A Multimethod Approach to Study the Variability of a Natural Hydrocarbon Seep and its Connection to the Surface

Mahdi Razaz, University of Southern Mississippi, School of Ocean Science and Engineering, Stennis Space Center, United States, Daniela Di Iorio, University of Georgia, Department of Marine Sciences, Athens, GA, United States, Binbin Wang, University of Missouri, Department of Civil and Environmental Engineering, Columbia, MO, United States and Samira Daneshgar Asl, University of California Santa Barbara, Geography, Santa Barbara, CA, United States
Natural hydrocarbon seeps occur commonly on the continental shelves throughout the world. In the Gulf of Mexico, the seeps release both liquid and gaseous hydrocarbons, with oily bubbles predominating offshore. Our work offers the first insight into day-to-day variability of bubble dynamics and hydrocarbon release rate at the source of a seep cluster in the GC600 lease block over 153 days. The results, obtained by processing videos recorded by a time-lapse video camera (VTLC), indicate that the emission rates based on a single measurement may contain 65% uncertainty when integrated over the 153 days. The vertical velocities measured with an Acoustic Scintillation Flow Meter (ASFM), at 20 mab for two weeks were 40% smaller than the measured rise velocity at the source. Assuming that the ASFM only measures the dispersed phase of the plume, the results suggest that the bubble-induced water flow contributes up to 40% of the bubble rise velocity close to the seep source. In addition to the direct measurements near the seafloor, migration paths of bubbles were simulated using the Single Bubble Module (SBM) within the suite of Texas A&M Oilspill Calculator (TAMOC) model. It was assumed that all the bubbles were coated with oil so the dissolution rates for the gaseous bubble constituents were modified accordingly. The simulations were performed for two 6-day periods for which the ambient currents throughout the water column were obtained from a nearby National Data Buoy Center station. Surfacing location of bubbles derived from simulations was in excellent agreement with the oil slick delineated from a satellite Synthetic Aperture RADAR (SAR) image. Also, comparing the two technique suggests that only larger bubbles (d > 8 mm at the source) contribute to the formation of a concentrated oil slick origin that was then advected downstream by wind and surface currents. Processing 19 SAR images led to an estimation of hydrocarbon discharge on the seafloor from the entire seep site. While processing the SAR data it is difficult to distinguish the oil slicks induced from geographically-dense sources, as a result, the SAR emission rates was 3.5 times larger than the VTLC observations.