| With a vast territory,China has a huge development prospect and space for the exploitation and utilization of the near surface.The realization of fast and fine detection in near-surface space is of great significance to the development of underground rail transit,the construction of integrated pipeline corridor,the search and rescue of geological disasters,and the detection of pollution sources.The airborne frequency-domain electromagnetic detection method with unmanned aircraft has the advantages of all airborne,convenient and efficient,and suitable for urban space detection.Since only conduction current is considered in conventional airborne electromagnetic exploration,the magnetizing current generated by dielectric permeability and high-frequency displacement current are usually ignored.As a result,there are large errors in the numerical simulation of models containing permeability anomalies and dielectric constant anomalies,which restricts the resolution and detection accuracy of near-surface.Frequency-domain airborne electromagnetic detection system consisting of the magnetic dipole source with a large transmit-receive distance,has the characteristics of large volume and heavy weight,and needs to use manned flight platform such as helicopter as instrument carriers.The flight cost is high and it is not suitable for the rapid exploration of urban space.Therefore,studying the displacement current,magnetizing current effects and the design scheme of airborne frequency-domain electromagnetic detection based on unmanned aircraft are the key to realizing rapid exploration of near-surface space.To solve the above problems,this paper is supported by science and technology development plan project of Jilin Province “Research on key technologies of UAV-based transition band all-airborne electromagnetic detection system” and the general project of National Natural Science Foundation of China “3D imaging method with high resolution of Ground-airborne frequency-domain electromagnetic survey based on reverse projection to sensitive region”,total current numerical modeling and multi-parameter extraction of airborne frequency domain with dual-source excitation are carried out.Based on the total current response that simultaneously considers conduction current,displacement current,and magnetizing current,airborne frequency-domain electromagnetic exploration with dual-source excitation design scheme suitable for unmanned aircraft is proposed,and the near-surface apparent resistivity,apparent magnetic permeability,and apparent magnetic permeability are completed.Finally,based on the research results,experiments were conducted using a dual-source excitation total current airborne frequency-domain electromagnetic exploration scientific research prototype independently developed by the research group,which verified the actual effect of joint extraction of total current multiple electromagnetic characteristic parameters.The main contents are as follows:(1)Aiming at the problem that displacement current or magnetizing current is ignored in airborne electromagnetic numerical simulation,resulting in large errors in the electromagnetic response of the multi-parameter anomaly model,a full current detection mode is proposed.The total current electromagnetic propagation equations of magnetic dipole source and central loop source are derived.The one-dimensional and three-dimensional high-precision numerical simulation of the airborne electromagnetic total current under the combined action of conduction current,displacement current and magnetizing current has been realized,and the response characteristics of different terrestrial electromagnetic characteristic parameters under the action of magnetic sources have been clarified.(2)Aiming at the problem that the magnetic source airborne frequency-domain electromagnetic exploration system is not suitable for flying over cities and the primary field cancellation accuracy is insufficient,a design scheme for the dual-source excitation airborne frequency domain electromagnetic device is proposed.Concentric and coplanar devices are used to separately excite the transmitting and compensating coils.Through coil structure design and transmitting,and real-time fine feedback of compensation current,small coil frequency domain detection with zero transceiver distance is achieved,which improves the cancellation accuracy of the primary field and improves the secondary field detection capabilities.(3)Aiming at the slow speed of joint extraction of multiple electromagnetic characteristic parameters,a method combining three-dimensional phase vector diagram interpolation and the Marquardt-Levenberg algorithm was proposed to jointly extract multiple parameters for apparent resistivity,apparent permeability and apparent polarizability.When the three-dimensional phase vector diagram interpolation method has overlapping phenomena and multiple solutions,it is combined with the Marquardt-Levenberg algorithm to jointly extract visual parameters.The comprehensive identification ability of near-surface differential resistivity,magnetic permeability,and polarizability has been improved.Finally,using the design scheme of the dual-source excited airborne frequency domain electromagnetic device proposed in this article,the research team developed a scientific research prototype and carried out a field experiment for UAV-borne dual-source excited aerial frequency domain electromagnetic detection.The total current electromagnetic response was obtained,and the near-surface apparent resistivity,apparent permeability,and apparent polarizability are quickly extracted through three-dimensional phase vector diagram interpolation and the Marquardt-Levenberg algorithm.In summary,this study further promotes the practicality of near-surface multi-parameter extraction based on total current response,and is of great significance to the development and application of UAV-borne frequency-domain electromagnetic exploration technology in rapid detection of urban near-surface space. |
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