Inversion of Rayleigh-wave ellipticity for shallow strong-velocity-contrast structures

Abstract

Rayleigh-wave ellipticity polarization, or horizontal-to-vertical ([Formula: see text]) spectral ratio, is particularly sensitive to shallow earth model parameters (e.g., S-wave velocity). We develop a novel Rayleigh-wave [Formula: see text] inversion method to delineate near-surface strong-velocity-contrast structures. In the new inversion method, the S-wave velocity model is perturbed until the synthetic Rayleigh-wave [Formula: see text] curve is fitted to the observed one based on the crosscorrelation misfit function. This criterion neglects the absolute amplitude of the observed and synthetic [Formula: see text] curves, which can eliminate the amplitude influence caused by inconsistent coupling of the horizontal and vertical geophones. Peaks and/or troughs in the [Formula: see text] curve are inverted with no need of picking. Numerical simulations indicate that the proposed [Formula: see text] inversion succeeds for models consisting of unconsolidated sediments overlying stiff bedrock with large velocity contrasts. We also find that the active Rayleigh-wave ellipticity estimation and inversion can be carried out for reconstructing pseudo-2D laterally varying models. Application to a real-world example suggests that a 2D model above 6 m in depth with large velocity contrasts is successfully reconstructed from Rayleigh-wave ellipticity data. This method provides an effective and efficient alternative for near-surface characterization.