International Geologiical Congress - Oslo 2008


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OSP-03 Ocean margin and ocean island sediment mass movements and their consequences: Where? When? Why? - Part 2


Using seismic reflection data to investigate free gas in a landslide area: An example from Finneidfjord, Norway


Eugene Morgan, Tufts University (United States)
Maarten Vanneste, NGI/ICG (Norway)
Oddvar Longva, NGU/ICG (Norway)
Isabelle Lecomte, NORSAR/ICG (Norway)
Laurie Baise, Tufts University (United States)
Brian McAdoo, Vassar College (United States)


On the 20th of June, 1996, approximately 1 million cubic meters of soil failed retrogressively in Finneidfjord, Norway. The location provides numereous geological and environmental conditions relevant for offshore geohazards and large-scale submarine landslides. Examples are slide-prone layers, lateral and vertical lithological variability, the presence of slide blocks, gas-containing sediments, and migration pathways, which could well indicate excess pore pressure.The multi-phase, retrogressive landslide initiated underwater, and, within a few hours, undermined a highway and several houses, claiming the lives of 4 people.
Whereas several factors likely contributed to this slope failure, we focus on the role that free gas played in destabilizing the slope. In a core from near the slide area, gas bubbles appear in the X-ray as vesicular spots. High-resolution 2D seismic profiles reveal the presence of a pronounced gas front, topped by a distinct, polarity-reversed reflection that can be traced over an area of ∼5 km2. The top of the gassy zone lies a few meters below the seabed, and close to the suspected failure plane. After processing the seismic data to correct for spherical divergence and to remove multiples, the gas reflection can be characterized in more detail by its amplitude, phase and frequency content. Continuous wavelet and Gabor transformations determine values for these two parameters with minimal user input. We attempt to convert the lateral variations in amplitude and frequency content along the gas reflection to infer the degree of gas saturation and investigate whether or not it can be used to estimate pore pressure in the submarine sediments. Interpolation of the network of single-channel seismic data estimates a 3-D model of the present-day gas front beneath the fjord bed. Coupling this with information about the seabed sediments may indicate how free gas weakens submarine slopes and thus facilitates slope failure.


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