地面磁共振测深分布式探测方法与关键技术

地面核磁共振方法(MRS)因具有对地下水探测定性、定量的特点而备受地球物理工作者关注.传统研究中,人们局限于一维探测方法,假设层状含水构造,导致复杂地质环境下难以确定井位、不均匀含水层小水体难以分辨的反演解释瓶颈.针对现有测量中的不足,本文提出了MRS二维分布式探测模式,依据激发场不均匀特性,定义了实际测量中的测线方位角α,推导了分布式接收线圈MRS响应核函数表达式,实现了二维正演计算,探索了α角与探测灵敏度之间的关系.在此基础上,首次将Occam方法用于MRS二维反演解释中,实现了磁共振断层成像MRT(magnetic resonance tomography).模型试算中,根据含水层位置以及环境噪声变化的磁共振响应,客观评价了分布式MRS探测适用范围.理论先行可推动仪器完善,本文通过分布式接收单元设计、接收线圈数量和匝数增加与调整、放大器参数自适应设置与矫正,成功研制了地面分布式磁共振探测系统,并进行了野外验证.本文的研究成果将为基岩裂隙水定位、堤坝渗漏灾害水源评价,喀斯特溶洞含水构造精确探测提供有力的科学支撑.

Theories and key technologies of distributed surface magnetic resonance sounding

Magnetic Resonance Sounding (MRS) has received much attention of geophysicists due to its qualitative and quantitative features in detection of groundwater. In traditional studies, people are limited to one-dimensional of instruments and interpretation methods based on layering aquifer models, which makes well location difficult in complex geological contexts and small water bodies in non-uniform aquifers not easy to identify in inversion interpretation. To solve these problems, this paper proposes a two-dimensional MRS distributed detection strategy. It builds on the non-uniform property of the excitation field, defines a detection profile angle α, and deduces a MRS response kernel function of distributed receiver coil. By realizing the 2D forward calculation, we explore the relationship between angle variations and detection sensitivity. In addition, it is the first time that Occam inversion is used for two-dimensional data interpretation of MRS, and magnetic resonance tomography (MRT) technique is accomplished. In the modeling calculation, the applicable scope of distributed MRS detection is estimated objectively by setting different positions of aquifers as well as the changes of environmental noise. Antecedent theory can promote instrument improvement. The research group successfully developed the MRS distributed detection system through designing distributed receiver unit, adjusting receiving coil number and the number of turns and setting amplifier adaptive parameters. This method can be used in groundwater evaluation. The results of this study can also provide a powerful scientific basis for positioning of fissure water, assessment of dam leakage water hazard and 3D accurate detection of karst aquifers.