Rayleigh-wave imaging of the loess sediments in the southern margin of the Ordos Basin by improved frequency–wavenumber transform

The southern margin of the Ordos Basin is covered by thick loess sediments. This area is typically characterized by frequent geologic hazards and is rich in energy resources. Identifying the velocity structure of the loess layer is of great significance for geologic hazard prevention and resource explorations in this area. In this paper, we utilize Rayleigh-wave data for imaging the shear-wave velocity (v(s)) of the loess sediments in the Dongjiahe area, in the southern margin of the Ordos Basin. The data collected in the Dongjiahe area is dominated by the higher-order mode Rayleigh-wave; the dispersive energy of the fundamental mode Rayleigh-wave is weak and suppressed in some frequency bands. An improved frequency-wavenumber (F-K) transform method which can enhance the dispersive energy and expand the frequency range of the weaker fundamental (or higher) mode Rayleigh-wave is proposed in this paper. Numerically simulated data are first utilized to show the processes and effectiveness of the improved F-K transform method. Then, we utilize the improved F-K transform to generate multi-mode dispersive spectrums and extract dispersion curves of the real-world data collected in the Dongjiahe area. Dispersion curves are inverted for v(s) structure using the Levenberg-Marquardt method. The pattern of the two-dimensional v(s) section of the study area indicates that the loess sediments mainly distributes as approximate flat layers with v(s) gradually increasing as the depth increases. In addition, the results also reveal the bedrock depth of some parts of the survey line.