DIRECT PHASE DETERMINATION OF S-WAVE VELOCITIES FROM ACOUSTIC WAVEFORM LOGS

A new technique for producing S-wave logs from borehole acoustic waves has been developed. The procedure is based on direct phase calculations for time-windowed waveforms obtained with an acoustic logging tool. A window is positioned over the S-wave portion of the signals, and window moveout is chosen to give zero-phase differences in a band of frequencies across the array of receivers. A major issue in application of this method is the error introduced by windowing and by interfering signals such as casing arrivals, residual P-waves, and modes propagating in the borehole. Examples using synthetic data are presented illustrating these errors and the means of reducing them. A capture effect, characteristic of phase methods, may be exploited to reduce the effect of interference.