Characterizing the borehole response for single-well shear-wave reflection imaging

Abstract

Single-well shear-wave imaging using a dipole source-receiver system is an important application for detecting geologic structures away from the borehole. This development allows for determining the azimuth information of the structures. Existing analyses, however, focus on the data received at the borehole axis and use the elastic reciprocity theorem to model the borehole radiation and recording. We have extended the existing analyses to model the radiation, reflection, and recording response of the borehole for azimuthally spaced receivers off the borehole axis. By treating the mirror image of the borehole source with respect to the reflector plane as a virtual source, the borehole reception problem was shown to be equivalent to the response of the borehole to the spherical wave incidence from the virtual source, which can be solved using the cylindrical-wave expansion method. An asymptotic solution using the steepest-descent method is obtained if the virtual source is far from the borehole. The analytical solution allows us to analyze the borehole response for azimuthally spaced off-axis receivers. The analysis results agree well with those from 3D finite-difference simulations. With this analysis, one can further model multicomponent shear-wave reflection data from the cross-dipole acoustic tool and study the azimuthal variation characteristics of the data. Our results reveal that, whereas the data characteristics are dominated by those of a dipole, nondipole responses due to the off-axis reception can be observed, the magnitude of the responses depending on the off-axis distance and frequency and on the formation elasticity. The nondipole response characteristics have the potential to resolve the 180°-ambiguity problem in the azimuth determination for dipole shear-wave imaging. Our findings, therefore, provide new information for shear-wave reflection imaging analysis and development.