Research On Broadband Low-noise Sensing Technology For Urban Towed Transient Electromagnetic Receiver

The urban modernization needs of China are increasing day by day,and the shortage of urban space resources is imminent.Mastering sufficient geological data reserves as a priori information for urban planning is a necessary prerequisite for the large-scale development of urban underground space.Therefore,the development and application of earth exploration instruments and technologies is of great significance.The transient electromagnetic method makes full use of the characteristics of non-destructive propagation of electromagnetic waves in underground media,and can realize high-resolution non-destructive detection of underground space.The traditional transient electromagnetic detection method uses the mode of large-loop fixed-point superposition detection,which cannot overcome the size limitation in urban environment,and is not convenient for efficient and rapid detection in urban areas.For this reason,small loop towed transient electromagnetic systems suitable for urban environments have emerged.Miniaturizing transient electromagnetic devices and placing them on mobile platforms enables rapid geological exploration in cities.However,the change of size and detection mode of the small loop towed transient electromagnetic device directly leads to a sharp decline in the quality of the received signal,and loses lots of high-frequency signals reflecting shallow information,resulting in a decrease in the depth and affecting the shallow detection accuracy.Therefore,it is necessary to optimize the urban towed transient electromagnetic receiving system by using broadband and low-noise sensing technology,so that it can not only broaden the frequency band of the received signal to retain shallow geological information,but also increase the signal quality and improve the detection performance of the transient electromagnetic receiving system.In view of the performance requirements of urban towed transient electromagnetic receiving system in bandwidth,noise floor and acquisition efficiency,this paper studies broadband low-noise sensing technology,designs a receiving system serving urban towed transient electromagnetic detection,and verifies the effectiveness and practicality of the system through experiments.(1)This paper introduces the detection principle and instrument composition of urban towed transient electromagnetic instruments,analyzes the engineering requirements and main problems faced by urban towed transient electromagnetic systems,and establishes the correspondence between detection problems and instrument composition.Taking the receiving system as the research goal,the broadband low-noise sensing technology is used to optimize the receiving system and formulate the overall design scheme.(2)Aiming at the shallow information loss caused by excessive parasitic parameters and insufficient receiving bandwidth of urban towed transient electromagnetic receiving coil,a wideband coil sensing technology based on cross-winding strategy is proposed.Firstly,the induction principle and parasitic parameters of the receiving coil are analyzed,and the mathematical model of parasitic parameters and coil performance is established.On this basis,the hive transient electromagnetic receiver coil designed based on the cross-winding strategy is proposed,and the parasitic parameter difference and coil bandwidth performance of the hive coil and the traditional coil are quantitatively compared by combining the COMSOL finite element simulation software,so as to clarify the performance advantages of the hive coil in transient electromagnetic detection.Finally,combined with the actual detection performance requirements,SOLIDWORKS is used to design the physical skeleton of the coil and wind the hive coil,and compared with the traditional transient electromagnetic coil,the frequency band of the hive coil can reach up to 213 k Hz,which is 60% wider than the traditional coil,which verifies the performance advantages of the hive coil in transient electromagnetic detection.(3)The problem of high noise floor of the amplifier of the urban towed receiving system is studied,and the low-noise JFET amplifier with independent current source is designed,so as to increase the resolving ability of the receiving system to late weak signals and improve the data quality of the urban towed transient electromagnetic device.Firstly,according to the small loop transient electromagnetic forward model,the noise floor requirements of the towed transient electromagnetic preamplifier are quantitatively analyzed,the performance of the low-noise amplifier device is compared,and the circuit design scheme with low-noise JFET as the core is determined.Secondly,the static operating point calculation of the JFET differential amplifier circuit is simplified by an independent current source,so that it can operate stably at the minimum noise operating point.A theoretical mathematical model of the circuit noise floor is constructed to optimize the external circuit parameters.Finally,a low-noise JFET amplifier with an independent current source is designed and manufactured,and the noise floor of the test circuit is as low as 0.6nV/(?)Hz in an electromagnetic shielding room.In lab tests,the effective data length can be increased to 800 μs,increasing the detection capability of deep signals by urban transient electromagnetic systems.(4)Based on the above key research content and key technologies,an urban towed transient electromagnetic receiving system was built to realize the optimization design of broadband and low-noise sensing of the receiving system.The system is built on the industrial control all-in-one machine with built-in high-speed data acquisition card,and in the Lab VIEW environment,according to the efficiency requirements of towed transient electromagnetic to develop an easy-to-operate upper computer data acquisition software,the program is written based on the "production-consumption" structure,and the data overlay storage function is added to optimize the acquisition and storage speed to meet the needs of rapid acquisition and storage of towed detection data.The operating efficiency of the host computer is higher than that of the original cycle structure.Combined with the urban towed transient electromagnetic broadband low-noise receiving system designed and produced in this paper,the detection experiment was carried out in Changchun.Experimental results show that the system can complete the detection of urban underground space in urban environment,which verifies the effectiveness and stability of the system.The advantages of wideband low noise technology in improving transient electromagnetic receiving systems are proved.The towed transient electromagnetic receiving system and related technologies studied in this paper can be used as a reference means to improve the towed transient electromagnetic detection ability,and lay a foundation for improving the urban towed transient electromagnetic detection capability to obtain urban underground space information.