Estimation of near-surface shear-wave velocity using multichannel analysis of Love waves (MALW)

摘要

As theory dictates, for a series of horizontal layers, a pure, plane SH wave refracts and reflects only SH waves, and does not undergo wavetype conversion as do incident P or Sv waves. This is why the shallow SH-wave refraction method is popular, abundant SH-wave refraction data were acquired, and usually works successfully in defining near-surface shear-wave velocities. The information used in the SH-wave refraction method is the first arrivals. Most SH-wave data also contain strong Love-wave energy. High-frequency Love waves are a type of surface wave formed by the constructive interference of multiple reflections of SH waves in the shallow subsurface. Unlike Rayleigh waves, the dispersion of Love waves is independent of P-wave velocity. Love-wave phase velocity of a layered earth model is a function of frequency and three groups of earth properties: SHwave velocity, density, and thickness of layers. In theory, a fewer parameters make the inversion of Love waves more stable and reduce the degree of nonuniqueness. Analyzing SH-wave data using Love-wave inversion for near-surface applications may attract as much attention as Rayleigh-wave inversion because it provides SH-wave velocities that are critical for anisotropy analysis. In this paper, the same inversion algorithm used for Rayleigh-wave analysis is applied to Love-wave data. Compared to Rayleigh waves, fewer unknowns in multichannel analysis of Love waves (MALW), in theory, make dispersion curves of Love waves are simpler, which leads to more stable inversion of Love waves and reduces the degree of nonuniqueness, and also because of being independent of P-wave velocity, "mode crossing" in an image of Love-wave energy is less common. Owing to a long geophone spread commonly used in SH-wave refraction survey, an image of Love-wave energy is clean and sharp, which makes picking phase velocities of Love waves much easier and more accurate. Real-world examples demonstrate the success of reprocessing SHwave data using Love-wave analysis.