Wave components of acoustic logs in transversely isotropic porous formations

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