MULTIPOLE SOURCES IN A FLUID-FILLED BOREHOLE SURROUNDED BY A TRANSVERSELY ISOTROPIC ELASTIC SOLID

In this paper, the acoustic field excited by multipole sources in a fl uid-filled borehole surrounded by a transversely isotropic elastic sol id is systematically analyzed and numerically simulated. Not only have the mode waves been analyzed thoroughly, but the propagation mechanis m of the critical refracted P and S waves corresponding to the multipo le branch cut integration for the transversely isotropic formation has also been investigated for the first time. In the presence of a fast or a slow transversely isotropic elastic solid formation, the componen t waves excited by a monopole, a dipole, and a quadrupole source have been studied in the time and frequency domains. It is found that the c ritical refracted arrival of the S wave is a dominant factor, while th e mode wave is not in the low-frequency multipole direct shear wave lo gging. When the formation is changed from isotropic to transversely is otropic, the amplitudes of the component waves vary significantly, but the variation of the cutoff frequencies of the mode waves and the res onant frequencies of the P and S waves is less pronounced.