| 基于双频磁共振的未知拉莫尔频率地下水探测方法 |
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| 引用本文： | 刘健楠,田宝凤,嵇艳鞠,蒋川东.基于双频磁共振的未知拉莫尔频率地下水探测方法[J].地球物理学报,1954,63(11):4244-4255. |
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| 作者姓名： | 刘健楠 田宝凤 嵇艳鞠 蒋川东 |
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| 作者单位： | 1. 吉林大学仪器科学与电气工程学院, 长春 130026;2. 地球探测信息仪器教育部重点实验室, 长春 130026 |
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| 基金项目： | 国家自然科学基金青年项目（41604083），面上项目（41674109）和中央高校基本科研业务费专项资金资助. |
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| 摘 要： | 在利用地面磁共振方法进行地下水探测时，要求发射频率与拉莫尔（Larmor）频率相同.由于地磁场的不均匀性和噪声干扰，实际测量时无法获得准确的Larmor频率，导致发生偏共振现象.如果假设磁共振或频率偏量未知时，反演得到的含水量和弛豫时间（T2*）将存在较大误差.为了解决未知Larmor频率情况下的准确探测问题，本文在频率环测量方案的基础上提出了双频磁共振探测方法，即只需知道Larmor频率的范围区间，通过区间外的两个频率进行偏共振激发，利用频率差值的一半作为频率偏量计算核函数，即可消除未知Larmor频率的影响.通过仿真实验，证明了在电阻率大于100 Ωm时，双频磁共振和偏共振的核函数偏差较小.对于双层含水层模型，双频磁共振和偏共振的信号偏差小于3 nV，因此得到双频磁共振信号可以用偏共振核函数进行反演.通过对假设磁共振、假设偏共振和双频磁共振反演结果的对比，可以得到在未知频率偏量和设定频率偏量改变时，双频磁共振均能得到准确的反演结果.但是，当环境噪声增加和电阻率降低时，反演结果的准确度降低.最后，通过长春市烧锅镇采集数据的反演结果与已知钻探资料进行对比，验证了双频磁共振探测方法的有效性和准确性.
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| 关 键 词： | 磁共振测深 拉莫尔频率 偏共振 地下水 |
| 收稿时间： | 2019-11-05 |
Dual-frequency magnetic resonance sounding method for groundwater detection under unknown Larmor frequency |
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| LIU JianNan,TIAN BaoFeng,JI YanJu,JIANG ChuanDong.Dual-frequency magnetic resonance sounding method for groundwater detection under unknown Larmor frequency[J].Chinese Journal of Geophysics,1954,63(11):4244-4255. |
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| Authors: | LIU JianNan TIAN BaoFeng JI YanJu JIANG ChuanDong |
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| Institution: | 1. College of Instrumentation and Electrical Engineering, Jilin University, Changchun 130026, China;2. Key Laboratory of Geo-Exploration Instrumentation, Ministry of Education, Changchun 130026, China |
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| Abstract: | When the magnetic resonance sounding (MRS) method is used to detect groundwater, the transmitting frequency is required to be the same as the Larmor frequency. Due to the inhomogeneity of the Earth's magnetic field and noise interference, the accurate Larmor frequency can not be obtained during field measurement, resulting in a phenomenon of off-resonance (off-MRS). If the on-resonance excitation (on-MRS) is assumed or the frequency offset of off-MRS is unknown, there will be a large error in the water content and relaxation time (T2*) from the inversion. In order to solve the problem of accurate detection under unknown Larmor frequency, this paper proposes a dual-frequency magnetic resonance sounding (DF-MRS) method based on the frequency cycling measurement scheme, which only needs to know the range of Larmor frequency, and performs double off-resonance excitation through two frequencies outside the range. Using half of the frequency difference as the frequency offset for calculating kernel function, DF-MRS can eliminate the influence of the unknown Larmor frequency. Simulation experiments show that the kernel function deviation of DF-MRS and off-MRS is small when the resistivity is greater than 100 Ωm. For the two-layer aquifer model, the signal deviation of DF-MRS and off-MRS is less than 3 nV, hence the DF-MRS signal can be inverted by the off-resonance kernel function. Comparing the assumed on-MRS, the assumed off-MRS and the DF-MRS inversion results indicate that the DF-MRS can obtain accurate inversion results when the unknown frequency offset and the set frequency offset are changed. However, as the ambient noise increases and the resistivity decreases, the accuracy of the inversion results decreases. Finally, the inversion results of the data collected at the Shaoguo Town in Changchun City are compared with the drilling to prove the validity and accuracy of the DF-MRS method. |
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| Keywords: | Magnetic resonance sounding Larmor frequency Off resonance Groundwater |
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