Rayleigh Wave Scattering By Underground Heterogeneity

The problem of underground heterogeneity is unavoidable in practical engineering.Unfavorable geologic body will affect building safety.In the course of construction,some special underground works need to be protected.Therefore,the detection of the underground heterogeneity is of great significance to the safety of the engineering quality and to the daily life.There are many ways to detect underground heterogeneity,but most of the methods are limited by many factors such as site environment.Rayleigh wave method has the characteristics of economy,portability and wide range of application.It is an economical and effective method for detecting underground heterogeneity.The excitation of the focal source on the surface of the medium produces a large energy Rayleigh wave in the shallow layer of the medium,and its energy is mainly concentrated in a wavelength range.The phenomenon of scattering will occur when Rayleigh waves meet with underground heterogeneity.The scattering wave causes the wave field to disturb and the characteristics of the wave energy propagation will also change.The analysis of the heterogeneity in the medium can be analyzed by the theory of scattering wave that is very trouble and inability to express heterogeneity.It is necessary to establish a two-dimensional axisymmetric model by using the numerical simulation method to analyze the scattering of the Rayleigh wave.The correlation between Rayleigh wave scattering and underground heterogeneity is studied by numerical simulation,and the relevant laws of Rayleigh wave detecting underground heterogeneity are obtained.Meanwhile,relevant field experiments are carried out to verify the conclusion of numerical simulation.The main research results are as follows:(1)When there is a heterogeneity,the encounter of the Rayleigh wave with the heterogeneity will be scattered,and the scattering wave leads to the disturbance of the energy in the surface wave field.It is demonstrated that the Rayleigh wave scattering phenomenon can be used to detect the heterogeneity in the numerical simulation method,and the shape of the heterogeneous body can be identified by the perturbation of spectral energy.(2)The influence of the depth,height and width of the underground heterogeneity in the scattering wave field is studied.The various wave components,propagation characteristics and wave mode transformation mechanism of Rayleigh wave scattering with different depth,height and width are discussed.By the regression analysis,the ratio of the wavelength of the buried depth to the disturbance is about 3/2.(3)The study of the influence of the scattering wave field on the geometric change of the underground heterogeneity.The geometric shape changes of the heterogeneity are divided into two cases: inflection point position change and no inflection point.A normal rhombus and a circle are used to simulate the underground heterogeneity.The study shows that the inflection point position change and no inflection point has no effect on the heterogeneity detection.(4)The sensitivity of the variation of physical parameters(density,Poisson's ratio,shear wave velocity)to the scattering wave field of a underground heterogeneity.Through the vertical particle velocity time history,whether the scattering energy the energy size can be identified,we get the relative sensitive amplitude of all physical parameters,and then get the main sensitive parameters.(5)Precast concrete slab cubes at the site of known geological conditions are used as underground heterogeneity,which are detected using the Rayleigh wave method.The feasibility of the field test results is verified by comparing with the numerical simulation results,The feasibility of theory and numerical calculation is verified by comparing with the numerical simulation results and field experiment results.The accuracy is verified comparing with field experiment data and Actual location of underground heterogeneity.