Figures 3 and 4 present the time-domain waveforms, amplitude spectra, and phase spectra for particle vibrations in shale, M-sandstone, and C-sandstone under different viscosity conditions.
The amplitude spectra indicate that the center frequency remains unaffected by viscosity, but the amplitude decreases as viscosity increases. Were there any non-linear deviations observed at higher viscosity values? How do these findings compare with theoretical predictions from the proposed spring-damping model?
The phase spectrum (Figures 4c and 5c) undergoes a significant shift at the center frequency. Was this shift consistent across multiple trials, and could it be linked to microstructural differences between rock samples?