| Magnetic Resonance Sounding (MRS or Surface NMR) is widely applied in the fielddetecting for groundwater exploration and high efficient evaluation of groundwater resourcesin arid and semi-arid regions of northern China. However, for the groundwater in thecomplicated geological condition, like fissure water in the bedrock and karst water in thesouthwest of China, MRS technology has many problems on low imaging precision, lowdetecting resolution and also low operating efficiency. Therefore, development of newhigh-precision2D/3D imaging method and high-resolution surface detection system has beenthe urgent demand in the field of MRS technology. Due to the defects of existed offsetmeasurement with coincident and separated loop, a new measurement with array loop isproposed in this thesis, which can replace the result of multiple measurements with separatedloops by only one measurement, and has the advantage of high-efficiency, high-resolutionand high-coverage. For the complicated groundwater in non-layered and non-uniform space,high-precision2D and3D imaging methods of water content as well as relaxation timeT*2are presented. On the basis of MRS imaging, array surface MRS system with multi-channelacquisition units is developed, and the results of field experiments and application cases aredescribed. The main research work and achieved results of this thesis are as follows:1. As1D MRS ignores the direction angle of loop layout, I introduce the rotation anglematrix to solve the MRS forward calculation problem with arbitrary angles of loop directionand the geomagnetic field. Moreover, I analyze the influence of the loop profile directionangel in2D measurements by simulation models.2. For the low resolution and efficiency of the existed2D measurements, I propose anarray loop configuration to carry out the fast and high-coverage measurement in2D profile.By analyzing the resolution radius, I conclude that array loop configuration can achieve2Dhigh-resolution detection by only one measurement, and the combination of rectangulartransmitter loop and half-over-lapping receiver loops is the optimal configuration.3. For the low efficiency and limited coverage in3D measurement, I take advantageof the diagonal measurement with array loops, which significantly improves the resolution inrespect to the coincident loop. It can detect a large coverage area of3D groundwater in highresolution with a small number of measurements. 4. To the issues of large number meshes, low precision imaging and instability ofalgorithm, I propose a non-uniform mesh generation mouthed based on the resolution radius,combine with discrepancy principle and L-curve method to select the regularization parameter,and apply Jacobain transform in objective function linearization and non-constraint spaceconversion. Finally, the stability high-precision MRS imaging of2D water content is solvedby the initial value inversion (IVI).5. By simulating and inverting a complicated groundwater model by IVI, I contrastimaging results of coincident, separated and array loop configurations, and find therectangular array loop in half-over-lapping measurement mode provides a high resolution of2D MRS even in the case of low signal-to-noise (SNR). The conclusion consists with thetheoretical analyses of2D resolution studies.6. Due to the storage of2D sensitivity kernels with multi-relaxation process in QTinversion using complete envelop data, I offer two solutions. One is to simplify the kernel inmono-relaxation process, and the imaging with two parameters, water content and singlerelaxation time, is inverted. In addition, the inversion results are more stable than IVI. Theother is base on the2D water cell inversion, which can achieve the water content andmulti-relaxation times imaging.7. For the low precision in the3D imaging based on uniform meshes and high-errorestimation of water volume, I use the non-uniform meshes with different density in the targetand the surrounding regions, and accomplish the3D water content imaging based on IVI. Thesummation and filed data from Einersberger Lake in Germany is tomography inverted inhigh-precision3D image, and the water volume is quantified with high accuracy.8. According to the composition and defects of existing single-channel surface MRSsystem, I develop multi-channel receiver units and control software for array loopconfiguration. By improving the rapid consumption circuit, the dead time of system reducesfrom38ms to15ms, and the data resolution of acquisition circuit and the signal conditioningcircuit are also improved. The structure of master with multi-client and hand-by-handconnection is used to achieve the high-precision synchronization control and high-speed datatransmission. These key technologies make the new system to catch up the most advantageinstrument in the world. Based on the study of the above contents, I complete the development of2D/3D MRSgroundwater imaging method and its array surface detection system. The main innovation liesas follow:The array loop configuration I proposed can achieve high-resolution profiledetection by one measurement in2D, and large coverage area detection by diagonaloffset measurement in3D, which solves the low-efficiency and low-resolutionproblem of traditional MRS measurement.The resolution radius I proposed is useful for the evaluation method of surface MRSdetection, and after the contras, I find the rectangular array loop in half-over-lappingmeasurement mode provides the highest resolution in2D. Based on the resolutionradius, I also general the non-uniform meshes in2D and3D for high-precisionimaging.I combine the regularization parameter selection methods with discrepancy principleand L-curve method and apply Jacobain transform in objective function linearizationand non-constraint space conversion to realize the stability high-precision MRSimaging of2D and3D water content by the initial value inversion (IVI).To the single relaxation and multi-relaxation properties of MRS signal envelope, Iimprove the QT inversion and propose Cell inversion to image the water content andalso relaxation timeT*2.The MRS imaging method and surface MRS system have been applied in engineeringpractice, like the groundwater source detection for Halatura iron ore in Mongolia, the damleakage detection in Huanren, Liaoning province of China, and the landslide stabilitydetection in Yantaicun Landsilde, Anhui province of China. The results of2D groundwaterimaging are consisting with the other geophysics methods, verifying the effectiveness andpracticality of the method and system. It provides a strong technical support for groundwaterexploration in southwest of China and complicated geological environment. |
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