A wavelet-based baseline drift correction method for grounded electrical source airborne transient electromagnetic signals
Yuan Wang 1 Yanju Ji 2 Suyi Li 1 3 Jun Lin 1 2 Fengdao Zhou 1 Guihong Yang 11 College of Electrical Engineering and Instrumentation, Jilin University, Changchun 130026, China.
2 Key Laboratory of Earth Information Detection Instruments, Ministry of Education, Jilin University, Changchun 130026, China.
3 Corresponding author. Email: lsy@jlu.edu.cn
Exploration Geophysics 44(4) 229-237 https://doi.org/10.1071/EG12078
Submitted: 4 December 2012 Accepted: 24 July 2013 Published: 4 September 2013
Abstract
A grounded electrical source airborne transient electromagnetic (GREATEM) system on an airship enjoys high depth of prospecting and spatial resolution, as well as outstanding detection efficiency and easy flight control. However, the movement and swing of the front-fixed receiving coil can cause severe baseline drift, leading to inferior resistivity image formation. Consequently, the reduction of baseline drift of GREATEM is of vital importance to inversion explanation. To correct the baseline drift, a traditional interpolation method estimates the baseline ‘envelope’ using the linear interpolation between the calculated start and end points of all cycles, and obtains the corrected signal by subtracting the envelope from the original signal. However, the effectiveness and efficiency of the removal is found to be low. Considering the characteristics of the baseline drift in GREATEM data, this study proposes a wavelet-based method based on multi-resolution analysis. The optimal wavelet basis and decomposition levels are determined through the iterative comparison of trial and error. This application uses the sym8 wavelet with 10 decomposition levels, and obtains the approximation at level-10 as the baseline drift, then gets the corrected signal by removing the estimated baseline drift from the original signal. To examine the performance of our proposed method, we establish a dipping sheet model and calculate the theoretical response. Through simulations, we compare the signal-to-noise ratio, signal distortion, and processing speed of the wavelet-based method and those of the interpolation method. Simulation results show that the wavelet-based method outperforms the interpolation method. We also use field data to evaluate the methods, compare the depth section images of apparent resistivity using the original signal, the interpolation-corrected signal and the wavelet-corrected signal, respectively. The results confirm that our proposed wavelet-based method is an effective, practical method to remove the baseline drift of GREATEM signals and its performance is significantly superior to the interpolation method.
Key words: airship, baseline drift, electromagnetic signals, grounded electrical source airborne transient electromagnetic (GREATEM), wavelet analysis.
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