基于地面核磁共振阵列线圈的二维QT反演方法研究

发布时间：2018-04-10 21:55

本文选题：核磁共振 + 阵列线圈　；参考：《吉林大学》2015年硕士论文

【摘要】：地面核磁共振测深技术凭借对地下水直接探测和定量评估的优势成为一种快速兴起的现代地球物理找水方法，近年来在探索开采地下淡水资源和预防水灾害方面得到广泛应用。但是，针对地质构造复杂、地下水非层状不均匀分布的二维核磁共振探测方法仍不成熟，存在发射/接收分离线圈多次测量综合成像耗时过长、工作效率低和二维初始振幅反演信号数据利用率低、只能得到含水反演结果的问题。为了解决这些问题，本文开展了阵列式测量模式的分辨率和成像效果研究；提出了利用全部的矩核磁共振探测包络信号进行反演的二维QT反演方法（Q-Time Inversion，简称QT），，这种反演方法能够有效利用所有激发脉冲和探测时间得到的核磁共振信号，完成弛豫时间和含水量双参数的核磁共振成像；最后利用阵列式测量模式进行数值仿真和对实测核磁共振数据进行数据处理，对比分析二维初始振幅反演方法、二维单指数QT反演方法和二维多指数QT反演方法的核磁共振成像结果。针对二维核磁共振探测时接收和发射线圈沿测线移动多次测量效率低的问题，根据阵列式测量理论，本文提出了三组不同的线圈类型和线圈尺寸的铺设方案，构造了一个具有多个含水体的复杂含水模型，分别计算和分析三组线圈的的分辨率和成像效果，结果表明阵列式测量模式能够提高工作效率，不同的线圈尺寸和测量模式具有不同的优势，根据实际需要采用不同的测量方案。针对二维初始振幅反演利用拟合得到的初始振幅进行数据解释，未考虑弛豫时间等信息的问题，基于核磁共振信号的指数特性和全部的包络数据，提出并推导了针对单个弛豫时间的二维单指数QT反演方法和针对多个弛豫时间的二维多指数QT反演方法，通过矩阵转换成一般形式并给出求解方法。二维QT反演（二维单指数QT反演和二维多指数QT反演）实现了含水量和弛豫时间双参数成像，能够更加准确地反应地下水赋存信息。通过构造数值模型，分别利用二维初始振幅反演、二维单指数QT反演和二维多指数QT反演进行数据解释，对比分析三种反演方法的成像结果。最后，利用德国北部的Eddelstorf地区阵列式测量核磁共振数据，分别采用二维初始振幅反演、二维单指数QT反演和二维多指数QT反演方法进行数据处理，与地质资料进行对比，验证二维单指数QT反演方法和二维多指数QT反演方法的有效性。采用阵列式测量模式和二维QT反演方法能够实现缩短探测时间、提高测量效率和对具有多种弛豫时间的复杂含水构造的弛豫时间和含水量双参数成像，并通过实例验证了测量方式和反演方法的优越性和实用性，对二维核磁共振探测技术的应用具有指导意义。
[Abstract]:Based on the advantages of direct detection and quantitative assessment of groundwater, surface nuclear magnetic resonance sounding (NMR) technology has become a rapidly emerging modern geophysical method for water prospecting. In recent years, it has been widely used in the exploration of underground freshwater resources and the prevention of water disasters.However, in view of the complexity of geological structure and the uneven distribution of groundwater, the 2D NMR detection method is still immature, and it takes a long time for multiple measurements and integrated imaging of transmitting / receiving separation coils.The problem of low efficiency and low utilization ratio of 2-D initial amplitude inversion signal can only obtain the result of water-bearing inversion.In order to solve these problems, the resolution and imaging effect of array measurement mode are studied in this paper.A two-dimensional QT inversion method called Q-Time inversion (QT) is presented, which uses all the moment nuclear magnetic resonance detection envelope signals for inversion. This inversion method can effectively utilize the nuclear magnetic resonance signals obtained from all the excitation pulses and the detection time.Nuclear magnetic resonance imaging with two parameters of relaxation time and water content is completed. Finally, numerical simulation and data processing of measured nuclear magnetic resonance data are carried out using array measurement mode, and the two-dimensional initial amplitude inversion method is compared and analyzed.Magnetic resonance imaging results of two dimensional single exponent QT inversion and two dimensional multi exponential QT inversion.In order to solve the problem of low measurement efficiency of receiving and transmitting coils moving along the measuring line in two-dimensional nuclear magnetic resonance detection, according to the theory of array measurement, three different types of coils and laying schemes of coil size are proposed in this paper.A complex water-bearing model with multiple water bodies is constructed. The resolution and imaging effect of three sets of coils are calculated and analyzed respectively. The results show that the array measurement mode can improve the working efficiency.Different coil sizes and measuring modes have different advantages, and different measurement schemes are adopted according to the actual needs.Based on the exponential characteristic of nuclear magnetic resonance signal and the whole envelope data, the initial amplitude is interpreted by fitting the original amplitude, and the relaxation time is not taken into account.Two dimensional single exponent QT inversion method for single relaxation time and two dimensional multi exponential QT inversion method for multiple relaxation time are proposed and derived.Two-dimensional QT inversion (two-dimensional single-exponent QT inversion and two-dimensional multi-exponential QT inversion) realizes two-parameter imaging of water content and relaxation time, which can reflect groundwater occurrence information more accurately.By constructing a numerical model, the imaging results of the three inversion methods are compared and analyzed by using two-dimensional initial amplitude inversion, two-dimensional single-exponential QT inversion and two-dimensional multi-exponential QT inversion.Finally, the nuclear magnetic resonance data are measured by array method in Eddelstorf area in northern Germany. The data are processed by two-dimensional initial amplitude inversion, two-dimensional single-index QT inversion and two-dimensional multi-index QT inversion, respectively, and compared with geological data.The validity of two dimensional single index QT inversion method and two dimensional multi exponential QT inversion method is verified.The array measurement mode and two-dimensional QT inversion method can be used to shorten the detection time, improve the measurement efficiency and image the relaxation time and water content of complex water-bearing structures with multiple relaxation times.The advantages and practicability of the measurement method and inversion method are verified by an example, which is of guiding significance to the application of two-dimensional nuclear magnetic resonance detection technology.
【学位授予单位】：吉林大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TH763.1

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