Joint inversion method of formation shear-wave anisotropy from logging-while-drilling acoustic data

Abstract

Most sedimentary formations with fine layers can be characterized as transversely isotropic media. The evaluation of shear-wave anisotropy is critical in logging-while-drilling (LWD) applications. We developed a joint method to simultaneously invert formation shear-wave anisotropy and vertical shear velocity using LWD monopole and dipole dispersion data. Theoretical analysis demonstrates that formation shear-wave anisotropy significantly affects the dispersion characteristics of Stoneley and formation flexural waves. The inversion objective function was constructed based on the change in dispersion characteristics and was weighted by the spectra of multipole waves. Numerical results using synthetic examples demonstrate that the joint inversion method can not only alleviate the non-uniqueness problem but also help improve the accuracy of the inversion results. The comparison of different signal-to-noise ratio inversion results proved that the weighted inversion method is more accurate and stable.