Evaluating gas-generated pore pressure with seismic reflection data in a landslide-prone area: An example from finneidfjord, Norway

On the 20th of June, 1996, a multi-phase landslide that initiated under water and retrogressed onto land ultimately killed four people, destroyed several houses, and undermined a major highway in Finneidfjord, Norway, an area with a known history of landsliding in the Holocene. Geological and environmental conditions inherent to the 1996 slide include excess fluid/gas pressure (particularly in gas-bearing sediment). In this study, we quantify pore pressures within the free gas accumulation at very shallow sub-surface depths using seismic reflection data. The gas front (a few meters below the seabed) produces a strong, polarity-reversed reflection, dramatically attenuating sub-surface reflections. On x-ray images of cores collected from the 5 km ² large gas zone, gas appears as vesicular spots. We use a previously published method incorporating continuous wavelet transforms to quantify attenuation produced by gas-bearing sediment. Taking the output from this method, and knowing or assuming values for other physical parameters, we invert for in situ pressure and equivalent thickness of the free gas layer. We compare our results to pressure data collected from a single piezometer penetrating the gas front. This analysis demonstrates the utility of seismic reflection data in analyzing the dominant parameter in submarine slope stability (i.e. excess pore pressure), which could be useful in assessing geohazards in similar geological environments.