Efficient processing power harmonic noise with fluctuation frequency in urban transient electromagnetic surveys

Power harmonic noise is the main noise interference source in transient electromagnetic (TEM) surveys in the urban environment. State of the art techniques, such as the traditional bipolar waves and superposition average without distorting the TEM signal, are mainly used in the TEM method to process power harmonic noise, but the effect of this method becomes very poor when the fundamental frequency of power harmonic noise fluctuates. To solve the problem, we propose the same phase and adjacent power harmonic noise subtraction method with bipolar waves of specific working frequency. Taking the fluctuation of harmonic frequency into account, the accurate fundamental frequency of power harmonic noise is obtained by the wavelet-Nyman, Gaiser, and Saucier estimator. In addition, we adjust the TEM working frequency and duty cycle according to the fundamental frequency to ensure that the power harmonic noises in the adjacent bipolar data have the same phase, and the adjacent bipolar signals are subtracted to suppress the power harmonic noise in one emitting cycle. In order to verify the effectiveness of the new method, we use a synthetic TEM signal embedded in simulation data and actual collected data for testing. The experimental results show that power harmonic noise with the fundamental frequency fluctuation can be removed efficiently without distorting the TEM signal, and the new method performs better than traditional standard methods.