TY - GEN
T1 - Characterization of the slope-destabilizing effects of gas-charged sediment via seismic surveys
AU - Morgan, Eugene
AU - Vanneste, Maarten
AU - Vardy, Mark
N1 - Publisher Copyright:
Copyright 2014, Offshore Technology Conference.
PY - 2014
Y1 - 2014
N2 - 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: gas saturation (concentration) and excess pore pressure as well as their uncertainty. 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 waveletbased 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, which give us estimates of parameter values and a sense of uncertainty about these estimates. We find good performance and agreeable values for gas saturation, but the model shows weak sensitivity to pressure, leaving large uncertainty in any excess pore pressure estimates. Implications for slope stability at Finneidfjord are discussed.
AB - 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: gas saturation (concentration) and excess pore pressure as well as their uncertainty. 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 waveletbased 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, which give us estimates of parameter values and a sense of uncertainty about these estimates. We find good performance and agreeable values for gas saturation, but the model shows weak sensitivity to pressure, leaving large uncertainty in any excess pore pressure estimates. Implications for slope stability at Finneidfjord are discussed.
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U2 - 10.4043/25196-ms
DO - 10.4043/25196-ms
M3 - Conference contribution
AN - SCOPUS:84945195853
SN - 9781632663870
T3 - Proceedings of the Annual Offshore Technology Conference
SP - 1296
EP - 1306
BT - Offshore Technology Conference Asia, OTC ASIA 2014
PB - Offshore Technology Conference
T2 - Offshore Technology Conference 2014, OTC 2014
Y2 - 5 May 2014 through 8 May 2014
ER -