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Oil Seep Hunting From Space: The Case of the Lower Congo Basin


We focus on the Lower Congo Basin (LCB) recognized for the abundance of natural oil slicks in order to establish relationships between natural oil seeps observed at the sea surface and seabed features responsible for active oil leakage. From a multi-temporal stack of SAR images, we show that oil leakage is intermittent with temporal occurrences from 5 to 80% depending on the sites. Oil slicks residence time can be low as seepage signal may be lost between two consecutive SAR scenes separated by 12 hours. The integration of in-situ mooring measurements allowed reconstructing the pathway of the upwelling oil plumes in the water column from their emitting point at a depth of ~1800m. The horizontal deflection of oil droplets falls within a circle of 1500 m radius around the active vent, with an ascent rate of oil coated bubbles between 3 and 6 cm/s. Thanks to those horizontal deflection assessments, SAR observations could be combined with bathymetric data allowing to investigate the morphological characteristics of the oil-releasing seabed features. This seafloor analysis reveals that active thermogenic seeps correspond to complex-shaped and/or clustered pockmarks or mounds. At contrary, funnel-shaped and lined-up pockmarks along syncline axes do not display any oil releases. High resolution sub-bottom profiler and bathymetry with 3D seismic data well document the geometry of a 2 km-long complex oil escape feature formed by a series of about 40 irregular and clustered pockmarks. Hydrocarbon seeps are usually associated with high impedance patches close to the seafloor. Numerous meter-size asphalt mounds are scattered up to 1000 m around the active hydrocarbon vents and several geophysical anomalies (high amplitude bodies, BSR's, …) characterize those structures in depth. Even if the study focuses on the LCB and that those results cannot be simply transposed in other parts of the world, the potential of hydrocarbon seeps recognition from SAR imagery hereby provides some recommendation to increase the success rate of thermogenic fluids to sample. Best oil seep candidates seem to correspond to complex-shaped pockmarks clusters and submarine mounds that have been found below the origin of recurrent oil slicks.