Phase-based dispersion analysis for acoustic array borehole logging data

Abstract

A phase-based dispersion analysis method for velocity (slowness) extraction from guided waves recorded by an acoustic borehole logging tool in a geological formation is presented. The technique consists of acquiring waveforms from an array of receivers distributed along the tool and constructing the dispersion characteristic by processing in the frequency domain and exploiting phase information to measure the travel time for each frequency component. The approach is nonparametric and completely data-driven and provides high resolution estimates that do not rely on velocity guesses or assumptions regarding the type of modes. Results are free of the aliases and spurious modes which are characteristic of some prior approaches. Examples of dispersion estimation curves are presented using synthesized flexural waves and field data from wireline dipole sonic tools; results are compared with those from the weighted spectral semblance (WSS) and amplitude and phase slowness estimation (APES) methods to demonstrate the effectiveness and utility of the proposed method.

Algorithm for processing dispersive waves from dipole sonic tool

“…The flexural mode must be slower than the shear wave mode in the formation (Tang & Cheng, 2004), so the procedure neglects every root at the right of the red vertical line in Figure 5. Figure 6 shows a dispersion curve obtained for a set of borehole and formation parameters (blue line), the amplitude spectrum (yellow dashed line) and the dispersion points observed in real dipole waveforms (dots with colors according to the amplitude spectrum), the last obtained by Phase Based Dispersion Analysis (PBDA) proposed by Assous (2014). Even though the PBDA method delivers data-driven dispersion estimates, flexural mode amplitude spectrum in slow formations usually shows a very low signal-to-noise ratio, making it difficult to obtain the shear wave velocity from extrapolating experimental data.…”