Hexapole and octupole modes in LWD and their use in DTS estimation

Acoustic wave fields in boreholes are often dominated by the fundamental modes. For example, the fundamental flexural and quadrupole modes often dominate the field generated by dipole and quadrupole sources, respectively. Formation elastic properties are typically estimated from these modes. However, when logging while drilling (LWD), the presence of a large tool mandrel may perturb the fundamental modes, particularly when the tool is eccentered or the data is contaminated by tool body waves, resulting in erroneous formation slowness estimates. Here we observe that higher-pole modes, including hexapole and octupole waves, are seen in LWD acoustic data. We demonstrate that the tool properties and hole size affect these high-pole modes less than the fundamental modes, and the high-pole modes provide an alternative way of estimating formation wave slowness. This is particularly helpful when the low-frequency portion of the fundamental waves are absent or contaminated by low-frequency drilling noise. We propose an approach for estimating shear-wave slowness (DTS) from these higher-pole modes, and show observations and processing results in both a test well and a prospect well with real drilling noise.