Radiation efficiency and energy distribution from a borehole dipole source in the vertical transverse isotropic formation

Dipole Shear-Wave Reflection Survey (DSRS) has gained wide application in identifying unconventional reservoirs in anisotropic formations. Previous investigations have illuminated how boreholes complicate the distribution of acoustic energy. However, these models have not accounted for the anisotropic context, rendering them insufficient for analyzing acoustic energy distribution and radiation efficiency in anisotropic formations. We derive expressions for energy flux and radiation efficiency from a borehole dipole source in the vertical-transverse isotropic (VTI) media using the Umov-Poynting vector and steepest-descent solution. Utilizing this approach, we systematically evaluate the sensitivity of anisotropic parameters to energy flux and radiation efficiency, unveiling intricate variations of these properties across frequency and anisotropic parameters. Our findings emphasize the substantial influence of formation anisotropy on energy distribution from a dipole source inside the borehole. Due to energy conversion between wave modes, five radiation wave modes are elicited by the dipole source, with the SH wave retaining its status as the prime candidate for DSRS in fast formations owing to its elevated radiation efficiency. Conversely, the qP-qP wave exhibits advantages over S waves in unconsolidated formations. A key distinction between isotropic and anisotropic media is the presence of dominant excitation frequencies in the low-frequency domain. The significance of dominant excitation-frequency bands is validated by field data, emphasizing their pivotal role. These results offer valuable insights for designing DSRS measurement strategies, which have broad application expectations for unconventional oil and gas exploration.