Illumination compensation for the adjoint-state first-arrival traveltime tomography in VTI media

Abstract

The sensitivity kernel of each parameter is angle-dependent in an anisotropic traveltime multiparameter inversion. To analyze the angle illumination in a transversely isotropic media with a vertical symmetry axis (VTI) by the adjoint state traveltime tomography (AST) method, a circular observation system is designed with receivers radially distributed at different propagated angles. Then, the nonuniform angular illumination in the gradient of different parameters in the VTI AST is revealed. The nonuniform angular illumination seriously hinders the simultaneous updates of multiple parameters toward the true solution and decreases the accuracy of the anisotropic multiparameter inversion. To overcome the illumination problem, a preconditioned AST method (PAST) is proposed by utilizing the diagonal elements of the approximate Hessian matrix as the angle illumination compensation for multiparameter gradients in the VTI medium. The PAST method not only compensates for angular illumination but also balances the ray density and eliminates gradient singular values, which improves the inversion accuracy for multiple parameters. The BP synthetic model test shows that the PAST method based on angular illumination compensation can invert the VTI multiparameter models more accurately. Therefore, the PAST inversion model can predict first-arrival traveltime information more accurately. Field data application in the East China Sea indicates that the inverted velocity of the proposed PAST fits well with the logging velocity, and the inverted anisotropic parameter agrees well with the Backus average values. Consequently, the image quality based on the PAST inverted models is slightly improved. It is proven that the proposed PAST method can better realize the near-surface multiparameter inversion of the VTI medium.