Estimation of near-surface velocity and seismic tomographic static corrections

A static correction approach of 3-D tomographic inversion is presented base d on surface-consistent model. This method can be applied to wide-line prof iles,crooked lines. as well as conventional 2-D and 3-D seismic survey geom etries. The accurate estimation of the thickness and velocities in shallow low-velocity zones is vital to proper static correction in rough topography and complicated low-velocity geological structure zones. We use a least-sq uares approach in conjunction with QR decomposition to reconstruct 3-D velo city model from the actual first-break times obtained from 3-D data, and th en calculat concurrently the long and short wavelength components of static correction based on the reconstructed velocity images. We analyze characte ristics of the first-break traveltime in complicated low-velocity layers. T o improve the inversion accuracy of velocity model, the various first-break times from direct,reflection and refraction waves are considered for deter mining the observed traveltimes and the theoretical model calculation. The fractal algorithm is applied to determining the first breaks, which overcom es the error caused by the differences of wavelet shapes and the leg-jump o f refractions. The method is capable of selecting large volume of first bre aks automatically. The ray paths and traveltimes are calculated with a 3-D ray tracer which does not require more CPUs for complicated geological mode ls. This method can determine the ray paths associated with minimum travelt imes regardless of the wave modes. The results indicate that the 3-D tomogr aphic static correction method is more effective for field data.