In cased-hole acoustic logging, estimating the formation velocity is often problematic when the casing is poorly bonded with the formation. The overwhelmingly large casing waves dominate the measured waveforms and overlap with the low-coherence, weak formation arrival, contributing to the failure of conventional semblance processing method. To tackle this problem, we have developed a filtered frequency semblance array waveform signal processing technique. The multiple filter technique is first used to filter the measured waveforms. We then apply the nth root stacking method to the filtered signals. Consequently, the coherence of the formation signal on the obtained 3D semblance correlogram is significantly enhanced and clearly separated with the casing waves. We have applied the method to process synthetic and field cased-hole acoustic waveform data. Our results indicate that the new method significantly enhances the coherence of the desired formation signal and simultaneously estimates the formation of the P- and S-wave velocity.

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