FDTD Method For Lossy,Dispersive Stratified Earth With Applications To Impulse Ground Penetrating Radar

Impulse ground penetrating radar is an important and phasic achievement of the development of transient electromagnetic (EM) theories and a result of a combination of time- domain measurement technique, nanosecond pulse generation technique, ultra-wide band antenna technique and signal processing methodology. It has many advantages such as nondestructive detection, high efficiency, low cost and high resolution imaging ability and has already become one of the most important tools in subsurface exploration. Also, it is a successful example for the applications of transient electromagnetic theory in many other areas such as ultra-wide band radar, ultra-wide band communications and so on. Yet, if one has an experience about the method, he will see many problems left for further investigations. For example, one can not tell the location, the constituent material and the size about an unknown target with an acceptable accuracy and exactness just by use of field test data. This arises from lack of knowledge about the transient EM interactions between radar antenna, inhomogeneous soil, and buried targets. Thus, a thorough and detail understanding of the physical rules about transient EM interactions in inhomogeneous soil is of great importance for solving these tough problems. It is prospective to extract as much valuable information as possible from the reflected radar returns based on the knowledge. The main purpose of the study presented in the paper is to reveal the rules of the transient EM scattering in this situation and to make a preparation for the establishment of simulation tools for impulse ground penetrating radar systems by using the methods of computational electromagnetics. Compared with other numerical tools, the finite-difference finite-time method (the FDTD method) is the most appropriate choice for the analysis of the transient EM interactions between the antennas, the lossy, dispersive, inhomogeneous soil, and the target(s). We begin with a careful check about the suitability, the method of implementations, and the performances of the algorithms. Some important results are obtained in the research work. The main contributions of the paper can be summarized as follows: 1. An analytical method of transient incident wave source conditions for total-scattered field formulation in stratified media is introduced using classical reflection and transmission relations under a time-harmonic incident plane wave and the fast Fourier transform (FFT) technique. The codes are confirmed at a variety of incident angles, both in transient field and in time harmonic field. The results show that the errors at the contiguous boundary due to this method are less than 3x101 relative to the magnitude of the incident field. 2. A method of the complex dielectric curve fitting from measured data based on minimum least square method and the first order Debye model is presented, which is validated by a group of typical soil conditions covering a frequency range of 5OMHz-l250 MHz. The result shows that the approximations are very good. The resulting parameters are then used in the investigation for the pulse propagating in dispersive media. 3. The performances of the Mur抯 second order absorption conditions, the GPML layer for lossy media, and the modified PML layer for dispersive media are investigated with special concern about the implementations for horizontal stratified media. The error distributions at the corners and at different media interfaces are explored. From theoretical derivations and numerical computations, one...