Estimating Shear-Wave Transverse Isotropy of Fast Formation From Borehole Flexural-Wave Measurements

In acoustic logging through a vertical borehole, the shear-wave transversely isotropic (TI) property has traditionally been determined from borehole monopole Stoneley-wave measurements. We have developed a methodology to determine the TI parameters from borehole dipole-flexural wave data. Our analysis shows that the Stoneley wave is sensitive to the TI property mainly in an acoustically slow formation, and the sensitivity diminishes when the formation become faster. The advantage of the flexural wave over the Stoneley wave is that the former wave is sensitive to the TI property in both slow and fast formations, provided the wave measurement is made in a broad frequency-range in which the flexural-wave dispersion characteristics from low to high frequencies can be utilized. By calculating the theoretical flexural-wave dispersion curve for the TI formation and using it to fit the measured wave dispersion data, we can simultaneously determine both the vertical and horizontal shear-wave velocities, from which the shear-wave TI parameter is obtained. Application of our methodology to field data processing shows that the flexural wave result is more accurate and reliable compared to the Stoneley-wave result for a fast formation. Our study thus introduces a novel application of dipole acoustic logging.