Study On The Measurement And Inversion Of The Speed Structure Of Agricultural Saturated Soil

Agricultural saturated soil is the soil with water content greater than 85%,which accounts for 5.56 % of agricultural land area.The velocity structure of agricultural saturated soil reflects the distribution of soil thickness and shear wave velocity,which directly affects the vibration characteristics of the site.It is an important basis for the vibration design and seismic design of facilities agricultural structure,farmland irrigation project and large agricultural equipment.Rayleigh wave exploration is a kind of non-damage detection method,has been widely used in the field of geotechnical exploration,detection and other fields.The Rayleigh wave dispersion curve of the site is obtained by observing the Micro-tremor signal through the surface array.As the aim,the shear wave velocity structure of the site is obtained by genetic algorithm and least squares method.In order to apply the Rayleigh wave exploration method to the agricultural engineering field and find the inversion method suitable for the speed structure of agricultural saturated soil,Daqing Longfeng wetland is chosen as observation site.The Micro-tremor signal is acquired by the surface array,and then the Rayleigh wave dispersion curve of the observation site is extracted by frequency-wave number method.On the basis of verifying the two forward calculation methods,the sensitivity of the model parameters is analyzed.The genetic algorithm and the least squares method are used to invert the shear wave velocity structure of the observation site,and the result of the inversion are compared with the simulation results.The following work were operated in this paper:(1)The observation instruments,observation arrays and the basic principle of the frequency-wave number method were introduced.The Rayleigh wave dispersion curve of observation site was obtained by calculating the frequency-wave number,which provides the basis for inversion of shear wave velocity.(2)In order to find the exact method of calculating the Rayleigh wave dispersion curve,the Knopoff method and the scalar transfer method are used to calculate the fundamental order and four high order Rayleigh wave dispersion curve of the three typical geological models.The Knopoff method and the scalar transfer method are compared to ensure the forward method is correct.Shear wave velocity,longitudinal wave velocity,soil thickness,soil density and other parameters will have a certain impact on the dispersion curve.In order to provide the theoretical basis for the follow-up inversion,Knopoff method was adopted to analyze the sensitivity of the above four factors.(3)The shear wave velocity structure of the site is inverted by using the genetic algorithm and Knopoff method with the objective function of the frequency-wave number method.The genetic algorithm is used to simulate the three typical geological models,and the genetic algorithms is verified.The shear wave velocity structure of the twenty-layer soil layer and the shear wave velocity structure of the four-layer soil layer are obtained by using the genetic algorithm to invert the measured sites.(4)The three typical geological models is inverted by the least squares method,and the least squares method is verified.The shear wave velocity structure of the four-layer soil layer is obtained by using the least squares method.