Higher Order Modes in Acoustic Logging While Drilling

Abstract

In multipole acoustic logging while drilling (LWD), the fundamental modes dominate recorded waveforms. Higher order modes may also appear and complicate the processing of LWD data. In dipole LWD measurements, the dipole tool mode is often not well separated from the flexural mode. This makes the shear wave measurement more difficult. We conducted theoretical and numerical analysis on dipole LWD logging responses. We found that hexapole mode may be present in the dipole waveforms. Laboratory dipole data show the presence of hexapole mode, which approaches asymptotically to the formation shear wave velocity. This observation supports our conclusion. We may make use of these higher order modes for accurate determination of formation shear wave velocity.