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1 Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Laboratoire Environnements Sédimentaires, Département Géosciences Marines, P.O. Box 70, 29280 Plouzané, France; Nabil.Sultan{at}ifremer.fr
2 Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Head Office, 155, rue Jean Jacques Rousseau, 912130 Issy-les-Moulineaux, France; Pierre.Cochonat{at}ifremer.fr
3 Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Laboratoire Environnements Sédimentaires, Département Géosciences Marines, P.O. Box 70, 29280 Plouzané, France; Florence.Cayocca{at}ifremer.fr
4 Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER), Laboratoire Environnements Sédimentaires, Département Géosciences Marines, P.O. Box 70, 29280 Plouzané, France; Jean.Francois.Bourillet{at}ifremer.fr
5 Total EP, Avenue Larribau, 64018 Pau Cedex, France; Jean-Louis.COLLIAT-DANGUS{at}total.com
Nabil Sultan received his Ph.D. in geotechnics from the Ecole Nationale des Ponts et Chaussées, Paris, France. He joined Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER) in 2000. His main interests are laboratory and field geotechnical tests, mathematical and constitutive models in geomechanics, and slope stability and flow in porous media.Pierre Cochonat received his Ph.D. in applied geology from the Université Scientifique et Médicale de Grenoble, France. He has 26 years experience in engineering geology and sedimentology. His main field of scientific interest has been the study of sedimentary processes on slope and deep-sea sediments. He is now deputy director of Ocean Research and manager of the Continental Margin Programme at IFREMER.
Florence Cayocca graduated as a civil engineer from Ecole Nationale des Ponts et Chaussées, Paris, France. After a master's degree in ocean engineering at the University of California, Santa Barbara, she completed her Ph.D. in morphodynamic modeling at IFREMER. Since then, she has worked there on sediment-transport numerical modeling on the slope and in coastal seas.
Jean-François Bourillet graduated in 1982 from the Ecole Nationale Supérieure de Géologie de Nancy, France. He joined IFREMER in 1982 for submarine mining exploration. His main interests are coring operations, bathymetry processing, slope stability, and sedimentology of the Gulf of Biscay (western Europe).
Jean-Louis Colliat received his Ph.D. in geotechnics from the University of Grenoble, France, in 1986. He joined Geodia in 1986, then Elf in 1991, becoming Total in 2000. His main interests are offshore geotechnical engineering, from soil investigation, design studies, and offshore installation works.
The Gabon continental slope is selected as a case study for slope-stability analysis because of evidence of previous slide activities. Different types of data were collected from the continental slope in the Gulf of Guinea off west Africa during Guiness and ZaiAngo surveys. The offshore investigation was carried out using swath bathymetry and associated imagery, deep-towed high-resolution subbottom profiles, side-scan sonar images, observation from remotely operated vehicle Victor, and Kullenberg cores. These data reveal different examples of seafloor instabilities commonly related to fluid-escape features. These slides occur on the continental slope at low declivities, showing that slope gradient has a secondary role on the marine slope instability with respect to external triggering mechanisms such as fluid flow, earthquake, shallow gas, and gas hydrates. One case of mass slide with small downslope displacement was studied on the Gabon slope.
In this work, a pseudo–three-dimensional slope-stability analysis (Sultan et al., 2001) was undertaken. Three scenarios of instability were tested to identify the possible trigger mechanism of the observed slide instability: (1) under static gravity loading, (2) under earthquakes, and (3) under upward fluid flow. Simulation results show that static stability of the area is satisfactory. However, the stability is very sensitive to fluid escape. These results agree with sonar images showing seepage features aligned along the upslope limit of the observed slide.
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