Inverse Analysis Of Dielectric Properties For Layered Inhomogenous Media

Ground Penetrating Radar (GPR) is used as a typical tool of electromagnetic non-destructure testing technologies. Because it is the fast, accurate and non-destructive, GPR has been widely used in geological explore, archaeology, traffic, civil engineering, environment, water conservancy, railway, etc, engineering fields. As the development of pavement in construction and maintenance, the air-launched GPR is widely used in construction quality control and pavement conditions evaluation. Currently, the research methods of GPR application focus on how to obtain the dielectric. Generally, the dielectric are calculated by the waveform amplitude reflected from the interfaces that the dielectric is obviously different between two layers. While the electromagnetic wave transmitting in pavement, it is assumed that the electromagnetic wave attenuation is fixed or ignored. A few methods that back-calculate dielectic constant are presented in half space or multi-layer system. But the model that is used to forecast the dielectric is homogeneous. In fact, many materials that are used in various engineerings are composites, which often are made up of three constituents or phases, such as solid, liquid, gas. The properties in every small space are different as the three phases mixtures vary. So the dielectric that predicted from the amplitude reflected from the interfaces can't represent the properties of a whole layer, and only represent the properties near the interface. The homogeneous predicting model will affect the precision of layer thickness detected from GPR, and it also could restrict the GPR application fields.According to the above, it is presented that the multi-layered inhomogeneous dielectrics are inversely analyzed, and how to use the inhomogeneous dielectric in pavement engineering testing by GPR. It is concluded as the followings.1. The foreward simulation that the electromagnetic wave propagates in multi-layered inhomogeneous media has been carried out.The 1-D and 2-D model of multi-layered inhomogeneous media are established, and the foreward simulations of electromagnetic wave propagates in this media are finished by using Finite Difference of Time Domain (FDTD) method. Regarded the pavement structures which are inhomogeneous as many homogeneous sub-layers, the forward model which is applied in inversion is established. Compared the homogeneous simulation, the inhomogeneous simulation is more close to the actual reflected waveform. 2. The dielectric properties back-calculation for multi-layered inhomogeneous media has been achieved.The dielectric properties back-calculation for multi-layered inhomogeneous has been accomplished by System Identification (SID) method and Genetic Algorithm (GA). But the SID is influenced on the seed dielectrics sometimes, and GA is a time-consuming method. So the GA-SID hybrid method is presented, this new method contains some advantages that it can't be affected on seed electric and it is high efficiency. Comparison analysis that the three methods are used to back-calculate the dielectrics from the same reflected waveform shows the hybrid method is more effective.3. Using the field correlation experiments between the back-calculated dielecteic and the concerned physics parameters, these parameters inside the layers could be detected by electromagnetic wave testing technology.The inhomogeneous dielectric properties are associated with some physics parameters' uniformity inside the layers. If the correlation between the dielectrics and these parameters has been obtained by field experiments, some new testing technologies have been achieved, such as the subgrade moisture testing vertically and horizontally, strength analysis of cement stabilized base, grade estmation of asphalt concrete. These testing technologies are based on the inhomogeneous dielectric analysis of a whole layer, they utilize all information of the reflected waveform besides the amplitude. They can extend the application of GPR.In this paper, it is brought out that the homogeneous dielectric properties can't describe actual material accurately. Based on electromagnetic theory, the model of FDTD that simulate the electromagnetic wave propagate in multi-layered inhomogeneous media is developed. And the three back-calculation methods that are SID, GA and GA-SID hybrid are accomplished to predict the dielectric constant of multi-layered inhomogeneous media. These works essentially find out the basic solution of GPR applications, they sufficiently apply the reflected waveform that connote the information of the material passed by electromagnetic wave. Combined the relationship bwteen the back-calculation dielectric and some physics parameters, the new testing technologies of construction quality control used by GPR are developed, such as base moisture testing, asphalt grade analysis, stabilized base strength survey. They effectively improve the electromagnetic non-destructive test technologies that detect pavement parameters by GPR.