New Applications In The Inversion Of Acoustic Full Waveform Logs - Relating Mode Excitation To Lithology

Abstract

Existing techniques for the quantitative interpretation of waveform data have been based on one of two fundamental approaches: 1) simultaneous identification of compressional and shear wave velocities; and 2) least-squares minimization of the difference between experimental waveforms and synthetic seismograms. Techniques based on the first approach do not always work, and those based on the second seem too numerically cumbersome for routine application during data processing. An alternative approach is tested here, in which synthetic waveforms are used to predict relative mode excitation in the composite waveform. Synthetic waveforms are generated for a series of lithologies ranging from hard, crystalline rocks (V[subscript p] = 6.0 km/s and Poisson's ratio = 0.20) to soft, argillaceous sediments (V[subscript p] = 1.8 km/s and Poisson's ratio = 0.40). The series of waveforms illustrates a continuous change within this range of rock properties. Mode energy within a characteristic velocity window is computed for each of the modes in the set of synthetic waveforms. The results indicate that there is a consistent variation in mode excitation in lithology space that can be used to construct a unique relationship between relative mode excitation and lithology.