An Improved Adaptive Filtering Methodology for Through-the-Earth Extremely Low Frequency Electromagnetic Signals

In through-the-earth extremely low frequency elec-tromagnetic explorations and communications, it is customary to deploy observation receivers with varying transmitter-receiver distances for synchronized observations. The signal-to-noise ratio (SNR) of receiving signals has intense relationship with the distance between the transmitter and receiver. Usually, when the distance is relatively large, it will cause obvious deviations or even errors in the subsequent processing and analysis. However, it is difficult for us to reduce noise without causing phase damage. In this paper, an improved adaptive filtering method was proposed, which took the electric field data with high SNR of the nearby receiver as the reference signal, and then extracted the components related to the reference signal in the electric field data with low SNR of the remote receiver, so as to reduce the local noise in the data of the remote receiver. The processing of simulated and measured data demonstrated that the proposed method did not damaged the phase and effectively improved the data quality.