【作者】 刘飞；

【导师】 林君；

【作者基本信息】 吉林大学 ， 测试计量技术及仪器， 2019， 博士

【摘要】 直升机时间域电磁探测方法（Time-domain electromagnetic method,TEM）具有快速高效、机动灵活、适应地形能力强、探测深度大、分辨率高等优点,非常适于我国复杂地形条件下资源快速勘查应用。然而,在实际探测过程中,直升机TEM获得的二次场目标响应信号微弱,低至10^-6 V量级,极易受噪声干扰;最主要的是,由于采用直升机移动式平台作为载体,在飞行运动过程中接收传感器存在机械振动,在外界磁场环境中便会产生动态噪声,严重干扰TEM目标信号。经过均匀半空间下层状异常体正演模型分析,当探测系统动态噪声水平从56.7nV/m²降低到10.4 nV/m²时,探测深度可增加80 m。由此可见,动态噪声水平是直升机TEM探测系统的决定性指标之一,开展动态噪声抑制方法研究具有重要意义和实用价值。目前,国外已经研发了多套商用直升机TEM系统,分为硬支架类型和软支架类型,相应的动态噪声抑制技术基本发展成熟并处于核心保密状态。而我国研究起步较晚,自主研发的硬支架系统动态噪声水平大致在±15 nV/m^2±35nV/m²,尽管达到国外同类型系统水平,但仍然存在很大的优化改进空间;对于软支架系统的研究处于起步阶段,缺乏相应的动态噪声抑制技术研究。因此,为克服直升机TEM系统的动态噪声问题,打破国外技术垄断,本文提出了基于机械共振方式的动态噪声抑制方法,并分别研制了与硬支架系统和软支架系统匹配的动态噪声抑制装置,实现了动态噪声的有效抑制。论文主要研究内容如下:（1）静态噪声决定直升机TEM系统的本底噪声,是实现探测系统低噪声水平的前提,因此,本文提出了基于噪声优化传感技术的直升机TEM系统静态噪声抑制方法。本文研究了差分方式的低噪声传感技术,通过最佳噪声匹配方法,优化设计了接收传感器的接收线圈及放大电路。经理论分析,接收传感器的噪声水平可达1.83 nV/Hz^1/2@1kHz。经屏蔽室环境实验测试,研制的接收传感器输入端等效电压噪声达到1.83 nV/Hz^1/2@1kHz,3 dB带宽可达71 kHz。经过探测系统野外地面测试,在4.5 h时间内,直升机TEM探测系统的静态噪声水平达到±1 nV/m²,漂移量为1 nV/m²;作为对比,国外同类型AeroTEM系统的静态噪声水平为±1 nV/m²,漂移量为3 nV/m²。（2）分析了直升机TEM系统的动态噪声产生机理和特性,提出了基于机械共振方式的直升机TEM系统动态噪声抑制方法。基于课题组前期研究基础,以硬支架直升机TEM系统为例,详细分析了动态噪声特性,得到以下结论。第一,动态噪声与TEM目标信号都由接收传感器产生,两者具有同源性;第二,接收传感器的运动形式包括大幅度低频率的摆动和小幅度高频率的机械振动两种,动态噪声主要由后者产生,并且动态噪声频率特性由接收传感器的机械特性决定。第三,直升机TEM系统接收传感器的机械特性受瞬变大电流情况下的一次场补偿限制,因此动态噪声是一个机电一体化的矛盾性问题。根据上述分析,提出了基于机械共振方式的直升机TEM系统动态噪声抑制方法。该方法的实现方案为,在瞬变大电流下一次场补偿的限定条件下,通过研制动态噪声抑制装置调节接收线圈的固有频率,使接收传感器工作于机械共振模式,从而改变动态噪声的频率分布,规避发射基频,最终实现动态噪声的抑制。（3）基于有限元的思想,将大尺寸的发射线圈和补偿线圈进行微元化处理与精细化计算,剖分了瞬变大电流情况下的一次场分布。然后基于精确求解的结果,限定了接收传感器的空间结构。针对硬支架直升机TEM系统,根据一次场计算结果,结合瞬变大电流特性,在发射线圈直径为12 m,补偿线圈直径为1.2m的条件下,接收线圈直径限定为为0.2 m,水平方向位移量为0.05 m;针对软支架直升机TEM系统,根据一次场计算结果,确定了发射线圈直径为25 m,补偿线圈直径为6 m的条件下,接收线圈直径限定为1.2 m。（4）基于一次场计算后确定的有限空间位移量,研制了与硬支架系统匹配的单自由度机械共振方式动态噪声抑制装置。经过地面测试,瞬变发射电流下降沿阶段,接收传感器的感应电压最大值为2.5 V,满足一次场补偿要求;此外,利用冲击法进行机械性能测试,接收线圈的固有振动频率被调节为8.6 Hz,规避了25 Hz发射基频。根据野外飞行试验结果,探测系统的动态噪声频率由传统方式的10-40 Hz调节为8.6 Hz,与接收传感器的固有振动频率吻合;此外,优化后的系统动态噪声水平达到±5 nV/m²,作为对比,传统方式下的系统的动态噪声水平为±15 nV/m^2±35 nV/m²,国外同类型系统的动态噪声水平为±20nV/m²。基于本文建立的均匀半空间下层状异常体正演模型,优化改进系统动态噪声水平的降低范围,可增加探测深度超过50 m,探测分辨力也大幅度提升。（5）研制了六自由度机械共振方式软支架系统动态噪声抑制装置。基于软支架直升机TEM系统接收传感器的机械独立性,以及同心方式下接收区域内一次场强度小且分布均匀的电磁场环境,研制了六自由度机械共振方式的动态噪声抑制装置。经过运动学和动力学分析,并通过仿真模型计算得到接收传感器的6阶固有振动分别为1.89、1.89、1.96、3.48、3.67以及3.67 Hz,远低于25 Hz发射基频;此外,经过振动模态分析,接收传感器对引入动态噪声的振动模式不敏感。利用冲击法以及模拟飞行方法进行机械性能测试,得到接收传感器的固有振动频率为3 Hz,符合理论设计值,远低于25 Hz发射基频。经过实际飞行试验,得到动态噪声频率约为3 Hz,实现了动态噪声与TEM信号的频谱分离;此外,系统动态噪声水平降低为±0.5 nV/m²,远低于硬支架直升机TEM系统动态噪声水平,可以获得更优的探测深度和探测分辨力。综上,基于本文提出的机械共振方式动态噪声抑制方法,同时解决了硬支架和软支架直升机TEM系统的动态噪声问题,性能指标达到甚至超过国际先进水平,提高了探测系统的探测深度和探测分辨力。本文的研究可促进我国直升机TEM探测技术的发展,为我国复杂地形环境的地球物理勘查提供了支撑。更多还原

【Abstract】 Helicopter time-domain electromagnetic method（TEM）is a fast,efficient and flexible detecting method,which is adaptable to the terrain and achieves a large detection depth and a high detecting resolution.It is very suitable for the resource exploration under complex terrain conditions in China.The TEM signal is very weak and easily disturbed in the flying detection mission,which is low to 10^-6 V.The motion induced noise（MIN）,which is generated because of the receiving sensor’s mechanical vibration in the magnetic field,interferes TEM signal seriously.The detection depth can be increased by 80 m when the MIN level is reduced from 56.7nV/m² to 10.4 nV/m²,based on forward model analysis of the layered anomalous body in uniform half space.The MIN level is a primary parameter