Characterization of the slope-destabilizing effects of gas-charged sediment via seismic surveys

Morgan, Eugene, Vanneste, Maarten and Vardy, Mark E. (2014) Characterization of the slope-destabilizing effects of gas-charged sediment via seismic surveys. In Proceedings of the Offshore Technology Conference 2014. Offshore Technology Conference. OTC-25196 . (doi:10.4043/25196-MS).

Abstract

Finneidfjord, Norway has a history of submarine slope failure and hosts areas of buried, gas-charged sediment. Using shipboard seismic survey data from Finneidfjord, we illustrate how novel seismic data processing techniques can yield estimates of geotechnical properties of this gas-charged sediment. This data processing involves two steps: 1) estimate the amount of seismic attenuation that a gas-bearing layer creates, and 2) fit a wave attenuation model to the observed attenuation. We accomplish the first step using a wavelet-based spectral ratio approach, and find the seismic quality factor (inversely related to attenuation) from the change in amplitude spectra across the gas layer. The novelty of this paper really comes in the second step, where we create a Bayesian hierarchical model that combines a wave attenuation model with a spatial random (Gaussian) process model. The advantage of this combination is that our estimates of the gas properties not only must conform to the observations of attenuation (quality factor), but must also smoothly vary over space, as one would expect of any natural process. Ultimately, we end up with posterior distributions of gas properties at points throughout our gas-bearing layer. We use these posterior distributions to probabilistically evaluate the role that gas plays in slope failures in Finneidfjord. This method has other applications in natural gas resource exploration and carbon sequestration.

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