A Study To Rayleigh Surface Wave Field Propagating Along The Surface Of Half-space With Layered Media By Physical Simulation

At present,the study of Rayleigh waves dispersion in layered media is mainly focused on numerical simulation and field experiments,but few on physical simulation.The physical simulation of wave field characteristics of surface waves in layered media will provide a more complete theoretical support for the application of surface waves in engineering nondestructive testing(NDT).In this dissertation,physical simulation research methods are used to study the propagation characteristics of seismic wave fields and Rayleigh surface wave systems for layered media models.The main contents are the following aspects:(1)Understand the basic theory of Rayleigh waves.The characteristics of velocity dispersion and elliptical polarization dispersion of Rayleigh surface waves,the extraction method of velocity dispersion curve,the calculation and extraction method of elliptical polarization dispersion are introduced in detail.(2)The design of the physical model experiment system and the two-dimensional physical models.This part mainly describes the ultrasonic waves can simulate the elastic wave by introducing the generation and characteristics of the ultrasonic waves,and introduces the similar law and the physical simulation test system that the physical simulation needs to comply with,the experimental model material.Then according to the first Fresnel radius theory with the limitation of observation conditions,the thickness of the model is studied to determine the thickness of the two-dimensional experimental model,which provides a theoretical basis for physical simulation(3)Study on physical simulation of Rayleigh wave velocity dispersion and elliptical polarization dispersion in homogeneous media.The physical simulation experiment of two-dimensional model of homogeneous half-space medium is carried out.By observing and analyzing the propagation characteristics of Rayleigh waves in homogeneous medium,and comparing with the theoretical analysis result,we can verify whether the experimental results are consistent with the theoretical analysis results.The feasibility of the experimental system and the experimental method is explained.(4)Study on dispersion characteristics of Rayleigh Surface Wave Velocity in horizontal layered Media.Two kinds of horizontal layered media models are made by selecting suitable experimental materials,one is a two-layer horizontal layered medium model with low velocity of upper medium and high velocity of lower layer.One is that the upper layer has a high velocity and the lower layer has a low velocity.Under these two geological models,three sets of models are made according to the thickness of overlying layers,and then the three sets of models under these two geological models are simulated experimentally tostudy refraction waves.The influence of reflected waves on dispersion characteristics.Through the research above-mentioned,the main conclusions obtained as follow:(1)The physical simulation results of two-layer layered media model with uniform half-space and different thickness show that in the Rayleigh wave physical simulation experiment,the thin plate model with thickness less than the first Fresnel band radius is used as a two-dimensional physical model.It is credible and feasible.Compared with the quasi-two-dimensional model with three-dimensional scale,the thin plate two-dimensional model has much higher modeling efficiency and lower modeling cost.(2)Physical simulation experiments show that for low-velocity layered media,the energy of the refracted shear wave is strong,and the constraint of the geological model is very important when it is based on the maximum energy pick-up velocity dispersion curve.In addition,the interference effect between the refracted shear wave and the Rayleigh surface wave can lead to local discontinuity in the frequency dispersion distribution of the surface wave energy.For high-speed overlaying layered media,the reflected shear wave has little effect on the Rayleigh surface wave velocity dispersion characteristics,which is mainly reflected in the smoothness or continuity of the Rayleigh surface wave dispersion curve.(3)To study the dispersion of Rayleigh surface wave elliptical polarizability,in order to avoid the interference of body waves,data collection and observation should be performed in far-field or surface-wave dominant areas.