Abstract

A 3-D tomographic inversion approach based on a surface-consistent model for static corrections is presented in this paper. Direct, reflected, and refracted waves are used simultaneously to update the near-surface model. We analyze the characteristics of the first-break traveltime in complicated low-velocity layers. To improve the accuracy for the velocity model, the various first-break times from direct, reflected, and refracted waves are considered for model inversion. A fractal algorithm which overcomes the error caused by wavelet shape differences is applied to pick first breaks. It also overcomes the leg jump of refractions. The method can pick a large number of first breaks automatically. The raypaths and traveltimes are calculated with a 3-D ray tracer that does not increase computation time for complicated geological models. Our method can determine the raypath associated with minimum traveltimes regardless of wave mode (direct, refracted, or reflected). We use a least-squares approach in conjunction with a matrix decomposition to reconstruct a 3-D velocity model from the actual first-break times obtained from 3-D data. Finally, long- and short-wavelength static corrections are calculated concurrently, based on the reconstructed velocity profile. The method can be applied to wide-line profiles, crooked lines, and 2-D and 3-D seismic survey geometries. The results applied to a real 3-D data example indicate that the 3-D tomographic static corrections are effective for field data.

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