| The acceleration of the urbanization process in China result in a shortage of overground space resources.However,reasonable utilization of underground space is conducive to the further development of urban construction.With respect to municipal infrastructure construction towards underground engineering,due to the complexity and invisibility of subsurface geological structure,accidents occur frequently in the construction.In particular,if the position of the pipes failed to be determined accurately,the pipeline leakage,rupture,even more dangerous situations will occur.Above all,it is of great significance to ensure the location of the subsurface pipelines before the construction by using valid geophysical exploration technology.With respect to urban reconstruction,Ground Penetrating Radar（GPR）is treated as an efficient tool that processed high resolution as well as non-destructive.It is utility in various of engineering applications,such as the monitor of underground facilities,geological disaster prevention and pipeline detection.The antenna is the key part of the whole radar system,its performance has a direct impact on the detection results of the radar system.Nevertheless,the traditional commercial radar antenna has a narrow working band,so the antenna needs to be changed frequently while detecting,finally lead to a low work efficiency.In this paper,most of our attention is paid on designing the ultra-wideband and high-gain performance of the antennas,to realizing the detection of the urban near-surface pipelines.In this paper,based on the theory of traditional bowtie antenna,an improved folding one with the performance of ultra-wideband is designed,and the feasibility of the system are proved to be effective through our experiments.The contents of the paper can be splitted into the following parts:（1）Firstly,the methods of detection in GPR is presented in detail,and presented thorough introduction of the way it works as well as its key performance indicators and the detection theory.Then performance of the antenna is emphatically analyzed with different center frequency and different antenna spacing on GPR detection.Based on the correlation theory,several parameters related to the performance of GPR antenna are summarized,which provides the evidence for design of the antenna.（2）Based on the design philosophy of bowtie antenna,the improved bowtie antenna is designed by the composite loading technology,that is,combining structure loading and resistance loading.By the methods of truncated arm,slotting and folding,the low frequency working bandwidth and the miniaturization of the size come to realize.Furthermore,the signal tailing degree of the antenna is reduced by resistive loading,and the directional gain and anti-interference ability of the antenna are improved by adding v-shaped reflection back cavity.Working in the frequency domain,the maximum bandwidth of the antenna can reach up to 720 MHz,while the minimum can be 75 MHz.Besides,it is also superior in the relative bandwidth for which reaches about 162%.The radiation main lobe protrudes in the frequency band,for which is superior in the radiation characteristics.The antenna is 480 mm in length,225 mm in width and 170 mm in height.This new designed antenna can successfully meet the requirements of the ground penetrating radar system for the detection of shallow underground pipeline.（3）For the part of the simulation of the shallow underground pipeline,several basic parameters that determined by the designed antenna are used to make the forward modeling.Then we obtained the datasets based on 2D forward modeling simulation by using gpr Max3.0 in shallow underground pipeline model with a single target and another with multi-target,respectively.So that the feasibility of the antenna is verified in the later measurement experiments.Direct wave removal and weak signal gain adjustment are processed for 2D forward modeling simulation data.On the other hand,Hilbert transformation is introduced here to obtain the instantaneous information of electromagnetic waves from forward modeling data.Therefore,the abnormal signalscan be analyzed from different aspects,promising the accuracy when judging the burial depth of the pipeline.Finally,the improved folding bowtie antenna with UWB is made and then tested in the indoor air and a sand box simulated with the distribution of underground pipelines,respectively.These experiments are carried out by using the step-frequency GPR system with the antenna designed antenna.The numerical tests illustrate that the whole system built in the paper can be considered as a valid approach to detect the shallow pipeline in urban underground space successfully,which also verifies the effectiveness of the GPR system and the superior performance of the antenna. |
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