Propagation of Flexural Waves in an Azimuthally Anisotropic Borehole Model

Flexural waves generated by a dipole source have been studied theoretically and used to estimate the shear parameters of a formation. The basic principles and main properties of flexural waves propagating in a borehole are reviewed in this paper. A mono/dipole transducer made of a PZT piezoelectric tube is used for laboratory experiments in borehole models. The radiation pattern of the dipole source is measured in a water tank. In order to simulate the hard and soft formations, measurements are performed in borehole models made of aluminum and IUcite, respectively. Experimental results are in good agreement with the theoretical dispersion characteristics. Measurements are also performed with the transducers in an azimuthally anisotropic borehole model made ofPhenolite XX-324. Both fast and slow flexural waves with different velocities are generated by a dipole source in the model. The flexural waves are related to the fast or slow shear waves in the anisotropic material. Experimental results show that the flexural wave splits into a fast and a slow component in an azimuthally anisotropic borehole; therefore, dipole acoustic well logging could be an effective means for estimating a formation's anisotropy.