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

摘要

I developed a new nonlinear inversion algorithm for estimating velocities from fully stacked reflection data and applied it to a field data set consisting of well logs from ODP Leg 170 and MCS data offshore Costa Rica. Results also include application to pre-stack seismic data responding to the presence of gas hydrates offshore Oregon. Two types of hydrate fabrics are identified. The method is based on a two-step procedure applying least-squares and preconditioned conjugate gradient algorithms. Mathematically, generalized inversion provides an optimum estimate of earth model parameters by minimizing the so-called cost (or misfit) function, which is a function of the data covariance matrix C_D and the a priori model covariance matrix C_M. The developed 2-step procedure solves the nonlinear inverse problems by first determining the two matrices C_D and C _M, which involves mapping the sensitivity of model smoothness and data error to the parameters σ_d and σ_m. The results from the data offshore Costa Rica show that almost every identified reflector of seismic data is very well matched by final synthetic seismograms, confirming that my estimates of velocities are fully consistent with the data.; The improved inversion method is extended to the inversion of pre-stack seismic data, which is applied to estimate seismic velocities of gas hydrate-bearing sediments in the Hydrate Ridge, offshore Oregon. In this experiment, preliminary V_p and V_s profiles obtained from OBS data by interactive velocity analysis are used as a starting model to estimate V_p from streamer data. The results of my inversion and interpretation of the gas hydrate zone reveals: (1) both streamer and OBS data show a strong BSR indicating the presence of gas hydrate above and free gas below; (2) interactive P- and S-wave velocity analysis of OBS data allows us to identify the presence of a P-SV “conversion surface”; (3) inverted velocity profiles show a low-velocity layer existing below the sea floor and above the normal gas hydrate; (4) two types of hydrate fabrics, massive and porous hydrates, were identified in the P-wave velocity profiles; (5) interpretation of the P-wave velocity, acoustic impedance and Poisson's ratio profiles allow mapping of the distribution of gas hydrates; (6) a series of faults in the accretionary complex under the ridge not only offer pathways for methane and fluid ascending from deeper layers but also control the distribution of the porous hydrates with low velocity below the seafloor.