Shear Slowness Determination From Dipole Measurements

Acoustic modes that are linked to the presence of the borehole are dispersive. That is, their phase slowness (the inverse of velocity) varies with frequency. Equivalently, the pulse wave shape changes as it propagates across the array. The Stoneley wave generated by a monopole source and the flexural wave generated by a dipole source are examples of dispersive borehole modes. The flexural wave is important because it provides shear in slow formations (i.e., formations in which the shear slowness, the inverse of shear velocity, is slower than the compressional slowness of the borehole fluid). Flexural mode dispersion makes shear slowness determination more difficult. The dipole flexural wave propagates at the shear slowness at zero frequency, but above this frequency, its phase slowness increases by an amount called the dispersion bias. The dispersion bias usually represents a small fraction of the measurement One approach would be to measure at very low frequency where the dispersion bias is negligible; this is, however, very difficult to achieve because of the extremely low excitation of the flexural mode at low frequency. In practice, dispersion of the flexural wave cannot be avoided and must be taken into account. Various methods can be used to measure shear from the flexural wave in frequency bands where the flexural energy is sufficient for an accurate slowness measurement, but where dispersion bias is not negligible: * Process the waveforms with traditional non-dispersive techniques in a narrow-frequency band to minimize frequency dispersion and then apply a correction for dispersion bias. The slowness time coherence (STC) processing in a narrow frequency band followed by dispersion bias correction is a robust technique, but it lacks flexibility. In some instances, the correction may be inaccurate when the model used to generate the correction tables does not fit the actual conditions. * Process the waveforms dispersively, making full use of the flexural wave dispersion and eliminating the requirement for dispersion bias correction. Dispersive processing is the maximum likelihood or least-mean-squared error solution and provides better results for uncertain wave spectra. It also allows a more accurate representation of the borehole condition (for example, the presence of very slow oil-base mud in the borehole). In this paper, dispersion curves obtained from mode tracking are presented. The different processing techniques are discussed and compared. Examples are given in formations of different types and discussed.