| There are nearly 100 million unexploded ordnance(UXO)such as landmines worldwide,which poses a great threat to the safety of human life.Because of its strong anti-interference ability,transient electromagnetic method has become an effective method for UXO.In the detection of UXO used transient electromagnetic method,the traditional off-time detection,because it is not affected by the primary field,the technology is simpler and more used.The secondary field response detected in off-time is almost ineffective for targets with small size and great depth due to fast signal attenuation and low signal-to-noise ratio at late stage.Compared with off-time response,on-time response of TEM system is more sensitive to high-conductivity targets,such as UXOs.Therefore,this article mainly studies the transient electromagnetic on-time response,and provides certain theoretical guidance and experimental basis for realizing transient electromagnetic on-time detection.First,this article introduces the basic working principle of the transient electromagnetic method and the theoretical response of a typical metal target(sphere).On this basis,the sphere response under different emission current waveforms is calculated,by comparing the on-time response and off-time response of the sphere under different conditions,theoretically analyze the transient electromagnetic on-time response characteristics and derive the basic theory of transient electromagnetic on-time detection.Then,this article explains in detail the design process of on-time sensor developed for on-time response detection.First,the sensor structure is determined according to the typical magnetic cancellation technology.Secondly,through a series of calculations and simulations,specific details such as the parameters of the magnetic cancellation coil,the waveform of the transmitted current and the implementation of the receiving acquisition system were determined.Finally,a series of effect tests are carried out on the on-time sensor produced to analyze the performance of the sensor.Finally,this paper uses the developed transient electromagnetic on-time sensor to measure the responses of 12 targets.Under the same experimental conditions,12 types of targets are classified,and their on-time response and off-time response are tested at different depths respectively Through the comparison of the two responses,the advantages and disadvantages of the on-time response are analyzed,and the detection advantage of on-time detection is explored. |
