| Airborne electromagnetic exploration is a kind of geophysical exploration method based on the principle of electromagnetic induction and has advantages such as terrain adaptability and high efficiency.It is suitable for large scale,high precision and high resolution exploration.Airborne electromagnetic flight measurements will introduce a large amount of noise,in which the amplitude of the coil motion noise is large.Therefore,it is important to analyze the characteristics of coil motion noise,which not only provides theoretical support for the development of the detection instrument hardware system,such as improving the mechanical structure,installation and suspension of the coil and reducing the attitude change and vibration effect of the pod,but also lay the foundation for soft-filter method.This article relies on the sub-project of the National Major Scientific Research Equipment Project,"Airborne Transient Electromagnetic System(All Aviation)Simulation,Data Processing and Inversion" and the National High Technology Research and Development Program(863 Program)."The key technology of Patch-type time domain helicopter airborne electromagnetic detection data processing and mapping".The paper analyzes the causes of the coil motion noise and deduces the calculation formula of the three-component coil motion noise,and studies the time-frequency characteristics and statistical characteristics of the z-component coil motion noise.The motion noise of the measured coil is suppressed by the ultra-late interpolation method and the minimum noise fraction method.The major findings and research contents are as follows:According to the mechanism of motion noise of airborne electromagnetic detection coil,the calculation formula of three-component coil motion noise is deduced.The threecomponent coil motion noise in the case of single changing attitude angle and simultaneous changing multi-angle is calculated by using the attitude angle roll,pitch and yaw obtained by receiving the coil in the flight test.In the case of a single attitude angle change,the motion noise of the x-component coil is independent of the change of the attitude roll.The motion noise of the y-component coil is independent of the changing attitude pitch.The motion noise of the z-component coil is independent of the change ofthe attitude yaw.In the case of simultaneous changes in multi-attitude angle,the calculated x-component coil motion noise is about ± 0.05 V,the calculated y-component coil motion noise is about ± 0.05 V,and the calculated z-component coil motion noise is about ± 0.03 V.Taking the calculation results of z-component coil motion noise and the coil motion noise measured by field flight as an example,the time domain and frequency domain characteristics of coil motion noise and its profile are studied.In order to study the relationship between the attitude angle and the coil motion noise,the time-frequency analysis of the pitch and the pitch and the z-component coil motion noise is carried out by using the short-time Fourier transform and the wavelet transform.The results show that the attitude angle roll and pitch and their rate of change will affect the frequency characteristics of the z-component coil motion noise calculation results.The statistical characteristics of coil motion noise of airborne electromagnetic are studied.Using the statistical histogram and probability distribution paper,the Gaussian properties of the data are analyzed,which proves that the attitude angle roll and the pitch,and the z-component coil motion noise and the measured coil are basically in accord with the Gaussian distribution.Based on the characteristics analysis of the coil motion noise in the airborne electromagnetic exploration,the super-late interpolation method and the minimum noise separation transformation method are used to noise suppression.The horizontal trajectory of each half-cycle data is connected and interpolated to form an approximate baseline,ie,the coil motion noise is simulated and then subtract it from the original data.The ultraadvanced interpolation method reduces noise level from ±8 m V to ±3 m V,the noise level with abnormal line profile is reduced from ±40 n T/s to ±25 n T/s.The noise reduction process of the original data is carried out by using the eigenvalue and eigenvector of the noise anomaly.The unknown noise is estimated as the important core technology of the minimum noise fraction.In this paper,the noise is estimated by the adaptive window width filter.The noise level of the abnormal line profile is reduced to ±10 n T/s without affecting the amplitude of the abnormal noise,and the method can effectively suppress the late channel noise without affecting the amplitude of the anomaly noise. |
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