Study On Near-surface LNAPL Pollution By GPR Method

Groundwater is a precious resource for human survival and the guarantee of water security is the premise of human health.However,due to the advanced industrial construction of human beings,which brings convenience and progress to human beings,but also causes a lot of serious damage to the ecological environment,the prevention and control of groundwater pollution has now become a common problem facing human beings.It is not suitable to use well sampling method for groundwater detection,which is not only expensive but also damages the formation structure.GPR as a mature geophysical detection tool,can be used for real-time monitoring of groundwater,and the Light non-aqueous phase contaminants detection.The ability of ground penetrating radar is required more and more,and the physical parameters of hydrogeology are expected to be obtained to make quantitative interpretation and analysis of groundwater and pollution.In this paper,we based on the multi-phase flow theory to get a percolation and migration model of LNAPL pollutants in soil,and the saturation distribution of hydrological parameters of LNAPL pollution migration is obtained.Then,the dielectric constant model is obtained by converting the hydrological parameters into the dielectric constant through the obtained dielectric model.Then,based on the finite difference time-domain method(FDTD),the ground penetrating radar response characteristics of LNAPL contaminated area are obtained and analyzed.Wave impedance inversion of GPR can be used to obtain the impedance parameters of strata.LNAPL pollution data are processed by wave impedance inversion method,including random media forward data and measured data.Impedance inversion accuracy of damped least square method and coefficient deconvolution method based on L1 norm is compared.Then,the impedance parameters can be converted into dielectric constant and water saturation information to estimate the location and distribution range of underground LNAPL pollution.After analyzing the research status of LNAPL pollution at home and abroad in the past two decades,it is found that the establishment of a good forward model is the basis,and the interpretation of physical parameters of medium obtained through parameter inversion is the trend.Therefore,this paper based on the theory of multiphase flow of light non-aqueous phase leakage of pollutant migration model,to get the saturation and hydrological parameters of the oil pollution model.then through the experience formula of the hydrological parameters and dielectric constant to get dielectric constant model,then,by using finite difference method(FDTD)for GPR forward modeling to get good reflection profile of oil pollution.We use impedance inversion method for data processing The impedance inversion accuracy of damped least square method and coefficient deconvolution method based on L1 norm is compared.Finally,the information of permittivity and water saturation can be used to determine the location and range of underground oil pollution.In this paper,the application of GPR in LNAPL pollutant detection is introduced from four aspects: establishment of hydrological model of oil pollution,GPR forward modeling,GPR data preprocessing and wave impedance inversion interpretation.It provides a more realistic model for GPR forward modeling and an effective theoretical basis for obtaining physical parameters of hydrogeology.