Flexural Waves in an Anisotropic Hard Formation Borehole

To investigate the propagation of flexural waves in a borehole surrounded by an azimuthally anisotropic hard formation, we made an ultrasonic borehole model of Delabole slate with very strong anisotropy. The axial and azimuthal acoustic fields generated by a dipole source were measured in the fluid-filled borehole. The results show that there are three dominant wave modes: fast and slow flexural waves and one we call a "flexural Stoneley wave. The phase velocities of the fast and slow flexural waves are lower than and close to those of the fast and slow shear waves of the formation, respectively. The phase velocity of the "flexural Stoneley" wave is higher than that of the borehole fluid. Regardless of the polarization of the dipole source, the particle motions of the fast and slow flexural waves are linear and in the direction of the fast and slow shear waves, respectively. The "flexural Stoneley" wave, similar to the normal Stoneley wave generated by a monopole source, is a tube wave with low frequency and high amplitude. Its main particle motion is in the horizontal plane perpendicular to the borehole axis. If the formation is azimuthally anisotropic, its particle motion is linear only when the source polarization is in the same direction as the fast and slow shear waves, and is elliptic everywhere else. Dipole acoustic well logging could be an effective and potential means for determining the anisotropy of a formation.