Numerical Simulation And Imaging Of The Airborne GPR For Subsurface Objects Detection

Ground Penetrating Radar is a kind of near-surface geophysical methodusing high-frequency electromagnetic waves to determine the laws ofcomposition of surface. Because of its excellent resolution, simple operation,the advantages of speed, it has been applied in various engineering problems.The operation platform of GPR can be separated into airborne, in-vehicle, holeunderground and multi-holes exploration, we often meet the latter fourcategories, but the airborne GPR is mainly used in surveying large areas or ininaccessible regions, for example, desert areas, permafrost areas or highmountain ranges. In addition, the airborne GPR could survey hydrogeologicalresource, map subsurface geomorphologic characteristics, detect buriedobjects and underground installations. Specially needs to be pointed, theairborne GPR is related to the detection of martial objects, such as areas oflandmine and unexploded ordnance.Airborne GPR came into existence in the 1960's with the development ofice profiling radars by the Technical University of Denmark, and the ScottPolar Research Institute of Cambridge. The primary interest at the time was inthe profiling of ice layers at depths of several thousand feet. Ultra-widebandradars were developed slightly later when the goal moved to producingprofiles and imagery of the near sub-surface in normal soils. Afterward more institutes manufactured GPR in abroad, for example, the GPR system thatHamburg-harburg technical university operated was used by The FederalInstitute for Geosciences and Natural Resource, The system was successfullyemployed during various campaigns in Antarctica, it was a successful trial thatproved the validity of airborne GPR.I use numerical simulation to simulate the mechanism of airborne GPRexploring underground objects, simulation is model experiment, when theresearchful system is expensive or there is big hazard in the experiment or thesystem needs more time to be understood with the changed parameters,simulation is an effective method. Using airborne GPR to detect undergroundobjects needs expensive cost, so I select the method of simulation. Thenumerical simulation method is FDTD, its main idea is to discrete the Maxwellrotation equation in time and space domain, then the partial differentialequations is replaced by the differential, and solve the differential equation, thevalue of the EM field of each cell could be calculated. It can be used widely,almost the entire electromagnetic and microwave technology in all fields aswell as other technological and industrial fields. For the sake of the need ofresearch, I simulate single and multi-objects'airborne GPR response underdifferent irregular topography, The simulation model include the cavity modelunder 2D horizontal and irregular topography, the model of multi-objects under2D horizontal and irregular topography.Later I deal with the common offset profile using migration imaging,because radar imaging is the focus of the reflected wave method, and it is readyfor obtaining geological image exactly. I use diffraction scan stacking migrationin the article, the diffraction scan migration founds the base of ray theory, and itcan let reflected wave return to real position automatically. According toHuyghens'principle, every reflection point underground is regarded as awavelet source, the diffracted wave that these wavelets sources bring could get to the topography and be received by the receiver, and the time-distance curvesare hyperbola. When we use diffraction scan migration, we compartmentalizethe underground space to grid, and regard every grid point as a reflection point.When there are reflection interfaces or reflection points, every trace's amplitudeis contiguous and in-phase stacking, the amplitude after stacking is increscent;Contrariwise, when there are no reflection interfaces or reflection points, everytrace's amplitude isn't contiguous and in-phase stacking, their amplitudescounteract one another, the amplitude after stacking is decrescent.Firstly, I use matlab code to produce the procedure of the irregulartopography, in order to integrate it with the numerical simulation software ofFDTD, thereby it produces the inner file that Gprmax can identify, these canfound base of the following irregular topography. Secondly, I simulate that theairborne GPR explores the model of cavity under the horizontal topography, theresult indicates that the common offset profile is clear, the following step ismigration imaging, first of all, suppose the medium is homogeneous mediumand the homogeneous medium is air, we can see the result, but the position ofthe cavity is errorless compare to the real position, then I use layered migration,and regard the air portion and the soil portion as homogeneous mediumrespectively, adopting the speeds of air and soil, the outcome arrivesanticipation aim, this model mainly proves the validity of the airborne GPRexploring. Thirdly, I simulate that the airborne GPR explores the model ofcavity under the big relief and small relief respectively, and complete migrationimaging, it indicates when the relief becomes big, even if we adopt layereddiffraction scan stacking migration, the position of the object is wrong, and theoutcome has false interferer, so I decide to adopt the migration after savvyingthe position of the irregular topography, apropos of how to know the position ofthe irregular topography, as the article refers, we may utilize pulsed laserranging or aerophotogrammetry,. When we use this method to complete migration, we arrive to anticipative effect, but compiling procedure is alsotroublesome. Fourthly, the article has a brief summary aims at the diffractionscan stacking migration: When we carry through the migration under thehorizontal topography, the divisiory grid and velocity are the most importantinfection factors. When we carry through the migration under the irregulartopography, in addition to the infection factors of the divisiory grid and velocity,the irregular topography also influences the result of imaging. If thetopography's fluctuant degree is big, the object imaging position's error alsobecomes big, and the outcome may has false interferer, so we need find the bestappropriate method according to the actual circs when migration. Fifthly, Isimulate that the airborne GPR explores the model of objects under the irregulartopography, and make it integrate the actual circs, the common offset profile'seffect is all right, and the imaging is also ok. Sixthly, I analysis the instrument'sdynamic range and the influence of the airplane's velocity.Integrating the above result we can know that the airborne GPR exploringthe underground objects is an effective geophysical method, and it is theessential complementarity to holding GPR, the airborne GPR will play animportant role in the future.