Anomalous dispersion of Stoneley waves in fluid-filled boreholes

For acoustic logging, the estimation of rock properties from Stoneley waves is of considerable importance. Stoneley wave is the interfacial wave propagating on the interface between the borehole liquid and the formation. In the majority of cases in physics, phase velocity decreases with frequency, so that this type of dispersion is referred to as normal dispersion. On the other hand, phase velocity increases with frequency. It happens less frequently in physics, and it is referred to as anomalous dispersion. However, anomalous dispersions are commonly encountered for borehole Stoneley waves. We studied influences of formation properties on the existence of anomalous dispersion of Stoneley waves, to investigate the correlations. It was found that the appearance of anomalous dispersion is sensitive to the shear velocity of the formation, which suggests that anomalous dispersion of borehole Stoneley wave could be related to the formation properties.