Method and system for calibrating acoustic receivers in borehole logging tools

Abstract

Monopole and quadrupole contamination on dipole measurements is one of the main problems for sonic logging tools that employ arrays of acoustic receivers to measure a borehole’s dipole mode. The amplitude and phase mismatch of the array of sensors in acoustic logging tools plays an important role in the quality of dipole measurements. Acoustic receivers often have different sensitivities, and different sensitivities to the same wave field result in a greater possibility of no dipole contamination. Even similarly or identically manufactured receivers tend to report different amplitudes and time measurements (i.e., amplitude and phase mismatch). In practice, the no-dipole modes are removed by making the measurements at different azimuth and extracting the dipole mode from those that reject the other contaminating modes. Therefore, to improve rock formation slowness estimation and downhole modal computation, it is necessary to calibrate acoustic logging tools by detecting and correcting the amplitude and phase mismatch of the individual sensors mounted in the logging tools. A method and system for calibrating acoustic receiver arrays mounted in a downhole logging tool and the factors that affect their sensitivity are discussed in this paper.