PROCESSING FOR SONIC WAVEFORMS

A method (Fig. 1) for creating a frequency or time domain semblance for use in conjunction with acoustic logging tools (100, 130, 140, 145). The frequency domain semblance (Fig. 2F) may be obtained by transforming an acoustic signa l (150) received at multiple depths into the frequency domain, combining the received waveforms corresponding to the different depths, and expressing the result in a graph with slowness and frequency axes. Another aspect of the invention is the treatment of two or more time domain semblances (Fig. 2B) a s probability density functions of the slowness for an acoustic signal. This enables the combination of time domain semblances from the same depth in the wellbore. Once combined the time domain semblances more accurately depict th e slowness of an acoustic wave through the formation at the selected depth. In addition, a related self-adaptation method to compress the waveform data downhole for storage or transmission is also disclosed.