Road Radar Electromagnetic Wave Finite Difference Time Domain Method To Simulate

Ground Penetrating Radar (GPR) systems are new type non-destructive tools using ultra-wide band antenna technique for detecting the structure of layers and objects buried in the earth. GPR special application in road is a system for testing road's thickness, voids, moisture, density etc. GPR has been widely used in road design, construction and maintenance, due to its speed, nondestructive qualities the capacity of providing continuous profile and high-resolution imaging ability. Since 1980s, its application has also become a vital technology in the worldwide. However at present, most of the achievements mainly focus on the measurement of pavement thickness. The current GPR data analyzing method for pavement thickness can only give the acceptable results for new asphalt concrete pavement, but for aged asphalt concrete pavement and cement concrete pavement, the accuracy is not satisfactory because of the theoretic modeling and data acquisition difficulties. The study on pavement compaction, moisture content and asphalt content is under exploration and the program for processing GPR data based on rigorous theoretic model cannot be found in literature.Finite-Difference Time-Domain(FDTD) method was first applied in the successfully simulation of PEC(Perfect Electric Conduct) by K.S yee in 1966. During the past decade, the FDTD method has become very popular due to its versatility in solving problems involving EM (Electromagnetic) wave transmitting, scattering and the analysis of antenna arrays. Additional advantages FDTD possesses over the other numerical methods includes:. inhomogeneous media is inherently modeled. lossy, non-liner and anisotropic media supported. boundary conditions are inherently represented. computer time and storage savedThis paper introduces the basic theory of FDTD by analyzing the characteristics of electromagnetic wave propagation in pavement, especially from 1-dimension FDTDequation, soft source, effective parameter of layered boundary and Absorbing Boundary Condition(ABC). These four aspects have the greatest significance in FDTD simulation of GPR. Then a forward air coupling GPR model is established. Moreover, the validity of this forward model is examined through several simulated and actual cases. This model develops a new concept for practical and accurate interpretation of GPR reflection data.The main achievements and conclusions of this paper are summarized below:1. The forward model of GPR electromagnetic wave propagation in pavement system is developed based on Maxwell Equation in electromagnetic theory. Then focused on four aspects of simulation: 1-dimension FDTD equation; useful soft source excitation; effective parameter of layered boundary (using the average of the parameters along the two sides of the interface) and 1-dimension Mur's Absorbing Boundary Condition(ABC).2. The dispersive characteristic of permittivity is analyzed in this paper. Based on the analysis of Debye model and suitable formulation method, the FDTD forward model in dispersive media is established, which makes the model is suitable applied not only innew asphalt but in aged concrete or asphalt pavement system.3. Analyzing how dispersive media characteristics influence EM wave propagation.The EM wave transmitting in weak dispersive and strong dispersive media is simulated in this paper. Through this case, we can see r and ers - s^ greatly influence the shapeof reflected wave and velocity of EM wave. Especially up to the certain depth, the reflected wave even cannot be recognized.4. Using this forward model, different kinds of pavement diseases(voids> highly moisture > incompact base) has been analyzed in this paper. With GPR, we can identify the air voids and incompact base problems through reflected waveform based on this analysis, which has the great engineering significance in road repair and maintenanceFDTD method based on 1-d Yee's algorithm was implemented in the GPR simulation. The results show the possibility and the accuracy of FDTD simulation of GPR in road system. The importance of this work is it settles the groundwork forsimulation of GPR in pavement system. So the future work will focus on the backcalculation of electromagnetic properties and thickness of pavement's layer from GPR data, which can develop a new algorithm to calculate the thickness of pavement's layer, and even develop a new way to measure the moisture and density of pavement's layer in non-destructive testing.