IEEE Magnetics Letters | 2019
Data Correction for Surface Nuclear Magnetic Resonance in an Inhomogeneous Geomagnetic Field
Abstract
In surface nuclear magnetic resonance, the geomagnetic field serves as the background field, and hydrogen protons in groundwater are excited with an alternating field at the Larmor frequency. However, in practice, an inhomogeneous geomagnetic field (IGF) or a magnetic anomaly caused by ferromagnetic materials will make the Larmor frequency nonunique, resulting in simultaneous on-resonance and off-resonance signals (composite resonance). If only the on-resonance or off-resonance model is used to interpret the composite resonance data, the accuracy of the inversion analysis would be adversely affected. To solve this problem, we derived an expression for the composite magnetic-resonance free-induction-decay (FID) signal based on a layered model. The influence of IGF on the characteristic parameters of the FID signal was analyzed with simulation models, which showed that, with an increase in frequency offset and gradient, the relaxation time decreased prominently. We demonstrate an iterative correction method based on a frequency-depth curve. Through simulated and measured data, we show that the water content and relaxation time are consistent with ground truth after correction and that data consistency is improved.