| Ground penetrating radar (GPR) is a kind of shallow geophysical technology fornondestructive detection by using electromagnetic wave, which is widely used in manykinds of near surface detection. The advantages of GPR include high resolution,non-invasive, high efficiency, and strong anti-interference ability and so on. But forlarge-scale problems or areas that are heavily vegetated, or dangerours areas such asbattlefield and minefield, conventional land surface GPR is inadequate. While the airborneGPR-the GPR system is mounted on a flying aircraft, is used as a potentially effectivealternative tool to conduct survey at these areas. The airborne GPR antennas usually shouldbe hanged in a high-speed flying platform with a certain distance from the ground. Theelectromagnetic (EM) wave from the transmitting antenna propagates in the air bygeometric spreading, and penetrates into the ground. Part of the EM wave reflects backwhen it encounters the discontinuous interface, the reflecting wave propagates back into theair; it could be recored by the receiving antenna. The recorded airborne GPR data could beused to infer the underground structure.Based on Maxwell equations, we have derived the discrete formula of finite differencetime domain in3D and2D domain, besides, the discrete expressions of FDTD in differentboundary area with uniaxial perfect matched layer (UPML) is derived. The absorptioneffect of UPML boundary condition is better than that of GPML, and UPML boundarycondition will be used in the numerical simulation of finite difference time domain in mythesis. In theory, FDTD requires10grids in a wavelength at least. As for airborne groundpenetrating radar simulation, especially for the3D numerical simulation of large scaleproblem, the FDTD method in the full domain is very memory consuming.In order to improve the computational efficiency of numerical simulation for airborneGPR survey, we use multi region technology. The modeling area is divided into two regions,the freespace region and the underground region. The analytical solution is used solve the EM wave propagation in the freespace region, while the FDTD solution is used to solve theEM propagation in underground medium, these two solutions are combined for solving theEM wave propagation, which is called multi-region finite difference time domain(MR-FDTD) technology. Numerical simulations show that RM-FDTD needs less memoryand calculating time compared with full FDTD, in addition, MR-FDTD could achievepretty nice precision. As for the airborne ground penetrating radar survey, antennapolarization, antenna height and the rougheness of ground surface will affect theelectromagnetic wave signal. The simulation results show that it is important to make theantenna polarization direction perpendicular to the target underground, make the flyingheight as lower as possible.Antenna is one of the most important parts for airborne ground penetrating radarsystem. We have simulated three typical radar antenna including axial helical antenna,bowtie antenna and dipole antenna. The results show that the axial helical antenna has gooddirectivity, high gain and broadband characteristic; however the geometry size for thehelical antenna is large, which will be affected by the wind. The bowtie antenna hasbroadband frequency range, and its manufacturing process is simple, however, its geometrysize is larger, and the shape of arms is the triangle which will be affected by the wind aswell.The electrical size of dipole antenna is small and usually equals half wavelength; thestructure of dipole atenna is simple which is suitable for the airborne GRP. When theresistivily loading technique and balun is employed to dipole antenna, we can obtainbroadband frequency range.Based on the Altshuler resistivily loading principle, we havemanufactured the resistor loaded planar dipole antenna, the test results show that thefrequency band of the manufactured antenna is pretty wide, besides the wavelet of theantenna is very clean, which could be used for transmitting and receiving antenna.We have setup a step frequency airborne ground penetrating radar prototype systembased on the handheld vector network analyzer (FieldFox-N9925a) and the resistor loadedplanar dipole antenna. Vector network analyzer is used to transmit and receive RFsignal,the loaded planar dipole antennas are used to emit and receive electromagnetic wave. At thesame time a data acquisition softwave for the step frequency airborne GPR is programed based on the visual programming environment Agilent VEE. The parameters setting andsweep trigger could be control by the software, acquired data could be saved to local disk.The wireless Wifi technology is introduced in the prototype system, which make it possibleto control the step frequency airborne ground penetrating radar system through wireless.In order to test effectiveness of the system, we have carried out two experiments. Theantennas are hanging in the air by rope for the first experiment, the reflecting waves fromthe ground and the underground could be clearly identified from the measured GPR profile;as for the second experiment, the prototype system including vector network analyzer,antennas, Wifi and so on are putted on the roof of a hard wooden framework, comparedwith the GPR data measured on the ground using100MHz antenna, we could found thereflecting signal from the underground targets. The experimental results show the airborneprototype system based on the FieldFox-N9925a and Wifi has the ability to detectunderground targets.Numerical simulations show that antenna flying height variation will bring thereflecting wave distortion in radar profile, we present a height correction method based onreal GPS data of flying antenna, the corrected airborne radar profile coincidents with theactual situation.The roughness ground surface would bring surface scattering clutter; strong scatteringclutter may cover the shallow target signal, which will make the radar data interpretationdifficult. In order to eliminate the influence of ground clutter, reverse time migration (RTM)technique which is widely used in exploration seismic is introduced to process airborneGPR data. We employ the2D TM model FDTD to model the EM wave forward andbackward propagation, and the cross-correlation imaging condition is used to get theimaging in the migration space.In order to test the proposed RTM method for EM wave,four2D model with different roughness surface of the ground are constructed, and thecommon offset airborne GPR data are synthesized.The traditional Kirchhoff migration andRTM technology are used to process the synthetic airborne GPR data. The migration resultsshow that the Kirchhoff migration method could focus some energy from scattering clutter,but the surface scattering clutters still exist in the migration results; as for the RTM imagingresults, the energy from scattering clutters are well focused, besides the shape and position of the roughenss ground surface match well with the real model. The migration results formRTM algorithm is better than of Kirchhoff migration method. |
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