of the helicopter TEM detection system.It is of great significance and practical value to reduce the MIN level.At present,a number of commercial helicopter TEM systems have been developed abroad,which are divided into rigid detection system and semi-rigid detection system.The MIN suppression methods are developed and kept as core technology.In China,the study of helicopter TEM method is laggard.The MIN level of the developed rigid detection system is in the range of±15 nV/m² to±35 nV/m².Although it reaches the level of similar systems abroad,there is still a large room for the optimization of the MIN level.The research on semi-rigid detection system is in the initial stage.The MIN suppression methods have not been developed.In order to reduce the MIN level of helicopter TEM system,a MIN suppression method based on mechanical resonance mode is proposed.The MIN level is reduced based on the proposed MIN suppression devices in the rigid and semi-rigid detection systems.The main contents of this paper are as follows.（1）The static noise level is the precondition of achieving low detection noise.A static noise suppression method of helicopter TEM system based on noise optimization technology is proposed in this paper.In this paper,the differential low noise sensing technology is studied.The receiver coil and amplifying circuit of the receiving sensor are optimized by optimum noise matching method.The theoretical analysis shows that the noise level of the receiving sensor can reach 1.83nV/Hz^1/2@1kHz.Then,equivalent noise voltage and the 3 dB bandwidth of the fabricated receiving sensor reach 1.83 nV/Hz^1/2@1kHz and 71 kHz,respectively.The field test shows that the static noise level of the helicopter TEM system reaches±1nV/m² with the drift value of 1 nV/m² in 4.5 hours.As a comparison,the static noise level of the AeroTEM system is±1 nV/m² with the drift value of 3 nV/m².（2）The generation mechanism and characteristics of MIN are analyzed.Then,the MIN suppression method based on mechanical resonance mode is proposed.The characteristics of the MIN is analyzed in detail based on the previous study of the rigid helicopter TEM system.Three conclusions are obtained.Firstly,the MIN and TEM target signal are both generated by the receiving sensor.Secondly,there are two motion modes of the receiving sensor,including large-scale low-frequency oscillation and small-amplitude high-frequency mechanical vibration.And the MIN is mainly generated by the vibration of the receiver coil.The frequency of the MIN is determined by the mechanical characteristics of the receiver sensor.Thirdly,the mechanical characteristics of the TEM receiving sensor are confined by the primary magnetic field compensation in the case of large bipolar transient current.The MIN is an electromechanical issue.According to the above analysis,the MIN suppression method based on mechanical resonance mode is proposed.The MIN suppression device,which can adjust the natural frequency of the receiver coil under the condition of primary magnetic field compensation,should be designed.The receiving sensor can work in the mechanical resonance mode under the MIN suppression device.Then,the frequency distribution of MIN is adjusted to avoid the base frequency of the TEM method,that the MIN can be filtered.The MIN level can be reduced.