Identification of a reversed-phase compressional in dipole borehole logging. Physical principle, theoretical modeling, and field examples

Dipole array sonic tools are particularly suited for estimating acoustic shear wave velocities in soft, unconsolidated, slow formations in boreholes. Dipole sources induce flexural waves, which are sustained in such formations. By contrast, monopole sources create pseudo-Rayleigh waves, related to shear wave velocity, which, in slow formations, refract away from the borehole and vanish before reaching the receivers. In addition, monopole sources create compressional and Stoneley waves. The typical order of these arrivals is compressional, shear (when present), and Stoneley. In an ideal situation dipole sources excite only borehole-flexural waves. The propagation velocity of the flexural wave is near that of the shear wave. Field experience backed by theoretical modeling indicates that in real situations dipole sources might also generate (reversed-phase) compressional and Stoneley waves. Theoretical simulations using well parameters taken from other logs render nearly identical wavetrains. Verification of the results with seismic measurements further validates the processing.