Field investigation of Love waves in near-surface seismology

Abstract

Seismic surface waves have the potential to yield important information about the subsurface. In particular, the spectral analysis of Rayleigh waves has been used to determine near-surface shear-wave velocities that are related to the elastic properties of the media through which they travel. Love waves have properties similar to Rayleigh waves; however, they have not been utilized in near-surface investigations. This study investigates the near-surface conditions, such as near-surface layer thickness and velocity contrast, necessary for producing recordable and usable Love wave data sets. Furthermore, it tests various acquisition parameters for optimum Love-wave survey design. In the frequency range (5-50 Hz) of typical near-surface surface-wave investigations the thickness of the surficial layer must be greater than 1 m to produce usable Love-wave dispersion curves. If layer thickness is less than 1 m the range of frequencies over which dispersion is observable becomes too high to be adequately imaged in field data with current capabilities. Based on this study the most significant factor in selecting survey parameters is the maximum velocity expected. The velocities in the case of a two layer model show that for large velocities the receiver line length must be of impractical lengths to sufficiently sample the wavefield. The theoretical and field data presented in this study show that Love-wave data can be used in near-surface seismology to determine subsurface shear-wave velocities. Future research should be conducted to study the effect of multiple layers on Love-wave data. Extending this case to multiple layers will also be the first step in developing an inversion routine to produce shear-wave velocity profiles.