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Chengshu Wang - Graduate Student Talk, Spring 2003

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Velocity estimation from seismic data
by nonlinear inversion and characterization of
gas hydrate deposits offshore Oregon

By Chengshu Wang, Ewing-Worzel Fellowship.

Abstract:
The aim of generalized inversion of seismic data is to estimate values of the elastic parameters such as P-wave velocity, S-wave velocity and density for lithology discrimination and direct detection of hydrocarbon. The inverted data permit characterization of a gas hydrate deposits, including identification of a low-velocity (gas-saturated) zone in the sediments above the gas hydrate.

Mathematically, generalized inversion provides the best estimate of earth model parameters by minimizing the so-called cost (or misfit between observed and computed seismic data) function, which is a function of the data covariance matrix CD and the a priori model covariance matrix CM. Matrices CD and CM (generally approximated by scalars σd and σm) introduce stability and robustness and thus have strong influence on the quality of the final inversion solution. Based on the preconditioned conjugate gradient algorithm, I have developed a 2-step procedure to solve our nonlinear inverse problem by first determining the two matrices:

  • First step - model smoothing: (1) run inversion for several possible values of (σd, σm) with a smooth starting model; (2) the error surface as a function of (σd, σm) is examined and the region of the error surface with very small value can be chosen as optimal (σd, σmd); and (3) any value of (σd, σm) from within that region generally produces a good data fit along with realistic smooth model.
  • Second step - data fitting: (1) use the optimal (σd, σm) to determine matrices CD and CM, and use the resulting model from the first step as the starting model; (2) run inversion with no smoothness constraint to get final model and corresponding data fit. This further improves data fitting and includes realistic high frequency variations in the final model.

I have applied this new approach to estimate seismic velocity profiles from both post-stack and pre-stack seismic data.

Inversion of post-stack seismic data generally yields reflection coefficients or impedance as a function of two way time. In this application, post-stack seismic data and density logs at selected locations along a 2D seismic line are inverted to estimate seismic velocities. The results show that almost every identified reflector of seismic data is very well matched by final synthetic seismograms and the density and inverted velocity profiles allow identification of major stratigraphic boundaries.

In our application, inversion of pre-stack seismic data is applied to estimate seismic velocities of gas hydrate-bearing sediments, offshore Oregon. Gas hydrates are recognized as a target for major future energy reserves, are believed to be a potential source of an important greenhouse gas, and are considered as a possible cause of massive slope destabilization. A simple indicator of gas hydrate is a bottom-simulating reflector (BSR), which marks the transition between hydrate-bearing sediments with high Vp above and the presence of free gas with low Vp below the bottom surface. A 3-D streamer and ocean bottom seismometer (OBS) survey in the Hydrate Ridge, offshore Oregon was carried out to image structures controlling the migration of methane-rich fluid and free gas and to map the gas-hydrate distribution. Preliminary Vp and Vs profiles obtained from OBS data by interactive analysis are used as a starting model to estimate Vp from the streamer data.

The results of my inversion and interpretation study in Hydrate Ridge demonstrate:

  • Both 3-D streamer and OBS data show a strong BSR indicating the presence of gas hydrate above and free gas below.
  • Inverted velocity profiles show a low-velocity layer existing below the sea floor and above the normal gas hydrate, suggesting a new geological model of gas hydrates.
  • Two types of hydrate fabrics - massive and porous hydrates - are identified, and inverted Vp profile shows a thick (40-60 m) porous hydrate.
  • Faults in an accretionary complex offer pathways for methane and fluid ascending from deeper layers, resulting in the formation of the porous hydrates with low velocity.