Velocity Analysis Of Multi-Receiver Full Waveform Acoustic Logging Data In Open And Cased Holes

Abstract

Average semblance and maximum-likelihood spectral analysis are applied to synthetic full waveform acoustic logging data to determine formation velocities. Of particular interest is the ability of these methods to resolve the P wave and pseudo-Rayleigh wave arrivals in data from poorly-bonded cased boreholes. In open-hole data the velocity analyses generally yield results within 0.15 km/s of the true velocities. For cased-hole models with no steel/cement bonding (free pipe), the measured P-wave velocities are 0.2 to 0.6 km/s less than the actual formation velocities, and the S-wave velocities are within 0.15 km/s of the true velocities. If the P-wave velocity is fairly close to the plate velocity of the steel pipe (within 0.7 km/s or so), then the P-wave arrival is not separated from the steel arrival by the semblance method, but the P wave is resolved by the spectral analysis. For cased-hole models with no cement/formation bonding (unbonded casing), accurate formation velocities are obtained if the velocities are not too great. When the formation P-wave velocity approaches the velocity of the casing arrival, the P-wave resolution is lost by both methods. Further work needs to be done to better define this limit.