Effects of borehole stress concentrations on dipole anisotropy measurements

We show both experimentally and theoretically how stress concentrations affect the velocity field around a borehole, and how the velocity field influences dipole anisotropy measurements. At low frequencies the dipole mode is sensitive to the far-field stresses primarily, so standard sonic log interpretation correctly yields the direction of maximum stress. At higher frequencies, the dipole mode is sensitive to near-field stress concentrations such that the fast polarization direction is aligned with the direction of minimum tectonic stress. These effects combine to produce a crossover in the dipole dispersion curves measured in the fast and slow directions. With broad-band dipole data, the dispersion crossover can be used as an indicator of stress-induced anisotropy dominating over weak intrinsic anisotropy.