| With the acceleration of urbanization in China,the overground space cannot continue to bear the expansion speed of urban construction,so the development of urban underground space has become the direction of land use development in the future.However,due to the natural and man-made defects of the stratum,accidents occur frequently in the construction process of urban underground space development.Therefore,it is of great significance to use geophysical exploration technology to understand structure in advance.Transient electromagnetic method has the characteristics of strong penetration,high imaging resolution and high detection efficiency.However,the depth of urban underground space is mainly distributed in shallow strata.The traditional transient electromagnetic detection method uses a large loop to lay,there is shallow detection blind area,and the urban road space is narrow,large-scale detection systems cannot be directly applied in the city.Due to its high detection efficiency,flexible operation mode and non-destructive detection characteristics,small-loop transient electromagnetic devices have gradually attracted attention in the field of engineering and environmental exploration,but the existing technology cannot meet the needs of actual detection.In addition to the turn-off time of the transmitting current,the primary field interference generated by the mutual inductance between the transmitting and receiving coils and the signal distortion caused by the transition process of the receiving coil are the main reasons that limit the shallow detection capability of the small transient electromagnetic device.In this paper,facing the shallow detection requirements of urban underground space,the key technology to improve the detection blind area is researched,and a transient electromagnetic detection system based on eccentric compensation in urban underground space is designed,and the effectiveness of the system is verified by test experiments.The main work contents of this paper are as follows:(1)The small loop transient electromagnetic detection method is researched,and the working principle and structural characteristics of the small-loop transient electromagnetic device are explained.Based on the basic theory of transient electromagnetic,a numerical analysis model based on the conductive loop is established.The signal response law of the small-loop device is analyzed through theoretical modeling,and the factors that limit the detection performance of the small loop device are summarized.The reason for the blind zone of shallow detection is found,which provides a basis for the overall design of the system.(2)Aiming at the problem of excessive primary field interference caused by the serious mutual inductance of the small-loop transceivers,in order to preserve the early signal characteristics as much as possible and shorten the shallow detection blind area,the Biot-Savart law is used to analyze the magnetic field strength of the area around the transmitting coil.On the basis of the distribution law,the design of the eccentric self-compensation structure for the weak magnetic coupling is proposed,the design principle of the eccentric self-compensation decoupling structure is introduced,and the mathematical model of the mutual inductance of the eccentric coil is established based on the Neumann integral formula,and the actual device may exist errors.A method for optimizing the coil eccentric position based on decoupling stability is proposed,the stability coefficient of the evaluation device's ability to resist errors is defined,and the detection sensitivity and primary field shielding ability are combined to determine the specific parameters of the system device structure.(3)In order to solve the problems of insufficient bandwidth of the traditional receiving coil and serious distortion of the secondary field signal,based on the analysis of the inductive principle of the coil and the influence of various parameters on the performance of the sensor,an equal interval differential hollow coil sensor design for shallow detection was proposed.The mapping relationship of sensitivity,SNR and resonant frequency to coil size and number of turns is established,and the design parameters of coil are determined with SNR and resonant frequency as constraints.The amplitude-frequency characteristics of the coil are analyzed,and the damping coefficient is optimized for the transition process of the secondary field on the coil.The 3 d B bandwidth of the optimized coil can reach 128 k Hz,which is54.2% larger than that of the conventional coil.According to the requirements of detection performance,a pre-amplifier matching the differential coil is designed.Based on the establishment of the circuit noise model,the noise source and the power density distribution of the noise are classified and analyzed.(4)Based on the above research focus and key technologies,a small-loop transient electromagnetic system with fast turn-off process,high sampling rate and simple man-machine interaction is built,and the integration and miniaturization of the transceiver system are realized.Based on the analysis of transmission current characteristics,the transmission inverter circuit with passive constant voltage clamp is designed,and the isolation drive circuit and logic control circuit are built according to the functional requirements of the transmission system.The receiving system is built by the high-speed data acquisition card and the embedded upper computer motherboard,and the data acquisition software is developed in the LabVIEW environment.Finally,the system developed in this paper is used to carry out indoor test and field detection experiment in urban underground space.The results in the urban area of Changchun city show that the system can detect underground space in the complex urban environment,which verifies the effectiveness and reliability of the instrument system.To sum up,the small loop transient electromagnetic detection technology and instrument system studied in this paper can be used as an effective detection method to evaluate urban underground space resources,which lays a foundation for the rational planning of urban underground space in China. |
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