2M‐SASW: Multifold multichannel seismic inversion of local dispersion of Rayleigh waves in laterally heterogeneous subsurfaces: application to the Super‐Sauze earthflow, France

Abstract

The approach presented here deals with the inversion of Rayleigh wave dispersion (SASW: Spectral Analysis of Surface Waves) in laterally heterogeneous media. The traditional method consists of computing the frequency versus phase‐velocity curve, known as the dispersion curve, from seismic records, then inverting to obtain a 1D shear‐wave velocity model. When media are laterally heterogeneous, S‐wave velocity changes lead to phase variations in Rayleigh waves in the offset dimension. This phenomenon can drastically alter the dispersion image, since it affects the local slopes of Rayleigh waves in the shot gather, and thus their phase‐velocity dispersion properties. In the case of multifold acquisitions (2M‐SASW: Multifold and Multichannel SASW), we redefine the manner in which the dispersion‐image calculation is formulated in order to compute the local dispersion stack (LDS). In principle, this consists of gathering receivers within a restricted window for a series of shots and computing the local dispersion images that are then stacked in order to improve the signal‐to‐noise ratio. Even though local dispersion diagrams are noisy because they are computed from a limited number of traces, the summation over multiple dispersion images enables the quality of the final LDS to be improved. Because it is related to the dispersion properties of the windowed wavefield, the LDS is an efficient input for the 1D inversion process. The method is tested on synthetic data to demonstrate its contribution compared to that of the traditional SASW technique. An application of the 2M‐SASW method to the Super‐Sauze earthflow confirms that is well‐suited to inverting the shear‐wave velocity from Rayleigh‐wave dispersion when high‐contrast media are considered.