A fast inversion analysis algorithm for the spectral analysis of surface wave (SASW) method

Abstract

Spectral Analysis for Surface Wave (SASW) is a widely practiced NDT method due to its ability to identify the shear velocity profile of subsurface layers. However, the SASW method is limited to point-to-point inspection because all data has to go through an inversion process, which is iterative and manual. Some automated iteration techniques were developed to improve the efficiency of inversion analysis. These attempts did not change the situation much because they were still based on the guess-and-check procedure incorporated with a forward analysis. In this paper, a new inversion analysis algorithm is proposed to estimate the shear velocity profile rapidly without performing conventional forward analysis. Unlike conventionally determining the dispersion curve with a stiffness matrix or something similar, the dispersion curve of a layered structure is assumed to be a weighted combination of the shear velocity profile. The weighting factors are determined according to the variation of particle displacement with depth for a specified wavelength of surface wave. Based on this assumption, a fast inversion algorithm is established to estimate the shear velocity profile from a given dispersion curve. No prior knowledge of the test site or personal expertise is needed because this method does not require the initial values of the layer depths and shear velocities. This new method allows the SASW method to be a fully automatic or even real-time reporting method for highway pavement detection. The accuracy of this fast inversion algorithm is verified by comparing the results to those of the conventional algorithm.