Ground-Airborne electromagnetic signals de-noising using a combined wavelet transform algorithm
Ground-Airborne electromagnetic signals de-noising using a combined wavelet transform algorithm
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发表时间：
2013
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通讯作者：
L. Su
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作者：
L. Su
飞艇载地面-机载时域电磁系统具有较高的探测深度和空间分辨率，探测效率高，飞行控制方便。但由于低空飞行受风向、气流、地形和地面温差的影响较大，前置接收线圈会切断飞艇高度、航迹和姿态变化所产生的地球磁场，产生电磁噪声、运动噪声和基线漂移。这些缺点会导致电磁数据的电阻率成像较差。因此，前置接收线圈会切断由飞艇高度、航迹和姿态变化引起的地球磁场，并产生电磁噪声和运动噪声，基线漂移。这些缺点会导致电磁数据的电阻率成像较差研究电磁数据的去噪方法对反演解释至关重要。由于有效电磁数据的频带和部分噪声同时出现，传统的滤波技术或去噪方法都有其局限性。为此，本文提出了一种组合去噪方法，该方法根据电磁数据的特点采用sym8小波基，利用小波多分辨率分析对基线漂移进行高阶逼近估计，并应用小波阈值收缩理论，采用MinimMaxi阈值的五级小波分解和硬收缩函数对其他噪声进行去噪处理，并分别采用理论响应和基于异常环路模型的实测数据对组合去噪方法进行了验证，最终结果证实了所提出的去噪方法是有效的去噪方法，这表明该方法对机载时间域电磁数据是适用和有效的。
Airship-based ground-airborne time domain electromagnetic system enjoys high depth of prospecting and spatial resolution,as well as outstanding detection efficiency and easy flight controlling.However,due to low-altitude flying is markedly affected by wind direction,air flow,landform,and the difference of temperature filed around ground surface,the front-fixed receiving coil would cut earth magnetic field,which results from changes of the altitude,track,and gesture of the airship,and causes electromagnetic and moving noises,baseline drift as well.The drawbacks mentioned above could lead to inferior resistivity image formation of electromagnetic data.Consequently,to investigate methods of removing noises of electromagnetic data is of vital importance to inversion explanation.With simultaneously occurred frequency band of valid electromagnetic data and part of the noise,traditional filter technique or noise-removing ways have their own limitations.Therefore,this study proposes a combined de-noising method,which adopts sym8wavelet basis according to the characteristic of the electromagnetic data,corrects the baseline drift using estimation of high-level approximation by applying wavelet multi-resolution analysis,and suppresses other noises by five-level wavelet decomposition with MiniMaxi threshold and Hard shrinking function by applying wavelet threshold shrinkage theory.The combined de-noising method is validated by adopting the theoretical response and wild measured data based on the Anomaly Loop Model,respectively.The final results confirm that the proposed method eliminates noises effectively,which indicates it is applicable and effectual to groundairborne time domain electromagnetic data.