Acoustic or sonic logging tools are often employed in wellbore environments for a variety of purposes, including formation measurements and material characterizations. In general, acoustic logging tools are disposed in a borehole and operate to generate/receive signals and measure signal responses (or signal dispersions) for different target modes. The signal responses or signal dispersions for a given target mode are further analyzed to determine various geophysical and mechanical properties of the borehole as well as adjacent formations. Signal dispersions are often characterized by a relationship between signal wave slowness (μs/ft) and signal wave frequency, which provides insight into material-types and geometric properties of the borehole as well as the adjacent formations. Put differently, the signal wave slowness (for a given target mode) depends upon its frequency. For example, signal responses from soft formation dipole logging may include both a leaky P mode and a dipole shear mode waveforms, where the dipole shear mode waveforms are typically more dispersive than the leaky P mode waveforms.
It is also appreciated certain target mode waveforms result in signal responses that indicate a low frequency asymptote or limit, and further, such low frequency asymptote approaches a shear wave formation slowness for the surrounding rock formation. As such, the low frequency asymptote may be used to estimate or otherwise indicate the formation slowness (or other properties) of the surrounding formation. However, certain challenges (e.g., noise, interference, scattering due to borehole radius changes, non-suppressed waves, etc.) hinder and complicate reliably extraction and validation processes, particularly extracting and validating signal dispersions at low frequencies.
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