Characterization of the Slope-destabilizing Effects of Gas-charged Sediment via Seismic Surveys

摘要

Finneidfjord, Norway has a history of submarine slope failure and hosts areas of buried, gas-chargedsediment. Using shipboard seismic survey data from Finneidfjord, we illustrate how novel seismic dataprocessing techniques can yield estimates of geotechnical properties of this gas-charged sediment: gassaturation (concentration) and excess pore pressure as well as their uncertainty. This data processinginvolves 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 waveletbasedspectral ratio approach, and find the seismic quality factor (inversely related to attenuation) fromthe change in amplitude spectra across the gas layer. The novelty of this paper really comes in the secondstep, where we create a Bayesian hierarchical model that combines a wave attenuation model with aspatial random (Gaussian) process model. The advantage of this combination is that our estimates of thegas properties not only must conform to the observations of attenuation (quality factor), but must alsosmoothly vary over space, as one would expect of any natural process. Ultimately, we end up withposterior distributions of gas properties at points throughout our gas-bearing layer, which give usestimates of parameter values and a sense of uncertainty about these estimates. We find good performanceand agreeable values for gas saturation, but the model shows weak sensitivity to pressure, leaving largeuncertainty in any excess pore pressure estimates. Implications for slope stability at Finneidfjord arediscussed.