We use residual moveouts measured along continuous full azimuth reflection angle gathers, in order to obtain effective HTI model parameters. The angle gathers are generated through a special angle domain imaging system, for a wide range of reflection angles and full range of phase velocity azimuths. To estimation the effective model parameters, we use the background HTI/VTI layered model, along with the values of reflection angles. We convert the measured residual moveouts (or traveltime errors) into azimuthally dependent NMO velocities. We apply a Fourier transform to convert the NMO velocities into azimuthal wavenumber domain, in order to obtain the effective HTI model parameters: vertical time, vertical velocity, Thomsen parameter delta, and the azimuth of the medium axis of symmetry. The method provides a reliability criterion of the HTI assumption. The effective model used in this approach is defined for a ID model with a set of HTI, VTI, and isotropic layers. The equivalent effective model yields the same NMO velocity as the original layered structure, for any azimuth of the phase velocity. The effective model approximates the kinematics of an HTI/VTI layered structure using only a few parameters. Under the hyperbolic approximation, the proposed effective model is exact.
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