Surface-wave inversion for damping ratio: do higher modes matter?
F. Wang (TU Delft - Civil Engineering & Geosciences)
K. Van Dalen (TU Delft - Civil Engineering & Geosciences)
E. Verschuur (TU Delft - Civil Engineering & Geosciences)
L. Huber (TU Delft - Civil Engineering & Geosciences)
R. Ghose (TU Delft - Civil Engineering & Geosciences)
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Abstract
Because dispersion of surface waves is mainly sensitive to stiffness and fundamental-mode damping inversion is depth-limited, attenuation at depth is typically poorly resolved in surface-wave inversion. To overcome this issue, the current study develops a determinant-based multi-mode surface-wave inversion workflow to estimate the material damping ratio in horizontally layered viscoelastic media. Based on frequency-dependent phase velocity and phase damping ratio, synthetic data are constructed in the form of complex wavenumbers. The misfit is defined as the mean absolute value of the row-normalized dispersion-equation determinant evaluated at the observed frequency-wavenumber pairs. A genetic algorithm inverts for layer-wise damping ratio (assuming equal P- and S-wave damping) by minimizing this misfit across all frequencies. In a 4-layer synthetic example (5–50 Hz), multi-mode inversion turns out to yield more stable damping profiles and lower errors than fundamental-mode-only inversion. This improvement is explained by sensitivity tests on a two-layer model, which show that especially under high impedance contrast, higher modes retain sensitivity to damping of deeper layers while the fundamental mode loses sensitivity. Future work will address robustness to velocity uncertainty and explore waveform-based misfits motivated by the observed amplitude sensitivity.