基于参考线圈和变步长自适应的磁共振信号噪声压制方法

核磁共振地下水探测仪的灵敏度高,接收到的纳伏级磁共振探测信号极易受到强工频谐波噪声的干扰,导致信号特征参数提取的准确度降低,影响反演解释的水文地质参数结果.为了解决这一难题,基于相关抵消的原理,针对全波磁共振信号,设计带有参考线圈的90°移相自适应噪声抵消系统,理论计算了参考线圈相对于探测线圈的距离,提出变步长LMS算法进行噪声压制.仿真结果表明,在不同的信号强度及不同的信噪比下,当信号与工频谐波干扰频谱不重合时,采用设计的自适应噪声抵消系统和变步长算法,信噪比可以提高到5.94 dB以上,初始振幅、弛豫时间特征参数的拟合误差在2.8%以内;当信号与工频谐波干扰频谱重合时,采用双向自适应滤波算法,信噪比可以达到5dB以上,初始振幅、弛豫时间特征参数的拟合误差在10%以内,可以满足实际应用的要求;实测数据处理进一步证明了方法的有效性.

Variable step adaptive noise cancellation algorithm for magnetic resonance sounding signal with a reference coil

Due to the high sensitivity of magnetic resonance sounding(MRS) instrument for groundwater exploration,the nV-level MRS signal received from the instrument is susceptible to interference of high industrial frequency harmonics,which leads to accuracy decrease in signal parameter extraction and influences the inversion result.To solve this problem,a 90° phase-shift adaptive noise cancellation system with reference coil for MRS full wave signal,based on the principle of correlation cancellation,is designed in this paper.The distance of reference coil away from the detection coil is theoretically calculated,and the algorithm of variable step adaptive noise cancellation is presented.The simulation results show that with different signal amplitudes and signal to noise ratios(SNR),adopting the adaptive noise cancellation system and variable step algorithm can increase SNR to 5.94dB or above,while the fitting error of initial amplitude and relaxation time is within 2.8%,when industrial frequency harmonics are not in coincidence with the signal frequency.However,when they are in coincidence,SNR can reach 5dB or above and the fitting error of initial amplitude and relaxation time is within 10%.The result of the variable step adaptive noise cancellation algorithm meets the requirement of practical application.Processing of field measured signals further proved the effectiveness of this algorithm.