Many earth formations are characterized as transversely isotropic (TI) media. In acoustic logging through a vertical borehole, the S-wave TI property has traditionally been determined from borehole monopole Stoneley-wave measurements, but the feasibility of shear-TI estimation from dipole flexural waves has not been fully investigated. We have developed a methodology to determine the TI parameters from borehole dipole-flexural wave data. Our analysis shows that the Stoneley wave is sensitive to the TI property mainly in an acoustically slow formation, and the sensitivity diminishes when the formation becomes faster. The advantage of the flexural wave over the Stoneley wave is that the former wave is sensitive to the TI property in the slow and fast formations, provided the wave measurement is made in a broad frequency range in which the flexural-wave dispersion characteristics from low to high frequencies can be used. By calculating the theoretical flexural-wave dispersion curve for the TI formation and using it to fit the measured wave dispersion data, we can simultaneously determine the vertical and horizontal S-wave velocities, from which the S-wave TI parameter is obtained. Application of our methodology to field data processing shows that the TI parameter estimated from the flexural wave is almost identical to that from the Stoneley wave for a slow formation. For a fast formation, the flexural-wave result is more accurate and reliable compared with the Stoneley-wave result. Our study, thus, introduces a novel application of dipole acoustic logging.

SUMMARY The Deep Sea Drilling Project (DSDP) or Ocean Drilling Program (ODP), and the recent International Discovery Program (IODP) usually acquire borehole geophysical data for planetary exploration. A recently developed shear wave imaging technique was applied to the acoustic waveform log data of the IODP U1473A hole to image a fault system about 40 m away from the borehole. The shear wave imaging not only allowed us to delineate the fault structure in the vicinity of the borehole, but also to determine the orientation of the fault system, providing much needed information for the interpretation of the tectonic situation of the well site. Theory and method for the imaging application are elaborated and data processing results are presented along with the supporting borehole geophysical data. Its successful application to the IODP U1473A hole suggests that the shear wave imaging technique can be applied to the previously recorded and future ODP/IOPD borehole acoustic data, promising the extraction of valuable geophysical and geological information.

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