Impulse noise suppression method for marine MT data based on frequency-domain Hampel identification

The marine magnetotelluric (MT) method utilizes the natural MT signal received by the ocean-bottom electromagnetic (OBEM) stations, and calculates the apparent resistivities and phases based on the impedance tensor estimation to investigating the sub-oceanic conductivity structures.The OBEM often generate noises that appear as irregular high-frequency pulses with particular frequency characteristics. Consequently, the impedance tensors of the affected frequencies deviate from the true values, which makes it difficult to interpret the data correctly. To mitigate the influence of the impulse noise on marine MT signals, an impulse noise suppression method based on frequency-domain Hampel identification is proposed. Firstly, the low-frequency energy of marine MT data is reduced by first-order differential pre-whitening to improve the noise recognition rate of the Hampel identifier. Then, the frequencies affected by the impulse noise in the four components of MT data are identified using the Hampel identifier in the frequency domain. Finally, considering the stability of MT impedance estimation, the noise frequencies in the four components are merged and universally amplitude-attenuated to suppress the impulse noise, so that the apparent resistivity and phase distortion can be corrected. Based on the periodic difference between MT signal and impulse noise, the autocorrelation function which can be used for measured data is introduced to evaluate the efficacy of the proposed method in suppressing impulse noise. Moreover, the proposed method is tested using the synthetic and measured MT data. The results show that the proposed method can effectively suppress the impulse noise in the marine MT data, reduce the periodic peaks caused by the noise in terms of the autocorrelation function curve, correct the apparent resistivity and phase distortion induced by the noise, and thereby enhances the quality of MT data .