Properties of the rock formation surrounding a fluid-filled borehole can be measured by sonic logging. Full-waveform recordings made by a receiver array are routinely processed to estimate the compressional and shear velocities. Other wave components, such as the normal modes, can be extracted by using two-dimensional spectral analysis techniques. A limited spatial array aperture constrains the resolution of techniques based on beamforming or Fourier analysis. The method discussed here employs the extended Prony method to do a high-resolution spatial-frequency analysis. Examples are presented to illustrate the method when tracing the velocity dispersion of the normal modes of the acoustic wavefield.