Active and passive source Rayleigh wave joint imaging of the shallow structure in the Caotan Camp area, southwestern Ordos Basin

Abstract

The shallow surface of the southern Ordos Basin is a thick Quaternary loess layer with a loose and unstable structure, which threatens the stability of local infrastructure and has a great impact on oil and gas seismic exploration in this area. Therefore, identifying the internal velocity structure of the loess sediments is of critical importance. In this study, joint imaging of active and passive source Rayleigh waves was utilized to reveal the shallow shear-wave velocity (vs) structure in the Caotan Camp area, southwestern margin of the Ordos Basin. Active source Rayleigh waves were recorded by long-spread geophone array and processed by an improved F–K transform algorithm to extract multi-mode Rayleigh-wave dispersion curves. The passive source seismic signals were collected by broadband seismographs, and the phase velocities at discrete frequencies of the passive source Rayleigh wave were calculated by Aki's spectral formula. A nested genetic and damped least squares (DLS) algorithm was used for the joint inversion of the active and passive source Rayleigh wave dispersion curves. The two-dimensional vs section under the survey line reveals the thickness and stratification of loess sediments in the study area. The joint imaging of the active and passive source Rayleigh wave also effectively increases the investigation depth, allowing to obtain the bedrock interface below the loess sediments.