Document Type : Original Research Paper

Authors

1 Ph.D. Student, Science and Research Branch of Islamic Azad University, Tehran, Iran

2 Assistant Professor, Science and Research Branch of Islamic Azad University, Tehran, Iran

3 Professor, Tehran University, Iran

Abstract

Magnetotellurics is an exploration method which is based on measurement of natural electric and magnetic fields of the Earth and is increasingly used in geological applications, petroleum industry, geothermal sources detection and crust and lithosphere studies. In this work, discrete wavelet transform of magnetotelluric signals was performed. Discrete wavelet transform decomposes signals into coefficients in multi-scales. Noise and signal portions are separable in multi-scale mode. Therefore, noise can be discarded in each scale; a threshold value is constructed dependent to coefficients of the scale then, the noise coefficients are discarded by thresholding the coefficients with the proper values. Proportional threshold values can be used to remove white and 1/f noise from time series. After that, a new signal is constructed using clean coefficients. This method is widely used in various fields of sciences from image processing to seismic studies. This work tried to show the effectiveness of this technique in decreasing pervasive noise from magnetotelluric signals. The results emphasized the advantageous effect of wavelet techniques in magnetotelluric data noise removing process.

Keywords

References
Chave, A. D. and Jones, A. G., 2012- The Magnetotelluric Method Theory and Practice: Cambridge University Press.
Chave, A. D. and Thomson, D. J., 1989- Some Comments on Magnetotelluric Response Function Estimation, Journal Of Geophysical Research, 94, 14215-14226.
Chave, A. D. and Thomson, D. J., 2004- Bounded influence estimation of magnetotelluric response functions, Geophys. J. Int., 157, 988-1006.
Chave, A. D., 2014- Magnetotelluric Data, Stable Distributions and Impropriety: An Existential Combination, Geophysical Journal International, 198(1), 622-636.
Chave, A. D., 2016- Estimation of the Magnetotelluric Response Function: The Path from Robust Estimation to a Stable MLE, Review paper, 23rd EMI workshop, Thailand.
Chave, A. D., Thomson, D. J. and Ander, M. E., 1987- On the Robust Estimation of Power Spectra, Coherences, and Transfer Functions, Journal Of Geophysical Research, VOL. 92, NO. B1, 633-648.
Donoho, D. L. and Johnstone, I. M., 1994- Ideal spatial adaptation by wavelet shrinkage, Biometrika, 81, 425-455.
Donoho, D. L., and Johnstone, I. M., 1995- Adapting to Unknown Smoothness via Wavelet Shrinkage, Jouunal of the American Statistical Association, Vol.90, No. 432, Theory and Methods
Egbert, G. D. and Booker, J. R., 1986- Robust estimation of geomagnetic transfer functions, Geophysical Journal International, 87, 173–194.
Egbert, G. D., 1992- Short Note: Noncausality of the discrete-time magnetotelluric impulse response, Geophysics, VOL. 57, NO.10, 1354-1358.
Egbert, G. D., 1997- Robust multiple-station magnetotelluric data processing. Geophys. J. Int., 130, 475-496.
Egbert, G. D., 2002- Processing and interpretation of electromagnetic induction array data. Surv. Geophys., 23, 207-249.
Gamble, T. D., Goubau, W. M. and Clarke, J., 1979- Magnetotellurics with a remote reference. Geophysics, 44, 53-68.
Garcia, X. and Jones, A. G., 2008- Robust processing of magnetotelluric data in the AMT dead-band using the Continuous Wavelet Transform, Geophysics, 73, F223-F234.
Goubau, W. M., Gamble, T. D. and Clarke, J., 1978- Magnetotelluric data analysis: Removal of bias. Geophysics, 43, 1157-1162
Mallat, S., 2008- A wavelet tour of signal processing, Academic press is an imprint of Elsevier.
Simpson, F. and Bahr, K., 2005- Practical Magnetotellurics: Cambridge University Press.
Soman, K. P., Ramachandran, K. I. and Resmi, N. G., 2010- Insightd into wavelets from theory to practice, Phi learning private limited, New Delhi-110001.
Stein, C., 1981- Estimation of the mean of multivariate normal distribution. Annals of statistics 9, 6, 1135-1151.
Trad, D. O. and Travassos, J. M., 2000- Wavelet filtering of magnetotelluric data, Geophysics, 65, 482-491.
Zhang, Y. and Paulson, K. V., 1997- Enhancement of signal to noise ratio in natural source transient magnetotelluric data with wavelet transform, Pure and applied Geophysics, 149, 405-419.