Forward And Inversion Study Of Induced Polarization Method For Detection Of Cr Heavy Metal Pollution Soil

With the acceleration of urbanization and the adjustment of industrial structure,the problem of soil heavy metal pollution has become increasingly prominent.Therefore,it is imperative to study accurate and effective rapid detection technology of heavy metal pollution areas.Traditional geochemical exploration methods for heavy metal pollution detection are inefficient and costly,and can only be used for qualitative or semi-quantitative analysis.With the development of geophysical exploration technology,the application of electrical exploration technology has been extended to the detection of heavy metal soil pollution,among which the DC resistivity method is widely used.The parameters of DC resistivity method are single and affected by many uncertain factors,so its accuracy and reliability are limited.Induced polarization method can be used to detect the pollution range and degree of heavy metal soil by detecting the multi-parameter data of IP effect according to the electrochemical difference between the polluted area and the unpolluted area.Therefore,this thesis takes Cr heavy metal contaminated sites as the research object,combines the distribution of contaminated sites and aims to quickly and accurately identify the pollution degree and pollution range,and carries out forward and inverse simulation of DC IP method and Dual-frequency IP method for detecting heavy metal soil pollution.(1)Using Finite element method to analyze the boundary value and variational problem of two-dimensional geoelectric field model of point source.The potential value of each node in semi-uniform space is solved based on COMSOL software.The accuracy of numerical solution and analytical solution are compared and analyzed to verify the reliability of forward simulation.(2)According to the basic equation of water flow movement and solute migration and transformation mechanism,using COMSOL software to establish a two-dimensional model of typical chromium heavy metal pollution field.By changing the pollutant concentration,rainfall,porosity and other parameters,the migration and change of pollutants under different working conditions is obtained.(3)The Schlumberger,Pole-Dipole and Dipole-Dipole arrays are used to simulate the chromium pollution field by DC IP method,and the smooth constrained least square method is used to invert the forward data.Analyzing the abnormal response characteristics of apparent resistivity and apparent polarization parameters to research the accuracy and existing problems of the inversion results of the three arrays.After adding voltage noise into the forward simulation data,the inversion is carried out.By comparing the error of apparent resistivity and apparent polarization with the real model,it is concluded that the symmetrical quadrupole device has better anti-noise performance.(4)By changing the parameters of Cole-Cole model,the amplitude,phase characteristics of the model are analyzed.Using Dual frequency IP method to simulate the electric sounding and forward modeling of the electric profile in the Schlumberger array.The zero frequency resistivity and polarization are selected as the inversion parameters under the least square theory to verify the feasibility of the inversion algorithm.