Seismic anisotropic inversion of multiple walkaway VSPs

Inversion formulae for two orthogonal walkaway VSPs are derived from the general weak anisotropic (WA) inversion formulae. Nine independent WA parameters can be determined if only qP wave is used. These nine WA parameters completely describe the anisotropic property of the medium at or near the receivers in the two profiles. The vertical and horizontal components of slowness vector are obtained by using the cubic spline smoothing function on the condition of least squares. If the medium is laterally inhomogeneous, the horizontal component of the slowness vector can be taken as a constrain condition. A method for calculating mass center is introduced to obtain polarization vector, which is simple, stable and no singular problem. Coordinate transform and least squares method are combined to get the WA parameters for orthorhombic medium and laterally homogeneous isotropic medium with tilted vertical symmetry axis (TTI) which are mostly close to the general anisotropic medium. Two walkaway VSP data from Java Sea are inverted and nine WA parameters at each receiver are obtained. Inversion results from theoretical and real data indicate that the method can be used to determine the anisotropic properties at each receiver in the walkaway VSP measurements. The method is often taken as the most direct and reliable one in the determination of anisotropic parameters in exploration seismology.