核磁共振测井的预极化分析与优化

Analysis and optimization of pre-polarization methodology for NMR logging

分析核磁共振预极化的必要性与设计原理,针对现有预极化方法不能兼顾较大T1范围与极化效率的问题,提出一种基于过极化、欠极化和调理磁场的分级快速预极化改进方法。利用Bloch极化方程,在特定运动速度与T1范围条件下完成预极化场的优化设计。结果表明,该方法不但可以有效节省核磁共振运动测量过程中的极化时间,而且具有适用T1范围广的优势,能够为核磁共振测井传感器与脉冲序列设计提供新的思路和重要依据。

The requirement for NMR measurement in moving state is an essential feature of NMR techniques in highly timeconstrained conditions,such as NMR logging,downhole and on-line NMR analysis. It’s of significance to introduce pre-polarization to shorten the waiting time,then to increase measurement efficiency in these NMR applications. The theoretical mechanism and design principle of NMR pre-polarization were discussed. Based on existing pre-polarization schemes,a fast three-step pre-polarization technique was proposed. The static B0 fields for pre-polarization consist of an over-polarization field,an under-polarization field and a field for adjustment. By employing a numerical simulation procedure based on Bloch equations,the individual fields in this scheme are optimized and improved under a given movement velocity condition. Efficiency and applicability analysis results demonstrate that this new pre-polarization design and its corresponding optimization methodology can improve the waiting time significantly for a wide T1 components range（ 0- 4. 5 s）,which can provide insights and supports for new moving NMR logging sensor and pulse sequences design.