Thomas J. Plona
Schlumberger-Doll Res., Ridgefield, CT 06776
Stress-induced dipole anisotropy exhibits a crossover in the principal flexural wave slowness dispersions oriented parallel and perpendicular to the farfield uniaxial compressive stress direction. This crossover phenomena is a result of borehole stress concentrations and can be used as a new technique for distinguishing stress-induced anisotropy from intrinsic anisotropy. Theoretical modeling (based on third-order elasticity) and laboratory measurements (from 10 to 60 kHz) have been made on a large block of berea sandstone subject to a uniaxial stress of up to 5 MPa. The two flexural mode dispersions are obtained by Prony's processing of an array of measured waveforms for dipole orientations parallel and perpendicular to the stress direction. The theoretical dispersions in the presence of biasing stresses are obtained from the solution of equations of motion for small dynamic fields superimposed on a static bias. Good agreement has been obtained between theory and experiment including a dispersion crossover phenomena unique to stress-induced anisotropy.