Orthorhombic models with a horizontal symmetry plane adequately describe seismic signatures recorded over many naturally fractured reservoirs. The inversion of wide-azimuth traveltimes of PP and SS (the fast and slow ) reflections are discussed for Tsvankin's anisotropic parameters and the azimuths of the vertical symmetry planes of orthorhombic media. If shear waves are not excited, SS traveltimes can be found from PP and PS (converted-wave) data, which makes the method applicable to offshore surveys.
The feasibility of parameter estimation is strongly dependent on reflector dip and orientation. For a horizontal reflector beneath a single orthorhombic layer, the vertical velocities and reflector depth cannot be found from conventional-spread reflection traveltimes alone. If the reflector is dipping, the inversion is ambiguous when the dip plane is close to one of the vertical symmetry planes of the orthorhombic layer above it. The parameter estimation becomes possible if the dip direction deviates by more than 10° from the nearest symmetry plane.
We apply multicomponent stacking velocity tomography to perform velocity analysis for stratified orthorhombic models composed of homogeneous layers separated by plane or curved interfaces. The tomographic algorithm, which operates with the normal-moveout (NMO) ellipses, zero-offset traveltimes, and reflection time slopes of PP- and SS-waves, is designed to build the orthorhombic velocity model in the depth domain by estimating the anisotropic parameters and the shapes of the reflecting interfaces. Numerical tests show that for layered orthorhombic media, it is necessary to put constraints on the vertical velocities to avoid instability in the inversion of noise-contaminated reflection data.