Near-surface anisotropic structure characterization by Love wave inversion for assessing ground conditions in Urban Areas

Abstract

In many geophysical applications, neglecting of anisotropy is somehow an oversimplification. The mismatch between prediction based on isotropic theory and near-surface seismic observations indicates the need for the inclusion of medium anisotropy. In this paper, surface wave (Love wave) dispersion properties are used to estimate the anisotropic structure of the near-surface layered earth, which is modeled as media possess vertical transverse isotropy (VTI), a reasonable assumption for near-surface sedimentary layers. Our approach utilizes multi-mode surface waves to estimate both the velocity structure and the anisotropy structure. This approach consists of three parts. First, the dispersion analysis is used to extract dispersion curves from real data. Second, the forward modeling is carried out based on the dispersion equation of Love wave in a multi-layered VTI medium. Dispersion curves of multi-modes, which are the numerical solutions of the dispersion equation, are obtained by a graphic-based method. Finally, the very fast simulated annealing (VFSA) algorithm is used to invert velocity structure and anisotropy structure simultaneously. Our approach is verified by the synthetic dispersion curve generated by a VTI medium model. The estimation of shear wave velocity and anisotropy structure of surface wave data acquired at Rentschler Field, an urban center site on sediments in the Connecticut River valley reveals a simple structure of the sediment layer over a bedrock half space. The results are verified by other inversion results based on different data set obtained on the same site. The consistency of inversion results shows the feasibility and efficiency of the approach.