（3）The primary magnetic field distribution in the case of large transient current is analyzed based on the idea of finite element method.The large-size transmitting coil and bucking coil are separated into many finite elements for the calculation.Then,the tolerant displacement of the receiver sensor is limited based on the calculation results.For the rigid detection system,the diameters of the receiver coil,bucking coil and transmitting coil are 0.2 m,1.2 m and 12 m,respectively.The horizontal displacement of the receiver coil is 0.05 m on account of the large transient current.For the semi-rigid detection system,the diameters of the receiver coil,bucking coil and transmitting coil are determined to be 1.2 m,6 m and 25 m,respectively.（4）A single degree-of-freedom mechanical resonance MIN suppression device for rigid detection system is designed within the limit of the tolerant space displacement.The ground test results show that the maximum induced voltage of the receiving sensor is 2.5 V,which meets the requirement of primary field compensation.In addition,the natural vibration frequency of the receiver coil is adjusted to 8.6 Hz by means of impact method,avoiding the TEM base frequency of 25 Hz.According to the field flight test results,the frequency of the MIN is adjusted to 8.6 Hz from previous 10-40 Hz,which is in accordance with the natural frequency of the receiver sensor.The MIN level of the optimized system reaches±5 nV/m².By contrast,the previous MIN level of the rigid detection system is in the range from±15 nV/m² to±35 nV/m².And the MIN level of the AeroTEM system is about±20 nV/m².According to the established forward model,the detection depth can be increased by more than 50 m,and the detection resolution can be greatly improved after reducing the MIN level.（5）A six degree-of-freedom mechanical resonance MIN suppression device is designed for semi-rigid detection system.The conditions of designing the MIN suppression device are the mechanical independence of the receiving sensor and the moderate electromagnetic field in the semi-rigid detection system.According to the kinematics and dynamics analysis,and the simulation model calculation,it is obtained that the six-order natural frequencies of the receiver coil are 1.89,1.89,1.96,3.48,3.67 and 3.67 Hz.The natural frequencies of the receiver coil are lower than the TEM base frequency of 25 Hz.Besides,the receiver coil is insensitive to inducing the MIN based on the model analysis inherently.The ground test results show that the natural frequency of the receiver coil is 3 Hz,which is separated from the 25 Hz.Then,the field flight results show that the frequency of MIN is about 3 Hz.The spectrum separation between the MIN and TEM signal are achieved.In addition,the MIN level of the semi-rigid detection system is±0.5 nV/m²,which is much lower than the MIN level of the rigid detection system.The detecting depth and detecting resolution can be improved.In summary,the MIN suppression method based on mechanical resonance mode is proposed to solve the MIN problem for the rigid and semi-rigid detection system.The parameter of the MIN level can reach or even exceed the parameter of the international advanced systems.The detecting depth and detecting resolution are improved.This study can promote the development of helicopter TEM detection technology in China,which can provide technology support for Chinese geophysical exploration.更多还原