Wave components of acoustic logs in transversely isotropic porous formations

The acoutic field excited by a monopole point-source was numerically investigated with a model of a transversely isotropic fluid-saturated poroelastic formation. The wavefields inside and outside the borehole are formulated in the frequency-wavenumber domain. All the poles of the acoustic function are obtained to correspond to several modes of guide waves: the normal modes (including Stoneley wave and pseudo-Rayleigh wave) and two leaky modes. The numerical results showed that the contribution of the lowest order leaky mode can not be ignored since its imaginary part has the same magnitude order as that of the normal mode. Moreover, the velocity of pseudo-Rayleigh wave at the low-frequency limit is not always equal to the shear wave velocity in transversely isotropic formations. Meanwhile, the excitation spectra and waveforms in the time-space domain are acquired for Stoneley wave, pseudo-Rayleigh wave and two-leaky-modes wave, contributed by the poles and two branchcuts (the quasi-shear and the quasi-fast-compressional branchcuts). The results showed that the contribution of leaky-mode poles can not be omitted in the full waveforms.