Polarization of Flexural Waves in an Anisotropic Borehole Model

Two modes of flexural waves can be generated by a dipole source in an anisotropic borehole. Their velocities are related to those of the fast and slow shear waves in the formation. The particle motions and the polarization diagrams of the fast and slow flexural waves are measured in borehole models made of phenolite materials with transverse isotropy or orthorhombic anisotropy. The experimental results show that the particle motion of the fast flexural wave is linear and in the same direction as that of the fast shear wave in the formation. The polarization direction of the fast flexural wave coincides with that of the fast shear wave and is independent ofthe direction of the dipole source. The particle motion of the slow flexural wave is nonlinear and elliptic. Its polarization direction and variation are dependent on the anisotropic material and the source direction. This means that the slow flexural wave is a more complicated wave mode rather than the simple mode where the particle motion generated by a dipole source is in the direction of the slow shear wave. The polarization characteristics of the fast flexural wave can be applied to determine the principal axis of an anisotropic formation by in-line and cross-line logging data. INTRODUCTION The rapid development of the dipole direct shear wave logging benefits from the fact that it can be applied not only in an open borehole, but also in a cased borehole surrounded by either a hard or a soft formation (Schmitt, 1989; Chen and Eriksen, 1991). It is a very useful means to explore the azimuthal anisotropy of a formation due to the polarization of dipole source and the receiver in the plane perpendicular to the borehole axis (Leslie and Randall, 1992). 3-1