基于频率环激发序列的地下水核磁偏共振探测方法研究

Magnetic Resonance Sounding (MRS) technology has the advantage of directly and quantitatively ground water detection, which is widely used in the fields of groundwater assessment and disaster water detection. However, due to uniform distribution of the geomagnetic field, the magnetic interference and high electromagnetic noise, magnetic off-resonance phenomenon is widespread, resulting in variation of on-resonance signal amplitude and phase. It usually causes inaccurate interpretation of hydrological information, particularly a serious limitation of resolution in the deep. In this project, aiming to the bottleneck that it is difficult to implement effective MRS on resonance measurement under the complicated geological conditions, we breakthrough the constraint of resonance condition, and propose a new magnetic off-resonance sounding method based on the frequency-cycling measurement sequence. We establish a comprehensive off-resonance model of MRS, analyze the characteristic of the signal response, and solve the selection and optimization problem of the excitation sequence parameters based on the L1 norm linear search algorithm. In addition, we carry out SVD method for the resolution analysis and complex signal inversion for the evaluation of the new method. Finally, simulation models and field measurements are used to verify the effectiveness and practicability of the proposed method. The research of this project gets rid of the reliance of Larmor frequency in traditional MRS method, achieve the accurate measurement of MRS signal in complicated environment, which improves the resolution of deep aquifers and the interpretation accuracy of hydrological parameters. The achievement has contribution on theory and application expansion, and will opens up a new development direction in MRS technology.

核磁共振（MRS）技术以其直接定量探测地下水的优势被广泛应用于水资源评价和灾害水探查等领域。但是，由于地磁场时空分布不均、磁性干扰及强环境噪声影响，偏共振现象普遍存在，导致共振信号幅度和相位发生改变，引起水文信息解释不准确和深层分辨率严重受限等问题。本项目针对复杂地质条件下难以实施有效MRS探测的瓶颈，突破共振条件的约束，提出基于频率环激发序列的MRS偏共振探测方法。在建立完备的偏共振模型基础上，分析信号响应特征，提出L1范数线性搜索算法，解决激发序列的参数选择和优化问题；同时，开展SVD分辨率分析及复数域反演评价方法研究；最后，通过仿真模型和野外实测验证偏共振探测方法的有效性和实用性。本项目的研究摆脱了传统MRS探测对Larmor频率的依赖，实现复杂环境下MRS偏共振信号的精确获取，提高对深部含水层的分辨能力和水文参数解释的准确度，为MRS探测理论拓展和方法应用开辟了一个新的发展方向。

由于地磁场在空间上分布不均匀，以及存在外界磁性干扰等问题，产生磁共振现象的拉莫尔频率难以准确确定，导致存在未知频率偏量的偏共振激发。本项目针对未知拉莫尔频率情况下准确探测地下水问题，提出地面双频磁共振探测方法。在估计的拉莫尔频率两侧，根据设定频率偏量发射进行两次偏共振激发，利用设定频率偏量计算偏共振核函数，对双频磁共振探测数据进行反演，从而消除未知频率偏量对含水量和T2*反演结果的影响。本项目针对地面核磁偏共振模型，提出了基于系统相位自动搜索和信号实部与虚部同时参与的复数QT反演方法，通过与幅度反演方法的对比分析，证明了基于复数QT反演方法的二维地面磁共振成像方法适用于复杂的地质环境。针对3D磁共振成像的研究和应用仍然受限于较低的测量效率和成像分辨率。本项目出一种新型的用于3D磁共振的阵列线圈布局，包含一个发射和接收同一线圈和一组接收线圈阵列，以实现高效地数据采集和成像。本项目还提出目基于探地雷达结构约束的二维磁共振成像方法，以实现MRS的高精度反演成像。通过长春市烧锅镇太平池水库和德国Schillerslage地区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，以及本项目提出的核磁偏共振模型和复包络反演方法的有效性和准确性，为地下水探测领域提供了新的技术手段。基于本项目的研究内容，任务执行期间发表SCI 论文15 篇，EI论文1篇，获得授权发明专利7 项。任务执行期间形成双频磁共振探测方法1套，完成DataProcessor数据处理和JLMRSInversion反演解释软件2 套，培养博士研究生3 名，硕士研究生3名。通过本项目的研究，有效改善了磁共振方法在隧道和矿井等极端地下工程应用中灾害水体的探测能力，使磁共振方法能够在更加复杂的环境下开展工作，无论对于提高磁共振方法的应用范围，还是为矿井安全生产提供可靠预警方面，都具有重要的实用价值。