The shear waves from a four-component cross-dipole acoustic tool were analyzed to provide an important method for the single-well acoustic imaging application. We analyzed the effects of wave radiation, reflection, and borehole acoustic response on the shear-wave reflection measurements. In particular, we studied shear-wave radiation from a dipole source and the wave’s reflection from a formation reflector, showing that the shear waves generated by a dipole source in borehole have a wide radiation pattern. This allows for imaging reflectors at various dip angles crossing the borehole. More importantly, the azimuthal variation of the shear waves, in connection with the multi-component nature of a cross-dipole tool, can be used to determine the strike azimuth of the reflector. The analysis results establish a theoretical foundation for the borehole shear-wave imaging application. The theoretical results are utilized to formulate an inversion procedure for field data processing. The field data application not only validates the theoretical results but also demonstrates advantages of shear-wave imaging.
Imaging near-borehole geological features using acoustic logging has become an important application in recent years. The imaging application in the past primarily focused on using compressional (P) waves from a typical full-wave monopole acoustic tool (e.g., Hornby, 1989; Li et al., 2002). Because the monopole tool is unable to determine the azimuth of a reflector, directional acoustic sensing techniques need to be developed. Haldorsen et al. (2006) showed the P-wave imaging result using multiazimuth sensors. Tang (2004) utilized the directional sensitivity of the P waves from a dipole acoustic tool to determine the reflector azimuth. This study investigates borehole acoustic imaging using shear waves from a four-component cross-dipole acoustic tool.
Recent advances in acoustic reflection signal processing (e.g., Tang et al., 2007) have significantly improved the ability to extract reflection signals from overwhelming acoustic waves that travel directly along borehole. Application of the technique to dipole shear wave logging data allows for extracting low-frequency shear-wave reflections from the data. The availability of the shear-wave reflection data stimulates a new interest in the shear-wave imaging application. The main objective of this study is to resolve the azimuth ambiguity and use the shear waves to image near-borehole reflector geometry. The results of the work provide an important application for the cross-dipole acoustic logging technology.
In the following, we analyze the radiation of shear waves from a borehole dipole source and the shear wave reflection from a near-borehole reflector, establishing the relationship between the reflector azimuth and the reflected wave amplitude for a four-component cross-dipole tool. Using the relationship, we develop analysis/processing techniques for determining the reflector azimuth and imaging the reflector. The application will be demonstrated using field data examples.
The analysis for the radiation of a dipole acoustic source in borehole is similar to analyzing a monopole source (e.g., Meredith, 1990). The result shows that the far-field radiation of the dipole source is equivalent to that of a single force in an elastic solid, with a radiation pattern (Ben-Menahem and Kostek, 1991) given by
