ANISOTROPIC TRAVELTIME TOMOGRAPHY OF DIFFRACTION ARRIVALS BASED ON EIKONAL EQUATION

摘要

Seismic diffractions provide wide angular illumination of the subsurface and, therefore, can supplement reflections in estimation of the parameters of anisotropic media. Migration velocity analysis of reflection data is usually performed by minimizing residual moveout in common-image gathers. This approach, however, cannot be directly applied to diffractions. Here, we propose to use the linearized eikonal equation to carry out traveltime tomography of diffraction arrivals in VTI (transversely isotropic with a vertical symmetry axis) media. The eikonal equation makes it possible to compute diffraction traveltimes along with their derivatives with respect to the medium parameters. To solve the linearized eikonal equation for VTI media, we employ an efficient and robust second-order finite-difference (FD) methodology based on the Fast Marching method. The accuracy of the developed technique is verified by computing the traveltime perturbations caused by Gaussian parameter anomalies embedded in a homogeneous VTI background. Another test of the modeling methodology involves perturbing the parameters of the structurally complex VTI Marmousi model. Then we perform traveltime tomography of transmission data generated for a VTI medium with Gaussian anomalies in the P-wave normal-moveout (V-nmo) and horizontal (V-hor) velocities. Finally, the tomographic algorithm is applied to diffraction traveltimes from scatterers embedded in the VTI Marmousi model. We use structure-oriented smoothing filters to condition the inversion gradients, which yields more geologically consistent velocity models. To evaluate the stability of the algorithm, this test is repeated using noise-contaminated traveltimes.