核磁共振2D地下水成像及3D正演研究

发布时间：2018-05-05 05:45

本文选题：地面核磁共振 + 地下水成像　；参考：《中国地质大学》2017年博士论文

【摘要】：地面核磁共振(Surface Nuclear Magnetic Resonance,SNMR)是目前唯一能够实现地下水直接定量探测的地球物理方法,且它在水文地质调查、地下水管理与污染监测、灾害水源超前探测等领域具有巨大应用潜力。与常规物探找水方法相比,SNMR技术具有高分辨力、信息量丰富、直接定量、非侵害性等优点,能有效解决一些常规物探方法不能解决的基本问题。自从首台核磁共振找水仪诞生以来,SNMR技术被30多个国家广泛应用于地下水探测和水文地质调查工作,在砂岩、粘土岩、石灰岩等不同水文地质环境下均获得了良好的勘查效果。特别是商用型仪器NUMIS和GMR问世后,SNMR技术正获得越来越多国家的认可和推广使用。目前,SNMR技术自身还处于快速发展阶段,仍然存在探测深度有限、抗干扰能力较弱、2D/3D含水目标体探测效果不佳、浅层应用时仪器笨重,数据处理与解释需提高等问题。比如随着野外勘探场景更多地涉及到复杂水文地质环境,仅用传统收发共圈(coincident,COI)测量模式的振幅信号进行反演解释的做法,对孤立含水体(如上层滞水)以及在复杂环境下(如山区,岩溶,裂隙水)的探测精度与效率就不理想,这限制了地面核磁共振技术的实际应用范围。针对传统收发共圈测量模式仅用振幅信号进行反演解释的不足,本文基于新信号响应公式,考虑椭圆极化和相位延迟效应,获得了三种线圈模式的2D灵敏度函数及其实虚部分布,从基本层面详细分析了三种线圈模式的横纵向分辨率特性,然后充分利用不同线圈模式灵敏度函数的差异化分布以及虚部信号对深部含水体的高灵敏特性,基于GN-CG算法,自编程序实现了不同线圈模式下SNMR二维振幅与复信号地下水成像。研究发现:COI模式能提供最好的纵向分辨率,半覆盖分离线圈模式(half-overlapping,HOL)具有较好的横纵向分辨率,边对边(edge-to-edge,E2E)模式由于收发线圈分离距较大而引起信号过度衰减,导致其纵向分辨率较差,且横向分辨率优势被淹没,优化组合多种线圈模式则能提供最佳的横纵向分辨率和探测效率;相对于振幅信号单独反演,复信号反演能显著提高地下水成像精度,提高深部分辨率,且在较强噪声条件下,复信号反演仍能获得较好的成像结果。为了拓展SNMR技术的应用范围,实现复杂水文地质环境下3D地下水探测,首先就必须深入开展3D正演研究,获得不同条件下3D含水体的信号响应规律,以及介质导电性、含水体深度、覆盖层厚度等因素对正演响应的影响。本文将SNMR技术最常用的圆形回线源视为有限个首尾相接的电偶源,进行源的加载。在实现场源的有效加载之后,基于电场总场矢量有限元法,计算出3D复杂电性结构下圆形回线在地下任意测点处的激发磁场。针对场源奇异性对回线附近测点数值解的不利影响,本研究采用非结构化四面体网格结合局部加密技术进行消除。并通过对比均匀半空间条件下的解析解和三维矢量有限元解,验证了本文算法的正确性。然后基于SNMR正演响应公式,考虑介质导电性引起的椭圆极化和相位延迟效应,利用自主开发的算法获得了典型三维含水构造的振幅和相位响应曲线,并进一步研究了介质导电性、含水体深度、覆盖层厚度等因素对信号响应的影响,最终实现了不同水文地质条件下三维复杂含水模型的正演模拟,为SNMR三维地下水勘查奠定了一定基础。
[Abstract]:Surface Nuclear Magnetic Resonance (SNMR) is the only geophysical method to realize direct quantitative detection of groundwater, and it has great potential in the fields of hydrogeological investigation, groundwater management and pollution monitoring, and the advance detection of water source in disaster water. Compared with the conventional geophysical prospecting method, SNMR technology is used. With the advantages of high resolution, abundant information, direct quantitative, non invasive, and so on, it can effectively solve some basic problems that can not be solved by some conventional geophysical methods. Since the birth of the first nuclear magnetic resonance water detector, SNMR technology has been widely used in more than 30 countries in groundwater exploration and water geological survey, in sandstone, clay rock, limestone and so on. Good exploration results have been obtained in different hydrogeological environment. Especially after the advent of commercial instrument NUMIS and GMR, SNMR technology is gaining recognition and popularization in more and more countries. At present, SNMR technology itself is still in the rapid development stage, and there is still limited detection depth, weak anti-interference ability, and detection of 2D/3D water target body. The effect is not good, the instrument is heavy in the shallow layer and the data processing and interpretation need to be improved. For example, as the field exploration scenes are more involved in the complex hydrogeological environment, the amplitude signal of the traditional coincident (COI) measurement model is retrieved and interpreted, and the isolated water body (such as the upper stagnant water) and the complexity are complex. The detection precision and efficiency of the environment (such as mountain area, karst, fissure water) is not ideal, which limits the actual application range of the ground magnetic