一种降低井下核磁共振振铃的新方法

核磁共振（NMR）测井仪在测量过程中，振铃噪声幅度一般很高，会影响回波信号的检测. 常用的交叉相位对脉冲序列（PAPS）虽然能有效地降低振铃影响，但由于振铃与频率有关，使PAPS叠加只能在同频率之间展开，大大降低了NMR测井的纵向分辨率. 利用回波信号和振铃噪声的相位特征，设计了一种脉冲序列，采用PAPS和回波间叠加相结合的方式降低振铃噪声. 研究表明，相对于PAPS技术，提高了测量效率，同时提高了回波串的信噪比和NMR测井的纵向分辨率.

A Novel Method for NMR Ringing Reduction under Downhole Condition

In NMR logging,the noise amplitude of ringing is often high,impacting echo signals detection.The phase-alternate pulse sequence(PAPS) technique is a conventional method to reduce NMR ringing.However,the amplitude of ringing is frequency-dependent,and the PAPS technique is effective at certain frequency,leading to a lower vertical resolution.Taking advantage of the feature that the echo signals change with the variation of RF pulse wherase the ringing noise keeps constant,we proposed a novel method,combining PAPS and stacking-between-echoes,to reduce NMR ringing under downhole condition.The results showed that this method has improved efficiency.The echo train measured by the new method had the characters of high SNR and high vertical resolution compared to that measured by the PAPS technique,and the stacking between the echo trains was not limited to one frequency.