Detecting lateral inhomogeneity using a 3D Rayleigh wave survey based on numerical simulation and on-site experiment

Abstract

To deal with the narrow field limitation for the 3D lateral inhomogeneity investigation in the geotechnical engineering, or in urban areas, the paper proposes a radial line survey grid in a cylindrical coordinate system for the Rayleigh wave method. Based upon the cylinder wave features of the wave and the Huygens' principle, the wavefield of the Rayleigh wave recorded by the survey grid can be regarded as a complete 3D wavefield record excited only by one virtual source at the origin, if the energy attenuation is discarded, and the effects of the multiple sources with the conventional 3D survey grid in a rectangular coordinate system are removed. The numerical method simulates the 3D wavefield for homogeneous half-space model and lateral inhomogeneity half-space model (represented by a cavity). The wavefronts of the 3D seismic records with the grid for the numerical models are imaged and the 3D dispersion features of the velocity and ellipticity of the Rayleigh wave are analyzed. Following these, an on-site experiment is carried out using the grid for a 3D detection of a sewage well. The results showed that the 3D seismic records in the far-source region acquired by the survey grid, can directly image the wavefront features of the Rayleigh wave and its distortion by the lateral inhomogeneity in a 3D domain. They also show that the anomalies of the 3D velocity and ellipticity dispersion of the local inhomogeneity are similar to that of water waves are scattered by a solid obstacle, which makes it much easier to recognize a cavity compared to that using the complex dispersion anomalies in a 2D profile or the conventional 3D survey grid in a rectangular system. Moreover, the grid can greatly reduce the work-cost of a 3D survey and be adapted well to narrow areas if the excited source is set at the origin of the radial lines.