We use a 3D Finite Difference (3DFD) method to simulate monopole wavefields in a singly-cased borehole with different bonding conditions. Modal dispersion curves and dispersion analysis facilitate the identification of propagation modes. We find that the casing modes are strong when interface I (interface between casing and cement) is partially or fully replaced with fluid. The amplitude dependence on fluid thickness is small which could lead to ambiguity in interpretation. The casing modes are different when interface II (interface between cement and formation) is partially replaced with fluid, because the modes propagate in the mixed material of steel pipe and cement and the velocities are highly dependent on the cement thickness. It would highly possibly misjudge cement quality because the amplitudes of these modes are very small and they propagate with nearly the formation P velocity. However, it is possible to use the amplitude to estimate the thickness of the cement sheath because the variation of amplitude with thickness is very clear. While the Stoneley mode (ST1) propagates in the borehole fluid, a slow Stoneley mode (ST2) appears in the fluid column outside the casing when cement is partially or fully replaced with fluid. The velocity of ST2 is sensitive to the total thickness of the fluid column in the annulus independent of the location of the fluid relative to the cement. By combining measurements of the first arrival amplitude and ST2 velocity, we propose a full waveform method that can be used to eliminate the ambiguity and improve cement evaluation compared to the current method that uses only the first arrival