Improvement of seismic travel-time inversion methods and application to observed data

Bin Wang, Purdue University

Abstract

This study consists of four research projects which deal with the improvement of seismic travel-time inversion methods and application to observed data. In project (1), KRTSP90 (Kenya Rift International Seismic Project, 1990) data were processed and interpreted using a travel-time inversion method to derive a two-dimensional crustal and upper-most mantle seismic velocity model across the East African rift in Kenya. In project (2), two inversion techniques (a) the regularization method and (b) stochastic inversion were compared. By comparing these two techniques, we have developed a new understanding of smoothing and damping from stochastic view point. Taking a stochastic view of damping and smoothing aids in selecting damping and smoothing factors to derive a more reliable model using inversion. In project (3), we demonstrate a problem associated with ray-based travel-time inversion when the conventional damped-least-squares method is used. The problem results from the non-uniform distribution of rays in the subsurface. With refraction data, and particularly when the surface source and receiver locations are non-uniformly distributed, the problem is even more severe. Two new approaches are proposed to improve the inversion results. One approach uses variable damping plus smoothing. The other is based on variable grid spacing. These improved inversion methods were tested using synthetic data and applied to the KRISP90 shallow data to demonstrate the utility of the new approaches. In project (4), an inversion scheme which simultaneously inverts normal-incidence reflection and refraction/wide-angle reflection data was tested and applied to the SAGE 90-91 seismic data. Stacked seismic reflection record sections provide high resolution images of complex structure as a function of two-way travel-time. Coincident refraction data provide better velocity information but less control on the configuration of reflecting interfaces. Simultaneous inversion takes advantage of both data sets and results in a more geologically meaningful model. The method was tested using synthetic data and applied to SAGE 90-91 data to derive a velocity model describing the western boundary of the Rio Grande rift at Abiquiu, New Mexico. The method allowed imaging of the complex fault structure and laterally heterogeneous velocities present in the sedimentary section associated with the rift boundary.

Degree

Ph.D.

Advisors

Braile, Purdue University.

Subject Area

Geophysics

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