Effects of borehole stress concentrations on dipole anisotropy measurements

Abstract

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.