Relationships Between Oil Seeps and Seabed Morphologies

Abstract

Abstract Owing to the permanence of a great quantity of natural oil seepages and to the variety of structures in the seafloor, the deep offshore of the Angolan margin is one of the most favorable areas in the world allowing a better understanding of the relationships between surface seepages and seabed morphologies. The analysis of radar satellite images at different dates shows that oil seeps form patterns of flower structures. Each petal of an isolated flower structure represents a single seepage, all of them converging toward an impact point at the sea surface that can be associated to an emission point or vent in the seafloor. The rising speed of the oil in a water column of 1800 m is 10 cm/s, which corresponds to a lateral drift of about 1000 m. Active vents in the seafloor correspond to submarine mounds and clustered pockmarks, whereas unit or single pockmarks are not associated to the leaking of oil. Introduction Synthetic aperture radar (SAR) sensors constitute a powerful tool to detect hydrocarbons at the sea surface because of the sensitivity of the microwave signal to the surface roughness (e.g., Williams and Lawrence, 2002; Zatyagalova et al., 2007). Oil slicks dampen the waves at the ocean surface and reduce the intensity of the backscattered radar signal, creating dark patches in the image. The distinction between natural oil seeps and look-alikes such as pollution and natural film slicks may be tricky with the sole use of SAR images. The categorization of oil slicks is merely based on the experience of the photo-interpreter how is able to recognize morphological and/or key features associated with either natural or man-made leakages. The relationships between surface seepages and the emission point on the seabed is not well constrained because of the difficulty in recognizing (i) the precise location of the impact points of the oil at the sea surface, (ii) the distinction between deep-sea active and non-active vents, and (iii) the lateral drifting of the " oil plume" in the water column. We focused our study on deep offshore of the Angolan margin which represents one of the most favorable areas for the study of natural oil seepage. The aims of this study are to map natural oil seeps from a collection of satellite radar images, to estimate the lateral drifting of the oil from the emission point to the surface impact point, and to determine the seabed morphologies related to active oil seepages from the analysis of bathymetric and seismic data.