Investigating stress induced anisotropy in boreholes

Low and high frequency flexural waves are generated with dipole transducers. From measurements made at receiving transducers which are oriented at two orthogonal directions in a horizontal plane normal to the borehole axis 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 intrinsic anistropy in the formation. Then, the low frequency information can be utilized to obtain values for formation shear stress parameters.