Measurements Of Shear-Wave Azimuthal Anisotropy From Ultrasonic Dipole Data

Abstract

Four methods for analyzing azimuthal anisotropy from dipole logging data are described and attempted in this paper. These techniques are based on the phenomena of flexural wave splitting in anisotropic materials and are analogous to the techniques used for vertical seismic profiling (VSP) data processing. The laboratory measured dipole data obtained with a scaled tool and a scaled borehole drilled in an anisotropic material (phenolite) are employed to simulate the flexural modes propagating in transversely isotropic (TI) formation with symmetry axis perpendicular to the borehole, and to examine and compare these methods. Amplitude and particle motion analyses of the laboratory data demonstrate that, under the conditions of our laboratory measurements and numerical simulation, only the polarization direction of the fast flexural mode is consistent in accordance with the fast principal direction of the anisotropic material. The slower mode, which is much easier to excite and is of much larger amplitude than the fast mode, turns out to be subject to interferences and is complicated; it has not been well-understood. The particle motion of this guided mode is highly elliptical, and its polarization direction always changes irregularly with the source orientations. The first three methods used in VSP data processing-the linear-transform technique, the technique of rotating the data matrix in the time domain, and the technique of rotating the propagator matrix in the frequency domain-do not work well for the case of flexural modes. The fourth method-determining the eigen-direction of a TI material by identifying the the polarization with polar energy spectrum-works best for the data used in this study.