Caveats in Multi-modal Inversion of Seismic Surface Wavefields

Abstract

We consider several examples demonstrating that the formal modal representation of surface wavefields often does not describe adequately observable wave parameters, such as the phase and group velocity dispersion of higher modes. The main reason for this is the existence in the medium of several waveguides or weakly coupled wavefields in the same waveguide. In such cases the separation of neighboring higher modes may be impossible, and observed dispersion curves may significantly differ from the ones predicted by the theory. From the example related to the studies of the crustal and upper mantle structure we found that the difficulty in the separation of first and second crustal higher modes can be overcome by applying a special inversion procedure. This procedure ignores the existence of a low velocity layer in the upper mantle when fitting the observable higher-mode dispersion curve to the one predicted by the model.

Shear‐wave velocity profiling at sites with high stiffness contrasts: a comparison between invasive and non‐invasive methods

“…The purpose is to mimic properties shown by the real data examples that will be introduced in the next section. With reference to Table , the earth model A05 is a four layer model after Asten et al (), the earth model LP06 is a two layer model after Levshin and Panza (), while the earth model L02 is a six layer model after Liang et al ().…”

“…In the case of high stiffness contrasts (Park et al ), fundamental mode dominance may be restricted to certain frequency ranges only and at normal sites with velocity increasing with depth the assumption that observed dispersion is dominated by the fundamental mode at all frequencies should be taken with caution. In contrast, in the case of velocity inversions, higher mode dominance is more frequently recognized (Forbriger ) and it is often difficult to accomplish satisfactory resolution for layers below the low velocity layer (Liang et al ), a problem that is well‐known in seismology (i.e., the ‘channel waves’ effect) due to low velocity zones in the upper mantle (Levshin and Panza ).…”

Addressing non‐uniqueness in linearized multichannel surface wave inversion

“…As is known, modal phase velocities are the only possible solutions of the Rayleigh‐wave eigenvalue problem. Particularly, in the presence of low‐velocity zones or velocity inversions (Kerry 1981; O'Neill et al 2003; Levshin and Panza 2006), higher modes may propagate with more energy than the fundamental mode and become dominant.…”

Multi‐channel analysis of surface waves (MASW) of models with high shear‐wave velocity contrast

“…Such higher-mode domination was demonstrated using models with high shear-wave velocity contrasts (Levshin and Panza, 2006;Cercato et al, 2010). One of the models suggested by Levshin and Panza (2006) consisted of two layers (Table 1) and had high contrast for both shear (V s ) and compressional (V p ) wave velocities. The 2-layer model parameters were used by two different modeling algorithms.…”

“…However, some researchers have reported that at some sites higher-mode energy can dominate over the fundamental mode at the low-frequency end of the spectrum (Park et al, 2005b, Ivanov et al, 2009. Such higher-mode domination was demonstrated using models with high shear-wave velocity contrasts (Levshin and Panza, 2006;Cercato et al, 2010). One of the models suggested by Levshin and Panza (2006) consisted of two layers (Table 1) and had high contrast for both shear (V s ) and compressional (V p ) wave velocities.…”