| 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/Hz1/2@1kHz.Then,equivalent noise voltage and the 3 dB bandwidth of the fabricated receiving sensor reach 1.83 nV/Hz1/2@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. |
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