The Differential-Phase Based Time- and Frequency-Semblance Algorithm for Array- Acoustic Processing and its Application to Formation-Slowness Measurement

摘要

An advanced, acoustic-array processing technique is presented that uses the measured phase difference between each array receiver's waveform signal in the time and frequency domain to obtain improved semblance (coherence) images and mode-slowness measurements. Standard phase-based semblance algorithms suffer from interference, and amplitude-based algorithms suffer from detecting weak amplitude mode occurrences. Because modern acoustic tools use broad-frequency source excitations to deliberately excite simultaneous multiple formation modes to determine multiple formation elastic properties, mode interference must be mitigated to measure all modes accurately. The differential-phase-based method discussed in this paper provides accurate formation mode-slowness information with high resolution for multiple modes and can also be used for error estimation of predicted formation slowness. The basis of this new method recognizes that, in an ideal case, a mode arrival will have a zero differential phase in a test slowness stack when the mode arrives at two receivers with that slowness and that phase is a nonzero value elsewhere. Continuing this phase analysis with each possible receiver pairing in the array and then stacking these paired results reduces incoherent noise. The benefit is both consistency of assumptions when producing time- and frequency-semblance maps and reduction in processing time to produce both maps compared to, for example, nth-root time semblance and Prony or matrix-pencil frequency semblance (neither of which can make use of any intermediate nth-root algorithm data). The application of this method is tested on actual data from a broadband multipole wireline tool run in a test well.