Design And Implementation Of Small Loop Transient Electromagnetic Receiving System

With the rapid development of economy,Chinese urbanization process is accelerating,urban population proportion is also increasing.The contradiction between supply and demand is becoming more acute due to the gradual decrease of urban per capita construction land,so the development of urban underground space has become an important means to solve the urban space congestion.However,in the process of developing underground space,accidents such as collapse,explosion,damage of cables and pipelines frequently occur.Therefore,it is necessary to use geophysical exploration technology to conduct geological investigation before the development to eliminate safety risks.The development depth of urban underground space is generally 0 ～ 100 m.At present,the typical geophysical detection methods for shallow underground space exploration generally include shallow seismic exploration,ground penetrating radar and transient electromagnetic method,etc.Due to the inherent limitations of the first two methods,the transient electromagnetic method will be adopted in this paper to solve the problem of urban underground space development.The traditional transient electromagnetic method is laid by large loop,and the shallow detection blind area is large,so it is not suitable for the development of shallow underground space in cities directly.Small loop transient electromagnetic detection system has the characteristics of small size,flexible working mode and high horizontal resolution,which has great advantages in urban space.However,the existing small loop transient electromagnetic receiving system has some problems,such as insufficient bandwidth of the receiving coil,large primary field coupling interference,and large background noise of the system,which lead to limited resolution and shallow detection ability.Based on the above problems,this paper will study the key technologies to improve the shallow detection capability of the system,and design a small loop transient electromagnetic receiving system that can carry out high efficiency and high resolution detection in urban environment.The main research contents are as follows:(1)The basic principle of small loop transient electromagnetic method and the characteristics of transient electromagnetic response signal are studied.The effects of transmitting current turn-off delay,primary field coupling interference and receiving coil transition on shallow detection blind area of small loop transient electromagnetic detection system are analyzed in detail,and the overall design scheme of small loop transient electromagnetic detection system is given.(2)The equivalent circuit model of the hollow coil sensor is established,and the influence of parasitic inductance and distributed capacitance on the resonant frequency and bandwidth of the coil is analyzed through theoretical calculation.The parasitic inductance changes when the key parameters such as the radius of the receiving coil,the number of turns and the radius of the conductor are changed.Based on the simulation results,an equal-interval multi-stage receiving coil is designed.The test shows that under the same conditions,the bandwidth of the equal-interval multi-stage receiving coil is improved significantly compared with that of the ordinary receiving coil,and more abundant high-frequency information can be obtained.(3)The principle and theoretical calculation method of primary field coupling are studied.It is found that reducing mutual inductance between transmitting coil and receiving coil is an effective way to weaken primary field interference.Based on this,the coupling model of transmitting coil and receiving coil is established,and the influence of the number of turns,radius and relative position of receiving coil on mutual inductance is analyzed.The coupling between transmitting coil and receiving coil is proposed by means of offset loop,and the optimal offset distance is given according to the experiment.(4)The low noise preamplifier circuit,gain programmable signal amplifier circuit and low pass filter circuit based on Sallen-Key structure are designed to effectively improve the signal-to-noise ratio;The main control circuit with low power consumption,low cost and high reliability is built by FPGA chip.Using the "producer-consumer" model,the upper computer software based on Lab VIEW was designed,and the prototype of the small loop transient electromagnetic receiving system was developed.The effectiveness of the system was verified through the indoor performance test and outdoor experiment of the electromagnetic shielding system.