全相位模型估计方法压制磁共振测深谐波噪声应用研究

Application of all-phase model estimation method for suppressing magnetic resonance sounding harmonic noise

磁共振测深是当前唯一直接找水的地球物理方法。磁共振测深找水仪探测接收性能灵敏度高,能够接收到纳伏(1 nV=10-9 V)级的NMR信号,但易受到环境电磁噪声的干扰,从而使得磁共振测深解释结果的准确性受到影响,往往不能反演出真实的地下含水层结构。针对这一问题,提出了全相位模型估计方法,对电磁噪声中比重最大的工频谐波噪声进行压制。高精度的谐波建模是建模对消法的关键,本方法利用存在时移关系的两段含噪NMR信号分别进行全相位FFT处理,直接取主谱线的相位值即为相位估值,利用主谱线的相位差可获取高精度地频率估值和幅值估值。与传统FFT相比,全相位FFT具有良好的抑制频率泄露特征。通过仿真实验发现,全相位模型估计法比传统的模型去噪法能够更加精确地找到工频谐波参数,精度提高一倍以上。通过实测数据检验发现,全相位模型估计法能够更好地应对实际噪声复杂多变的情况,有效地压制工频谐波的干扰,提高MRS方法反演解释的准确性。

Magnetic resonance sounding is currently the only direct geophysical method of groundwater exploration.The instrument of magnetic resonance sounding has high sensitivity for detecting and receiving performance,and can receive nanovolt(1nV=V)NMR signals,but is susceptible to environmental electromagnetic noise.This restrains the accuracy of the magnetic resonance sounding interpretation results which cannot be reversed to the actual underground aquifer structure.This study proposes an all-phase model estimation method to suppress the power frequency harmonic noise with the largest specific gravity in electromagnetic noise.High-precision harmonic modeling is the key to modeling cancellation.This method uses two-stage noisy NMR signals with time-shift relationship to perform all-phase FFT processing.The phase value of the main line is taken as the phase estimation.The phase difference of the main line can be used to obtain high-precision frequency estimation and amplitude estimation.The all-phase FFT has good frequency leakage suppression characteristics compared with the conventional FFT.Through simulation experiments,we find that the all-phase model estimation method can obtain the power frequency harmonic parameters more accurately than the traditional model denoising method,and the precision is more than doubled.Through the test of the measured data,we find that the all-phase model estimation method can better cope with the complex and variable conditions of the actual noise,effectively suppress the interference of the power frequency harmonics,and improve the accuracy of the MRS method inversion interpretation.