection model, weathering, and fate data. The inputs of surface oil advection model derived from: 1) The volume distribution of floating oil during the DWH discharge quantified by 166 SAR images, 2) Modeled wind time series from the North American Mesoscale Forecast System (NAM), 3) Ocean currents from the HYbrid Coordinate Ocean Model (HYCOM). Evaporation of volatiles from surface oil was simulated by Oil Spill Contingency And Response (OSCAR) model. Daily magnitude and spatial distribution of aerial dispersant application and burning operations were obtained from publicly available databases. At each time step these weathering and fate data were subtracted from the modeled distribution of oil volume on the water surface. Results were compared to SAR images of DHW oil spill in order to verify the amount of oil which was 1) suspended below the surface or buried through sedimentation, 2) washed ashore, and 4) resurfaced through time.