Some properties of fundamental acoustic modes when logging while drilling

The properties of the fundamental modes generated by acoustic monopole, dipole and quadrupole sources<br>in the LWD environment are reviewed in the presence of homogeneous slow isotropic formations. The<br>techniques that can be used to determine the formation shear wave velocity are also discussed. For dipole<br>data, interference between the formation and tool signals requires the inversion of the mode velocity at<br>intermediate frequencies. The reliability of such process increases with decreasing formation shear wave<br>velocity. The fundamental quadrupole mode phase velocity starts with the formation shear wave velocity at a<br>cut-off frequency at which very little energy is present and then decreases toward the Scholte velocity, like<br>the flexural mode. Such features may require a dispersion analysis/correction depending on the fluid annulus<br>thickness and the signal to noise ratio in the low frequencies.