| Abstract: | Conventional methods for determining Poisson's ratio from full-waveform acoustic logs rely on measurement of the shear velocity. This measurement is subject to error from contamination by multiply reflected P-waves and pseudo-Rayleigh waves which arrive at the same time as the shear head wave. In slow formations where the shear head wave and pseudo-Rayleigh waves are not excited, the determination of formation shear wave traveltime must rely on other methods. A new method is presented for determining Poisson's ratio and shear velocity. This method can be implemented by recording the components of particle displacement on the borderline wall. The particle trajectory in the compressional head wave is rectilinear and is deflected with respect to the direction of propagation. The deflection is a sensitive function of Poisson's ratio, varying by more than 30 degrees over the range of Poisson's ratio encountered in rocks. In the polarization method all the measurements are made on the compressional head wave, the spatial resolution is greater than for velocity measurements, and the deflection is greatest in soft formations. To compare the different methods in extracting the attenuation (Q⁻¹) from full-waveform acoustic logs, the model Q's are incorporated in the algorithm. The algorithm is tested by recovering the model Q of the compressional head wave form synthetic waveforms. For a wide-band source, windowing the compressional head wave and using the spectral-ratio, slope method recovers model Q accurately. For a narrow-band source, windowing most of the P-wave train and using the spectral-ratio, peak-amplitude method recovers model Q approximately. The discrepancy between the extracted value and the model Q increases when the receiver spacing, z, increases in a high attenuation zone and when the dominant frequency of the source function increases. |
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