Theoretical dispersion curves for borehole real-valued wave modes in vertically transverse isotropic formations

Abstract

The dispersion curves of real-valued modes in a fluid-filled borehole are widely used in acoustic well logging. The accurate dispersion curves are the precondition of theoretical analysis and inversion process. Generally, these curves can be obtained by solving the conventional dispersion equation for isotropic formations and most vertically transverse isotropy (VTI) formations. However, if the real-valued solutions exist when the radial wavenumbers for the formation quasi-P and quasi-S equals to each other, the existed methods based on the conventional dispersion equation could lead to incorrect results for some VTI formations. Few studies have focused on the influence of these real-valued solutions on dispersion curve extraction. To remove these real-valued solutions, we have proposed a modified dispersion equation and its corresponding solving process. When solving the dispersion equation, the Scholte wave velocity of VTI formation at high frequency is used as the initial guess. The two synthetic examples including fast and slow VTI formations validate that these real-valued solutions do not contribute to the wavefield, and the new dispersion curve extraction method is suitable for all kinds of VTI formations. Consequently, the method can provide reliable dispersion curves for both theoretical analysis and anisotropic parameters inversion in VTI formations.