GIS-based mapping for marine geohazards in seabed fluid leakage areas (Gulf of Cadiz, Spain)
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ORIGINAL RESEARCH PAPER
GIS-based mapping for marine geohazards in seabed fluid leakage areas (Gulf of Cadiz, Spain) Ricardo Leo´n • Luis Somoza
Received: 11 March 2010 / Accepted: 3 May 2011 / Published online: 11 June 2011 Ó Springer Science+Business Media B.V. 2011
Abstract This paper applies, for the first time in offshore deepwater, a method based on geographic information systems for seafloor susceptibility assessment as a first approach to marine geohazard mapping in fluid leakage areas (slope instabilities, gas escapes, seabed collapses, pockmarks, etc.). The assessment was carried out in a known seabed fluid-flow province located on the Iberian margin of the Gulf of Ca´diz, Spain. The method (based on statistical bivariate analysis) creates a susceptibility map that defines the likelihood of occurrence of seafloor features related to fluid flow: crater-like depressions and submarine landslides. It is based on the statistical index (Wi) method (Van Westen in Statistical landslide hazard analysis. ILWIS 2.1 for Windows application guide. ITC Publication, Enschede, pp 73–84, 1997), in which Wi is a function of the cartographic density of seafloor features on ‘‘factor maps’’. The factors selected monitor the seafloor’s capability to store and transfer hydrocarbon gases and gravitational instability triggers: geology-lithology, gas hydrate stability zone thickness (temperature, pressure– water depth and geothermal gradient), occurrence of diapirs, proximity to faults or lineaments, and slope angle of the seafloor. Results show that the occurrence of seafloor features related to fluid flow is highest where the factors ‘‘gas source and storage’’ and ‘‘pathways of fluid escape’’ converge. This means that they are particularly abundant over diapirs in contourite deposits, in the vicinity of faults, and inside theoretical gas hydrate stability fields thinned by warm undercurrents. Furthermore, the submarine landslides located on the Palaeozoic-Toarcian basement are not R. Leo´n (&) L. Somoza IGME, Geological Survey of Spain, Rios Rosas 23, 28003 Madrid, Spain e-mail: [email protected]
related to fluid leakage. This methodology provides helpful information for hazard mitigation in regional selection of potential drill sites, deep-water construction sites or pipeline routes. It is an easily applied and useful tool for taking the first step in risk assessment on a regional scale for vast areas where fluid leakage may be present, the geological model is known, and the geologically hazardous features have already been mapped. Keywords Susceptibility maps GIS Gulf of Cadiz Marine geohazard
Introduction Seepage-related geomorphic seafloor features are direct indicators of hydrocarbon migration that provide a sign of hydrocarbon potential in deep sediments (Heggland 1998). Fluid expulsion may play a role in potential instabilities on slopes (Prior and Coleman 1984; Evans et al. 1996; Yun et al. 1999; Cochonat et al. 2002) and involve a risk for human activities (Sultan et al. 2001; Elverhøi et al. 2002). Seafloor fe
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