A model for a fluid-filled borehole with an LWD tool is used to identify and analyze the monopole, dipole, and quadrupole modes that are present. The modes can be classified into three groups and have dispersion behavior that is influenced predominantly by the geometry and material properties of the three borehole layers (inner fluid column, tool, and annulus fluid column). However, this simple dependence gets complicated in regions of the frequency-wavenumber plane, where modes related to different layers interact and exchange their dispersion characteristics. The dipole modes exhibit the effects of interaction below 2 kHz and above 20 kHz.