Estimating phase velocity and attenuation of guided waves in acoustic logging data

Phase velocity and attenuation of guided waves have been estimated from acoustic log-ging data recorded by a receiving array. The method uses data from multiple sources and successive depths yielding more accurate estimates than could be obtained with data from one source and one depth. The inversion requires two steps: (1) all traces are transformed into the frequency domain with a fast Fourier transform, and (2) at each frequency a modification of Prony's method is used to estimate the guided waves ' char-acteristics which include amplitude, attenuation coefficient, and wavenumber (which yields the phase velocity). An important assumption underlying this technique is that the formation, borehole fluid, and tool are homogeneous along the receiving array. Application of this method to synthetic data shows that the phase velocity and at-tenuation of the tube and pseudo-Rayleigh waves are accurately estimated at many frequencies. With noisy synthetic data, the phase velocities are correctly determined, but the attenuation estimates, being sensitive to noise, are accurate only when the am-plitudes are high. Using data from multiple sources and successive depths suppresses the noise effects and improves both estimates. The amplitude estimates are important because they roughly indicate the reliability of the velocity and attenuation estimates. From laboratory and field data, the velocities for the guided waves are accurately pre-dicted even when the amplitudes are low. The attenuation estimates are good when the amplitudes are high but degrade as the amplitudes diminish. 8 Ellefsen et aI