Powerline noise is a severe interference source in urban or mine transient electromagnetic (TEM) surveys. TEM systems generally adopt the synchronous detection scheme to suppress powerline noise. However, considerable powerline noise residue still remains even after synchronous detection when the powerline frequency fluctuates differing from its nominal value (50/60Hz).
In order to solve this problem, Jang Chol Jin, a researcher at the Institute of Nano Science and Technology, has proposed a quantitative suppression method for powerline noise taking into account the instability of powerline frequency. The method is based on the adjustment of base frequency and optimal choice of stacking-times.
He mathematically represented a sufficient condition for powerline noise suppression by synchronous detection. It consists of two equations; one is related to base frequency and the other is connected with stacking-times. He first derived the mathematical relationship between frequency estimation accuracy and residual noise amplitude and stacking-times. Based on it, he developed an efficient algorithm to determine the optimal stacking-times. The algorithm takes as input the powerline noise estimates and the noise tolerance limit. By adjusting base frequency and determining the optimal stacking-times, he reduced the powerline noise residue after synchronous detection to below the desired tolerance limit.
The experimental results show that the method achieves quantitative suppression of unsteady powerline noise and prevents measurement time loss due to excessive stacking without any damage to effective signals.
You can find the details in his paper “Quantitative suppression of unsteady powerline noise in transient electromagnetic surveys: Adjustment of base-frequency and optimal choice of stacking-times” in “Review of Scientific Instruments” (SCI).