A DISPERSIVE-WAVE PROCESSING TECHNIQUE FOR ESTIMATING FORMATION SHEARVELOCITY FROM DIPOLE AND STONELEY WAVE-FORMS

A model-guided dispersive-wave processing technique has been developed to estimate formation shear-wave velocity from borehole acoustic logg ing waveforms. These waveform data can be the Stoneley waves in monopo le logging or the flexural waves in dipole logging. In this technique, the waveform recorded on a given receiver is compared to the waveform from a second receiver that is numerically propagated to the given re ceiver's position using a trial formation shear-wave velocity. The num erical propagation step uses the proper dispersion relation for the wa ve mode (dipole or Stoneley). The phase difference between the two wav eforms is minimized by varying the shear velocity. The velocity value that minimizes the phase difference is chosen as the final shear veloc ity at which the waveforms attain the optimum phase match. In this pro cedure the dispersion effect is automatically accounted for by using t he model theory and is demonstrated by a comparison with the results o f the semblance method. Using this technique with a multiple-shot sche me to process array acoustic logging data, formation shear velocity ca n be estimated to the resolution of one receiver spacing [typically 0. 5 ft (0.1524 m)]. This result has been demonstrated by a field example in which the improved resolution in the shear velocity log is convinc ingly supported by the same character in the density log measured at o ne receiver spacing intervals. Shear velocity logs obtained using fiel d Stoneley and dipole sonic data are also compared to demonstrate the ability of the technique to obtain high-resolution formation shear vel ocity logs from the dispersive logging waveforms. The new technique ma y find very useful applications in the determination of formation shea r-wave properties using acoustic waveform logging.