| ABSTRACT:Ground penetrating radar (GPR) is an important kind of shallow geophysical methods which uses reflection, diffraction and other propagation characteristics of high frequency electromagnetic wave to detect the structure and characteristics of the underground medium, it has been widely used in many areas of engineering survey and nondestructive testing, and showing a strong vitality and broad application prospects.At present, GPR data interpretation is still qualitative and mainly rely on manual judgment and experiences, which is liable likely to produce a false interpretation, there are still many gaps between the high requirement of GPR resolution in engineering exploration area. Therefore, study on GPR forward modeling and the GPR reflection image characteristics of complex geological target are of great significance to improve the GPR data interpretation level. Moreover, GPR forward modeling is the basis of GPR inversion, the speed of forward modeling determines the efficiency of the inversion algorithm, high efficiency and precision of forward modeling algorithm helps to improve the inversion interpretation techniques. Meanwhile, with the characteristics of easy to carry out, rapid data-sampling, large amount of sampling and high precision, this method gains great popularity for the geophysical workers, but with the disadvantage of large energy loss, shallow detecting depth, GPR method makes some major rivers can’t use this method in geophysical exploration. Therefore, improving the GPR techniques and detecting depth in waterborne area is of great practical significance.The thesis carried out a series of GPR forward modeling research, which including Finite Difference Time Domain (FDTD), Alternating Direction Implicit Finite Difference Time Domain (ADI-FDTD) and Mixed ADI-FDTD sub-grid technology. Combined with the many years engaged in hydraulic engineering of GPR application, the author improved the GPR techniques in waterborne area and summarized some important technical measures about reservoir sedimentation detection with GPR, this will provide a scientific basis for the promotion and application of GPR technology in hydraulic engineering. The paper was funded by a joint support of the National Natural Science Fund (41074085), the Special Foundation for Shenzhen Water Utilities(2011-200), the main contributions of the thesis are listed as follows:(1) Based on the Maxwell’s equations, the constitutive relations of medium and Yee’s grid, the two-dimensional and three-dimensional GPR modeling FDTD equation was derived both in dispersive and non-dispersive medium, as well as the Courant-Friedrich-Levy (CFL) numerical stability condition and the formula of numerical dispersion, which will lay a theoretical foundation for the application of FDTD algorithm in high resolution GPR forward calculation.(2) Elaborated the principle of second order approximate Mur Boundary Condition, the Uniaxial Perfect Match Layer (UPML) and the Complex-Frequency Shifted Perfect Match Layer (CFS-PML) Boundary Condition, the thesis derived and discussed the difference formula of these Boundary Conditions. Using the snapshots of GPR wave field propagation in homogeneous medium, compared the large area without truncation, with no boundary conditions added at truncation, three different boundary conditions of5cases of the reflected wave strength at the border, which is attempt to show the better absorption effect of the UPML and the CFS-PML boundary condition. In order to compare the UPML with the CFS-PML, a slim model with double-layer medium was set, from the perspective of the wave snapshots and the global reflection error, which is attempt to show that the CFS-PML is more efficiently in dealing with evanescent wave, low frequency wave and grazing wave absorption.(3) The two-dimensional and three-dimensional FDTD forward modeling program of GPR was compiled with the tool of Matlab and C++, and using a three-layer model with analytic solution to validate the correctness of the program and the effectiveness of the algorithm, then the program is applied to some effective examples,in which the relationship between the dielectric constant, electrical conductivity and propagation,the relationship between the depth of target, the frequency of GPR and the resolution of GPR both has been systematically analysed.(4) The thesis designed a two-dimensional GPR complex geoelectric model, a three-dimensional GPR typical geoelectric model with three targets added, both two-dimensional and three-dimensional forward modeling were carried out with the compiled program, from the obtained results such as section, wave snapshots and three dimensional figure, it systematically reflected the echo characteristics of target, deepened the understanding of GPR reflection section, which helps to understand the propagation characteristics of GPR in medium.