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Removal of Co-frequency Powerline Harmonics from Multichannel Surface NMR Data
- Publisher: European Association of Geoscientists & Engineers
- Source: Conference Proceedings, 23rd European Meeting of Environmental and Engineering Geophysics, Sep 2017, Volume 2017, p.1 - 5
Abstract
Powerline harmonics is often the dominant noise source in surface NMR data and can completely overwhelm the NMR signal. Several methods have been successfully implemented for removal of powerline harmonics including notch filtering, multichannel filtering and model-based subtraction. However, the performance of these methods can be problematic when one of the powerline harmonic components has a frequency close to or coincident with the Larmor frequency, referred to as a co-frequency harmonic. Removal of the co-frequency harmonic can distort the NMR signal causing erroneous estimates of water content and relaxation rates. To solve this problem we propose an extended method of multichannel model-based subtraction of powerline harmonics. In this method, the co-frequency component is modeled in the primary coil and a reference coil on noise-only data recorded immediately prior to the NMR excitation. The phase and amplitude relationships between the components measured in the two coils are calculated. The relationships are used to predict the co-frequency harmonic component parameters in the primary coil during the NMR decay based on NMR signal free, synchronously recorded reference coil data. The mathematical framework is developed and we give examples of the efficiency of the method based on synthetic NMR signals embedded in noise-only data.