ABSTRACT

Gabon continental slope is selected as a case study for backanalysis of slope instability because of evidence of previous slide activity. Different types of data were collected from the Gabon continental slope in the Gulf of Guinea off of West Africa during Guiness1 and ZaiAngo surveys (joint projects between IFREMER and ELF-EP). They include swath bathymetry and associated imagery, deep towed highresolution sub-bottom profiles, side-scan sonar images and Kullenberg cores. These data reveal different examples of seafloor instabilities often related to fluid escape features such as cold seeps, pockmarks, mud volcanoes, carbonate concretions, etc. Fluid flow in slope sediments plays an important role in sedimentary processes. The heterogeneity of the hydro-mechanical characteristics of the sediment can greatly modify slope flow fields, effective-stress fields, and slope stability. Consequently, it was appropriate to evaluate the stability as a spatially varying quantity. The extensive submarine zone (20 kmx15 km) makes it impossible to carry out a real 3D slope stability analysis. Therefore, the General Formulation2, which fully satisfies equilibrium conditions, is used to evaluate the safety factor of the marine slope in twodimensional vertical cross-sections and it is interpolated to the three-dimensional area.

Three scenarios of instability were tested in order 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. On the other hand, 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.

Introduction

In the last 3 decades, a special attention has been paid to submarine slope instability. Industrial and scientific concerns have instigated numerous studies regarding design of marine constructions, offshore structures and pipelines. Data are now available on instability of submarine slopes in different environments ranging from shallow-water and near-shore areas to continental slopes and deep ocean. Most often, slumps involve a large volume of material as opposed to on-shore landslides (Magdalena River delta 3 108 m3, Suva, Fiji 1.5 108 m3, Valdez, Alaska 7.5 107 m3, Orkdals Fjord 107 m3, Sagami Wan 7 1010 m3)3, (Storrega 5.6 1012 m3)4. Submarine slides can take place on very gentle slopes, even less than one degree. For example, the slump of the Mississippi River delta occurred on a slope with an angle of only 0.5°. Unfortunately, while special attention has been focused on the description of new sites of submarine slides5 through 11, little attention has been paid to the comprehension of the processes at the origin of slope instabilities.

In this paper a new methodology developed to assess the risk of marine slope instability was used in order to understand the origin of the slide activity observed on the Gabon slope. Based on geophysical data and geotechnical properties, several sediment facies are differentiated and classified

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