(5) Summarizes the characteristics of ADI-FDTD algorithm:one time step iteration in FDTD algorithm is replaced by two sub-time steps iteration in ADI-FDTD algorithm, and two sub-time steps is alternately implicit and explicit. Carried out the stability and numerical dispersion relationship analysis of ADI-FDTD algorithm with TM wave, which is attempting to prove that the ADI-FDTD is unconditional stable even though a larger time step was chosen. Meanwhile, two and three dimensional ADI-FDTD GPR modeling iterative calculation formula was deduced.(6) The paper proposed a GPR forward algorithm which based on Mixed ADI-FDTD grid technology, and emphatically discusses the transition and cohesion, the exchange and update of the field value between coarse grid and fine grid, etc. Relying on the advantages of ADI-FDTD computation efficiency, not bounded by CFL condition, with simple programming process, the core of the ADI-FDTD algorithm is that in case of physical parameters change dramatically area by adopting the ADI-FDTD with fine grid mesh, while,other areas by adopting the conventional FDTD algorithm.(7) Combined with GSSI company processing software Radan, the paper compiled a2D and3D forward modeling program of GPR based on Mixed ADI-FDTD grid algorithm, using this program to lead a series of forward modeling, the model including two-dimensional and three-dimensional geoelectric model with lateral heterogeneous medium, compared with the conventional FDTD coarse grid and fine grid calculation results, which is attempt to show that ADI-FDTD algorithm is a method of satisfactory balance between accuracy and timeliness, which lays the foundation for GPR inversion research with high precision and high efficiency.(8) The ADI-FDTD algorithm of GPR forward was applied to seven different typical river sedimentation thickness model in hydraulic engineering, compared with the geoelectric model and forward simulation results, to find the radar characteristics of object boundary, to guide the interpretation of the GPR section, and aim of proving that the GPR method is feasible in the river sedimentation thickness detect application.(9) The thesis studied the techniques of waterborne GPR, the new techniques improved the detection depth and detecting accuracy, increased the application of GPR in hydraulic engineering by means of improving the antenna coupling reaction, which is aimed at minimizing the reflection energy attenuation and interference at surface of waterborne, all of which will effectively solve the sedimentation detection problem in the reservoir, river and other hydraulic area detection, and provide a more sufficient basis for the hidden trouble diagnosis in the hydraulic engineering.Through the earnestly studies, the research conclusions are as follows:(1) Proved the better absorption effect of the UPML and the CFS-PML boundary condition. Meanwhile, demonstrated that the CFS-PML is more efficiently in dealing with evanescent wave, low frequency wave and grazing wave absorption.(2) The two-dimensional and three-dimensional FDTD forward modeling program of GPR was compiled with the tool of Matlab and C++, and the2-D&3-D Mixed ADI-FDTD sub-grid algorithmic of GPR forward modeling was compiled.(3) Proved that the ADI-FDTD is unconditional stable even though a larger time step was chosen. And2-D&3-D ADI-FDTD GPR modeling iterative calculation formula was deduced.(4) Demonstrated that in case of physical parameters change dramatically area by adopting the ADI-FDTD with fine grid mesh, while,other areas by adopting the conventional FDTD algorithm, and proved that the ADI-FDTD algorithm is a method of satisfactory balance between accuracy and timeliness, which lays the foundation for GPR inversion research with high precision and high efficiency.(5) Carried out a series of forward modeling in seven different typical river sedimentation thickness model with the research result above, compared with the geoelectric model and forward simulation results, to find the radar characteristics of object boundary, to guide the interpretation of the GPR section, as well as to reduce the misjudgment, and improves the interpretation process.(6) With the improved antenna coupling reaction in waterborne GPR, the techniques minimizied the reflection energy attenuation and interference at surface of waterborne, and improved both the detection depth and detecting accuracy, which expanded the application range of waterborne GPR in water operation area(such as underwater coating thickness detection of large rivers problem can become easily solved). |
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