Dispersion energy analysis of Rayleigh and Love waves using finite-difference modeling

Abstract

Surface-wave analysis methods have been effectively and widely used to determine shear (S) wave velocity. To separate and identify different dispersive modes of surface waves accurately is one of key steps using surface-wave methods. We analyze the dispersion energy of Rayleigh and Love waves based on staggered-grid finite-difference modeling in 2D isotropic elastic media with horizontally homogeneous layered models. Results demonstrate that when there is a low velocity layer (LVL) in an earth model, LVL-guided waves will generate and possess energy on dispersive images, which can interfere with the dispersion energy of Rayleigh or Love waves. The dispersive energy looks like jumping from the fundamental mode to higher modes on dispersive images because of dispersion curves (whatever the fundamental or higher modes) of LVL-guided waves being lack of energy in a high-frequency range. If the S- wave velocity of the LVL is higher than the surface layer, the energy of LVL-guided waves only contaminates higher mode energy of surface waves and there is no interlacement with the fundamental mode of surface waves. While if the S-wave velocity of the LVL is lower than the surface layer, the energy of LVL- guided waves may interlace with the fundamental mode of surface waves. This may cause misidentification for the fundamental dispersion curve of surface waves and produce significant errors in the inversion.