Anisotropic Depth Velocity Model Estimation Above the Receiver Array in Walkaway and 3D VSP Data

This paper presents a new method for interval VTI anisotropic parameter estimation from walkaway VSP data. The method utilizes duplex downgoing multiples (multiples with two reflections: from the boundary and free surface) for estimation of a layered depth velocity model above the borehole receiver array. A three-term NMO approximation function is used to estimate NMO velocity and a non-hyperbolic parameter. Assuming that the interval vertical velocity is known from zero-offset VSP data, Dix-type inversion is applied to the three parameters to invert them to interval Thomsen anisotropic parameters ε, δ above the receivers. Modeling results show reasonable accuracy for the interval Thomsen parameter estimates above the VSP receiver array. For 3D or multi-azimuthal walkaway survey, this approach is generalized for a model with horizontal homogeneous orthorhombic layers.