Back-Analysis of Submarine Slumping in the Gabon Continental Slope: A Pseudo-3D Approach

摘要

Gabon continental slope is selected as a case study for backanalysisrnof slope instability because of evidence of previousrnslide activity. Different types of data were collected from thernGabon continental slope in the Gulf of Guinea off of WestrnAfrica during Guiness1 and ZaiAngo surveys (joint projectsrnbetween IFREMER and ELF-EP). They include swathrnbathymetry and associated imagery, deep towed highresolutionrnsub-bottom profiles, side-scan sonar images andrnKullenberg cores. These data reveal different examples ofrnseafloor instabilities often related to fluid escape features suchrnas cold seeps, pockmarks, mud volcanoes, carbonaternconcretions, etc. Fluid flow in slope sediments plays anrnimportant role in sedimentary processes. The heterogeneity ofrnthe hydro-mechanical characteristics of the sediment canrngreatly modify slope flow fields, effective-stress fields, andrnslope stability. Consequently, it was appropriate to evaluaternthe stability as a spatially varying quantity. The extensivernsubmarine zone (20 kmx15 km) makes it impossible to carryrnout a real 3D slope stability analysis. Therefore, the GeneralrnFormulation2, which fully satisfies equilibrium conditions, isrnused to evaluate the safety factor of the marine slope in twodimensionalrnvertical cross-sections and it is interpolated to thernthree-dimensional area.rnThree scenarios of instability were tested in order tornidentify the possible trigger mechanism of the observed sliderninstability 1) under static gravity loading, 2) under earthquakesrnand 3) under upward fluid flow. Simulation results show thatrnstatic stability of the area is satisfactory. On the other hand,rnthe stability is very sensitive to fluid escape. These resultsrnagree with sonar images showing seepage features alignedrnalong the upslope limit of the observed slide.