Research On Electromagnetic Detection Method Of Urban Underground Metal Pipelines Based On Resonance Difference Technology

With the progress of society and economic development,China’s urbanization process continues to accelerate,and pipeline leakage problems caused by urban underground infrastructure construction emerge in endlessly.Therefore,it is imperative to use geophysical methods to detect underground metal pipelines before construction.Geophysical methods can realize the non-destructive detection of urban underground metal pipelines,which attracts more and more attention.At present,the mainstream non-destructive geophysical detection technologies of underground metal pipelines include ground penetrating radar and frequency domain electromagnetic method.Due to the high transmitting frequency,the detection depth of GPR is limited when it is applied in urban environment,especially the buried depth of underground metal pipeline in old urban area is large,so it is difficult to obtain good detection effect.Frequency domain electromagnetic method can realize the full coverage detection of underground space pipeline area by adjusting the emission frequency,which has a great development prospect in the safety and efficient construction of urban underground infrastructure.There are two main problems in the application of frequency domain electromagnetic detection technology in urban underground pipeline detection: one is that the transmitting coil and receiving coil have strong coupling interference of primary field mutual inductance,which makes the effective signal be covered by the primary field signal,even cause the data damage of the receiving sensor.Secondly,compared with the field environment,the noise interference in urban environment is very serious,which leads to the signal submerged in the noise,and it is difficult to extract effective data.In view of the above problems,this paper presents a method of electromagnetic detection for underground metal pipelines in cities based on resonance difference technology.The differential coil sensor is used to solve the coupling interference caused by the transmitting coil to the receiving coil.The differential balance technology is used to weaken the mutual inductance coupling effect between the receiving and receiving coils.In view of the serious noise interference problem in the city,LC resonance technology and locked phase amplifier technology are used to improve the signal-to-noise ratio,and the amplitude and phase information of electromagnetic signal are effectively extracted.Finally,based on the above technology,the prototype system of the electromagnetic underground pipeline detection principle in frequency domain is designed,and the effectiveness of the system is verified by the experiment.The main contents of the paper are as follows:(1)The frequency domain electromagnetic detection method is studied,and the working principle of the frequency domain electromagnetic device is described.Based on the equivalent circuit model,the eddy current response of metal target in the detection process is analyzed.The characteristics of eddy current sensor are analyzed through theoretical derivation.The limitations of frequency electromagnetic detection in urban environment are analyzed.(2)In order to solve the problem that the urban noise is serious and the collected data signal is submerged in the noise,the processing method of weak electromagnetic data is studied.Considering the superior filtering performance of lock-in amplifier technology in single frequency signal detection,a digital quadrature vector lock-in amplifier is designed,which can filter the noise and extract the amplitude and phase information of electromagnetic signal at the same time.The simulation noise and field noise are added to the simulated secondary field signal to test the signal extraction ability of the PLL amplifier,which provides a guarantee for the later data interpretation.(3)The processing method of weak electromagnetic data is studied.Considering the superior filtering performance of lock-in amplifier technology in single frequency signal detection,a digital quadrature vector lock-in amplifier is designed,which can filter the noise and extract the amplitude and phase information of electromagnetic signal at the same time.The simulation noise and field noise are added to the simulated secondary field signal to test the signal extraction ability of the PLL amplifier,which provides a guarantee for the later data interpretation.(4)The design of frequency domain electromagnetic detection instrument system is completed.Firstly,the hardware circuit of the transmitter is designed,including oscillation circuit,full bridge inverter circuit,drive circuit,control circuit,to realize the controllable emission of excitation source;the receiver is designed,including preamplifier,signal conditioning circuit,low noise power supply,and the upper computer software is designed with the selected high-speed data acquisition card to realize the data acquisition Set,display and store.Finally,the whole system design is completed,and the reliability of the detection system is verified by combining with the indoor detection experiment.The experimental results show that the electromagnetic detection system based on resonance difference technology can effectively detect the underground metal pipelines in cities,and provide reliable data guarantee for urban construction planning,pipeline maintenance and emergency rescue.