Numerical Studies of Effects of Dipping Structures on Horizontal/Vertical Spectral Ratios

Abstract

Shallow S-wave velocity structures commonly have been estimated from phase velocity curves and horizontal to vertical (H/V) spectral ratios in microtremor exploration techniques. However, such estimations assume that underground structures are horizontally stratified. Therefore, if the structures were irregular instead, the estimations would have errors owing to perturbations in the wavefield from the irregular boundary. In this study, we examined the influence of the dipping structure on the observed wavefield and the H/V spectral ratios based on numerical calculations. As a result, we observed that the peak frequency in the H/V spectral ratios shifted toward lower frequencies along the dipping structure as the structure deepened. We also observed a complex amplitude change in the H/V spectral ratios along the dipping structure. The peak became obscure near and on the dipping structure. It was likely that the waves reflected within one wavelength of the issued receiver were likely to have significant effects on the observed wavefield and to disturb the H/V spectral ratios according to a sensitivity analysis. Our result indicates that dipping structures could be estimated by the peak shift in H/V spectral ratios derived from a spatially dense observation. However, it should be noted that the peaks sometimes become obscure due to the wavefield perturbation from the dipping structure mainly within one wavelength.