Mitigating Narrowband Noise Sources Close to the Larmor Frequency in Surface NMR

A common noise source in surface nuclear magnetic resonance (NMR) is narrowband noise sources that occur at frequencies close to the Larmor frequency, such as power-line harmonics or other sinusoidal signals of unknown origin. These noise sources can lead to significant perturbation on the estimated NMR signal and degrade the accuracy of subsurface characterizations. We demonstrate that the spectral analysis envelope detection scheme, where the envelope is estimated using the discrete Fourier transform for a suite of sliding windows, can be modified to mitigate the effect of narrowband noise sources. Selection of an appropriate length rectangular window is shown to allow one to effectively place a stopband at frequencies very close to the Larmor frequency, where the stopband may be selected to coincide with narrowband noise sources. The result is a significant reduction in the influence of the narrowband noise source on the estimated envelope, without introduction of distorting transients on the signal. Synthetic and field results are presented to illustrate the effectiveness of the proposed approach, as well as identify limits on the required resolution of the narrowband noise source's frequency.