A curve-fitting technique for determining dispersioncharacteristics of guided elastic waves

摘要

We have developed a novel curve-fitting method to esti-mate dispersion characteristics of guided elastic waves andinvestigate its application to field wireline and logging whiledrilling (LWD) acoustic data processing. In an elastic wave-guide such as a fluid-filled borehole with a logging tool, thefrequency dispersion of a guided-wave mode is characterizedby a monotonically varying dispersion curve bounded by itslow- and high-frequency limits. The detailed behavior of thecurves relates to various elastic/acoustic parameters of thecomplicated waveguide structure. The novelty of the pro-posed technique is that it simulates the multiparameter dis-persion curve using a simple analytical function that has onlyfour parameters. By adjusting the four parameters to fit theactual wave dispersion data, the wave's dispersion character-istics can be satisfactorily determined. The result of this sim-ple approach leads to several important applications in acous-tic logging. The first is to correct the dispersion effect in theshear-wave velocity from wireline dipole acoustic logging.The second application obtains P-wave velocity from the dis-persive leaky compressional-wave data from wireline orLWD measurements. Third, the technique is applied to obtainshear-wave velocity from LWD quadrupole shear-wave log-ging. Finally, the technique is applicable to layered wave-guide structures encountered in surface seismic exploration.