Robust Estimation of Shear Slowness from Quadrupole Mode in LWD Environment

Quadrupole acoustic logging has is to measure shear in the logging-while-drilling (LWD) environment in the oil field service industry today. A robust estimation of shear slowness from quadrupole acoustic logging requires an inversion method that takes into account the dispersion of the quadrupole mode, including tool-presence effects. This inversion method was developed to adopt a semblance processing methodology. The inversion optimizes formation shear slowness and also includes optimization of mud slowness as an internal parameter to compensate for effects of undetermined mud slowness. The forward modeling of quadrupole dispersion behavior is based on a homogeneous isotropic (HI) formation model which already includes the tool-presence effects. An automatic selection method of frequency band is designed to minimize errors from unknown borehole parameters. The multi-shot processing mode enhances the vertical resolution of the shear slowness log. Thus the two-parameter inversion becomes more robust in a wide range of LWD environments. Testing with finite difference method waveforms validated the inversion method and the field results show very good agreement with wireline measurements.