A method and apparatus for estimating the slowness of a geological formation using waveforms recorded by an acoustic borehole logging tool. Phase slowness coherence is extracted from the recorded waveforms after transformation to the frequency domain (804) at various frequencies (806), and this is converted into a formation slowness curve (808) which can be used as an objective function from which an anomaly, such as a local maximum or minimum (810), can be identified as representative of an estimate of the formation slowness.