STRESS-INDUCED AZIMUTHAL ANISOTROPY IN BOREHOLE FLEXURAL WAVES

A nonlinear elastic model for acoustic waves in a stressed medium is u sed to calculate tectonic stress-induced changes in borehole flexural dispersions. Our theoretical analysis shows that a horizontal uniaxial stress in the formation causes a crossover in flexural dispersions fo r the radial polarization aligned parallel and normal to the stress di rection. This crossover in flexural dispersions is caused by stress-in duced radial heterogeneities in acoustic wave velocities that are diff erent in the two principal stress directions. Other sources of borehol e flexural anisotropy caused by finely layered dipping beds, aligned f ractures, or microstructures found in shales, exhibit neither such rad ial heterogeneities nor flexural dispersion crossovers. Consequently, a crossover in flexural dispersion can be used as an indicator of stre ss-induced anistropy. In this situation, the fast shear polarization d irection coincides with the far-field uniaxial stress direction. The a nalysis also yields an expression for the largest shear stress paramet er in terms of the fast and slow seismic shear-wave velocities with sh ear polarization parallel and perpendicular to the far-field stress di rection.