3D 3-C full-wavefield elastic inversion for estimating anisotropic parameters: A feasibility study with synthetic data

Abstract

Traveltime-based inversions cannot solve for all of the anisotropy parameters for orthorhombic media. Vertical velocities cannot be recovered simultaneously with the dimensionless anisotropy parameters. Also, the density cannot be solved because it does not affect the normal moveout of P and S reflections. These limitations can be overcome using full-wavefield inversion for anisotropy parameters for orthorhombic media and for transversely isotropic media with vertical and horizontal symmetry axes. Tsvankin’s parameters and the orientation of the local (anisotropic) coordinates are inverted from three-component, wide-azimuth data sets containing P reflected and PS converted waves. The inversions are performed in two steps. The first step uses only reflections from the top of an anisotropic layer, whichdoes not constrain the trade-offs between the vertical velocities, the anisotropies, and density, as shown by parameter correlation analysis. The results from the first step are refined by using them as the starting model for the second step, which fits reflections from the top and bottom of the layer. The properties of the target layer influence the amplitudes of top and bottom reflections as well as the traveltime of the bottom reflections; when all these data are used, the inversion is highly overdetermined and all model parameters are estimated accurately. When Gaussian noise is added, the inversion results are very similar to those for the noise-free data because only the coherent signal is fitted. The residual at convergence for the noisy data corresponds to the noise level. Concurrent inversion of data from multiple sources increases the azimuthal illumination of a target.