NONAXISYMMETRIC ACOUSTIC FIELD EXCITED BY A CYLINDRICAL TOOL PLACED OFF A BOREHOLE AXIS AND EXTRACTION OF SHEAR-WAVE

The nonaxisymmetric acoustic field excited by a cylindrical tool place d off the axis of a fluid-filled borehole is investigated. It is shown that the acoustic fields are composed of infinite numbers of the mult ipole fields with different orders. In cases in which the medium outsi de the borehole is an isotropic homogeneous elastic solid and a two-ph ase medium, the dispersion and excitation of the mode waves such as St oneley waves, pseudo-Rayleigh waves, flexural waves, and screw waves a re studied. The full waveforms in the time domain are also simulated a nd analyzed. It is pointed out that the components of the symmetric an d nonsymmetric acoustic fields could be strengthened or weakened by ad ding and subtracting the two full waveforms logged in some opposite az imuths in a horizontal plane. A method of extraction of the shear wave is presented, and the relationship of the nonsymmetric acoustic field to the shear wave logging is analyzed. (C) 1996 Acoustical Society of America.