Comment on “A numerical investigation of the acoustic mode waves in a deviated borehole penetrating a transversely isotropic formation”

In the paper “A numerical investigation of the acoustic mode waves in a deviated borehole penetrating a transversely isotropic formation” by Liu et al. [1] numerical experiments are reported for a dipole sonic logging scenario with a deviated borehole penetrating a VTI anisotropic medium. In such a situation it is important to clearly define the meaning of group and phase velocities as this has led to much confusion in the literature as discussed by Miller, Horne and Walsh [2].