Identification of stress induced anisotropy in formations

Abstract

Low and high frequency flexural waves or their equivalents are generated with dipole or other source transducers. From measurements made at receiving transducers which are oriented at two orthogonal directions in a horizontal plane normal to the borehole axis, and via known processing techniques, the received signals are transformed into arrivals as a function of frequency such that the principal polarization directions and the magnitudes of the maximum and minimum wave velocities at those directions are determined at different frequencies. If the maximum velocity of the relatively low frequency flexural waves are in a first principal polarization direction, and the maximum velocity of the relatively high frequency flexural waves are in a second principal polarization direction which is substantially normal to the first principal direction, uniaxial stress in the formation is attributed to stress induced azimuthal anisotropy as opposed to an instrinsic anistropy in the formation. Then, the low frequency information can be utilized to obtain a value for a formation shear stress parameter such as c.sub.456 T.sub.23.sup.max /c.sub.66, where c.sub.456 is a third order elastic constant of the formation in Voigt notation c.sub.66 is the formation shear modulus and T.sub.23.sup.max is the magnitude of the formation shear stress in a direction forty-five degrees from the maximum far-field compressive stress direction.