Analysis And Dielectric Parameters Of The Layered Medium Half-space Rayleigh Wave Mode Inversion

It is a topic in the fields of the seismic exploration, geo-technical engineering and the ultrasonic detection to inverse the properties of the stratified media in half space by using the Rayleigh waves. The propagation modes and dispersion curves of the Rayleigh waves are presented based on the analysis of the experimental data in stratified half-space media. The parameters of the stratified media are inversed by using the genetic algorithm (GA) based on the fundamental and/or higher modes of the Rayleigh waves observed. In this text, the numerical simulation and the experimental analysis are investigated in detail.In our numerical simulation, the Rayleigh waves were excited by the explosion of a point source. This method is widely used in the seismic exploration. The dispersion of the Rayleigh waves of the stratified media was analyzed with the method of the frequency-wavenumber (f-k) analysis. The observed dispersion curves were affected by the number and spacing of the receivers used, and the distance between the point source and the first receiver. Certain quantitative rules for these spacing and distance are given. It was shown that dispersion curves obtained using f-k analysis agreed with those mode-jumping dispersion curves derived from the change of the excited intensity dominated mode with the frequency. For the stratified media with S-wave velocity increment from the top to the bottom, it is good enough only to consider the fundamental mode in inversion. But for the stratified media with the low-velocity layer in between, the zigzag dispersion curves caused by the mode jumping must be considered. In the inversing the stratified media, the objective functions in the GA are selected properly also based on the relation of the excited intensity denominated modes to the frequency and the correct inversion results can be obtained.In our ultrasonic experiment, the Rayleigh waves were generated by a normal force disk source on the surface. Three stratified half-space models(homogeneous half-space, two-layered media with the S-wave velocity increment and the three-layered media with a middle layer of low-velocity) are investigated. All experiments were carried out in the digitalized multi-channel transceiver system developed by our laboratory. After the experimental data of the Rayleigh waves were processed by using the f-k analysis, the correct dispersion curves were obtained in good agreement with those from the theoretical prediction. And the parameters inversed by using the GA are in agreement with those of real media.