EFFECTS OF BOREHOLE STRESS-CONCENTRATIONS ON DIPOLE ANISOTROPY MEASUREMENTS

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