摘要:
传统的核磁共振激发场的计算中,很少考虑三维地下介质的电性变化以及发射线圈姿态对激发场的影响,笔者借助地面核磁共振三维有限元正演模拟,开展了核磁共振三维响应影响因素分析.首先,基于三维有限元方法实现了非均匀介质条件下地面核磁共振激发场的计算,以便于分析介质电性变化对核磁共振响应的影响;其次,在源的加载过程采用近似函数来等效场源的作用,使之能够适用于不同发射回线激发的情况,进而利用旋转矩阵实现垂直地磁场分量的计算;最后通过不同条件下核磁共振响应的计算,分析了盖层电阻率、含水体背景电阻率、局部不均匀体以及发射线圈姿态等参数对核磁共振信号的影响.计算结果表明:核磁共振响应对高阻盖层电阻率变化不敏感;低阻含水体背景对核磁共振响应的影响较大;异常体规模越大,对核磁响应的影响越大,对于一定规模的异常体,低阻条件下的核磁共振响应差异明显;与电性参数变化相比,不同的发射线圈姿态对核磁信号影响更大,野外应考虑线圈姿态影响.
Abstract:
In view of the problem that the traditional calculation of surface nuclear magnetic resonance (SNMR) excitation field seldom considers various three-dimensional subsurface electric properties and different forms of launcher,the authors carried out surface NMR three-dimensional finite element modeling which premeditates the impact of a conductive medium.First,based on the three-dimensional finite element method,the authors realized the calculation of surface NMR excitation field under the condition of inhomogeneous medium.An approximate function was adopted to replace field source,so that it could be applied to different cases of transmitter loop.Then,the vertical component of excitation field was obtained through rotation matrix.Finally,the NMR response under different conditions was calculated.Several typical models were designed to analyze the effect of cap rock resistivity,formation resistivity,inhomogeneous medium and transmitter coil posture on SNMR signal.Numerical simulation shows that nuclear magnetic resonance is not sensitive to the change of the high cap rock resistivity.Low formation resistivity has great effect on SNMR response.The larger the inhomogeneous medium,the greater the SNMR response.For a certain size of the local inhomogeneous body,the difference between the SNMR response curves under the condition of low resistance is obvious.The coil posture should be considered in the field because it considerably impacts the NMR signal,and the coil orientation should be considered because the influence of the coil orientation on the magnetic signal is greater than electrical parameters.
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