resonance technology. The paper is based on the new signal response formula, considering the elliptical polarization and phase delay effect. The 2D sensitivity function and the virtual part distribution of the three coils mode are analyzed in detail from the basic level. Then the differential distribution of the sensitivity functions of different coils and the Gao Ling sensitivity characteristic of the virtual part signal to the deep water body are fully utilized, and the self compiled program is implemented based on the GN-CG algorithm. SNMR two-dimensional amplitude and complex signal groundwater imaging under different coil modes. The study shows that the COI model provides the best longitudinal resolution, the semi cover separation coil mode (half-overlapping, HOL) has a better transverse resolution, and the side pair edge (edge-to-edge, E2E) mode causes the over attenuation of the signal due to the larger separation distance of the receiving and receiving coils. As a result, the vertical resolution is poor, and the lateral resolution superiority is submerged. The optimal combination of multiple coil modes can provide the best transverse resolution and detection efficiency. Compared with the amplitude signal, the complex signal inversion can significantly improve the accuracy of the groundwater imaging, improve the deep part discrimination, and the complex signal inversion under the strong noise condition. In order to expand the application range of SNMR technology and realize the detection of 3D groundwater in complex hydrogeological environment, first of all, we must carry out the 3D forward research to obtain the signal response law of 3D containing water under different conditions, as well as the positive response of the medium conductivity, the depth of the water body, the thickness of the cover layer and so on. In this paper, the most commonly used circular return source of SNMR technology is regarded as a limited first and end connection source, and the source is loaded. After the effective loading of the field source, the excitation magnetic field of the circular return line under the ground is calculated on the basis of the field vector finite element method, which is based on the singularity of the field source. This study uses unstructured tetrahedral meshes and local encryption technique to eliminate the negative influence of the numerical solution of the measured points near the loop. The correctness of the proposed algorithm is verified by comparing the analytical solutions of the uniform half space and the three-dimensional vector finite element solution. Then, based on the SNMR forward response formula, the conductivity of the medium is considered. With the effect of elliptical polarization and phase delay, the amplitude and phase response curves of typical three-dimensional water bearing structures are obtained by using the autonomous development algorithm, and the influence of dielectric conductivity, depth of water body and thickness of cover layer on the response of the signal is further studied. Finally, the three-dimensional complex water cut model under different hydrogeological conditions is realized. Forward modeling lays a solid foundation for SNMR 3D groundwater exploration.

【学位授予单位】：中国地质大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P631;P641.7

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