地面核磁共振找水反演

简要介绍了地面核磁共振找水的正演理论方法,采用高斯求积并结合连分式展开的方法对核磁共振核函数中包含双重贝塞尔函数的积分核进行了数值积分,进而计算出精度较高的核函数值,在此基础之上,讨论了导电性对地面核磁共振信号的影响.基于奇异值分解算法,对核磁共振找水理论模型进行了反演研究,在不导电的情况下,重构出了比较理想的含水率分布.总结并改进了模拟退火算法,用改进的模拟退火算法分别对覆盖层高阻和导电条件下的核磁共振人工合成数据进行了反演,试验结果表明,利用改进的模拟退火算法反演地面核磁共振是可行的,而且反演结果较稳定,收敛速度较快.     

地面核磁共振信号相位求取电阻率

椭圆极化现象是电磁场在导电介质中传播时产生的,为得到准确的相位信息,在计算激发磁场时应考虑椭圆极化。本文研究了地面核磁共振(SNMR)信号相位求取电阻率的基本方法,利用SNMR信号通过广义逆反演法得到电阻率未知情况下的含水率及层位分布,作为相位反演电阻率的先验值,通过模拟退火法实现对电阻率的反演。对SNMR的研究解决了通过相位对电阻率的求取,有利于进一步利用相位信息提高含水层和含水率反演的精度。      

核磁共振与瞬变电磁三维联合解释方法

传统核磁共振地下含水量解释多采用基于均匀半空间或层状导电模型的一维反演,分层给出地下含水信息.然而,这些方法忽略了地下复杂电阻率分布信息对结果的影响,也不能很好地反映局部三维含水构造.本文从三维电介质中核磁共振响应的正演理论出发,提出首先利用瞬变电磁数据进行基于等效导电平面法的快速电阻率成像,然后将成像结果作为核磁共振三维反演的电性模型,进行联合解释.激发磁场的分布采用有限元法直接求解,通过引入伪δ源实现电流源的加载,并强加散度条件排除了三维磁场模拟中"弱解"的影响.针对核磁共振灵敏度矩阵的病态性和数据中存在的干扰信号,提出考虑罚项的非线性拟合目标函数,利用线性化方法进行核磁共振反演.模型数据表明该方法能较准确反映地下三维含水构造,实测算例进一步证明了方法的有效性.本研究将促使核磁共振方法在岩溶、裂隙水、孤立水体等复杂水文地质条件及隧道、矿井灾害水源探测等方面得到有效应用.     

层状导电介质中地面核磁共振响应特征理论研究

水平层状导电大地中层状含水层地面核磁共振响应的数值计算技术涉及到导电介质中回线源 磁场的计算以及地下含水层中质子磁矩在线圈中产生感应信号的体积积分. 文中采用直接数 值积分技术,对具有振荡核函数的Hankel变换进行积分,以求取回线源磁场的径向与垂向分 量,计算并研究了回线产生的径向与垂向磁场分量随空间位置的变化规律. 基于磁场空间分 布特点,利用不等间距空间剖分技术计算地面核磁共振的体积积分,模拟了不同模型的地面 核磁共振响应并讨论了其影响因素. 结果表明,结合能对任意层状导电介质中磁场进行稳定 快速计算的直接数值积分技术与不等距空间模型剖分技术,可正确模拟地面核磁共振响应. 导电性是产生地面核磁共振信号相位的先决条件,但影响响应振幅强度与相位的因素还有含 水层的埋深、厚度以及装置大小等.     

1D SNMR吉洪诺夫正则化反演方法研究

地面核磁共振（SNMR）技术是目前世界上直接用来寻找地下水的技术。本文在讨论反演基本问题的基础上,用共轭梯度法实现了核磁共振的一维正则化反演。通过反演理论数据、噪声数据和实测数据,说明该方法的可靠性。     

地面可控源频率测深三维非线性共轭梯度反演

讨论了地面可控源电磁勘探三维非线性共轭梯度反演的可行性以及反演过程中考虑场源的必要性.反演采用非线性共轭梯度反演方法.反演过程中,模型响应利用交错网格有限差分技术计算.反演数据采用与发射源平行的电场x分量E_x.利用层状导电模型作为背景,设计了两个理论模型进行数值试验:第一个模型中包含两个电阻率异常,以检验反演的有效性;第二个模型中,在测区外设置了一个低阻异常,以考察源的信息在反演中的作用.两个模型的反演分别从层状背景模型开始,迭代120次后终止.数值试验结果表明,(1)非线性共轭梯度反演所获得的电阻率分布和理论模型吻合较好;(2)非线性共轭梯度算法收敛速度较慢,需要较多的迭代次数完成反演;(3)对于可控源频率电磁勘探,必须考虑源位置信息.因此,本文采用考虑场源信息的地面可控源非线性共轭梯度反演方法能完成真正意义上的可控源频率电磁测深数据的反演.     

地面核磁共振技术发展述评

核磁共振技术在医学、地质等方面都有广泛的应用.运用核磁共振技术找寻地下水资源,开创了用地球物理方法直接找水的先河.地面核磁共振技术是目前唯一能直接探测地下水的地球物理方法.本文通过对核磁共振技术在国内外的应用和发展现状的介绍,总结了地面核磁共振技术现在存在的问题及其影响,同时提出了该技术今后的发展前景.     

利用SE求取SNMR横向弛豫时间方法研究

自旋回波作为测量T₂时间的重要脉冲序列,由该脉冲系列交变磁场激发而获得的核磁共振信号不受磁场不均匀性的影响,这对于以地磁场作为稳定磁场的SNMR方法来说,可以减小地磁场不均匀对NMR信号的影响,得到真实、可信的T₂值,提高信噪比。将自旋回波信号应用到地面核磁共振找水方法中求取T₂分布,在国内尚属首例,本文借鉴核磁共振测井中的多指数反演方法,采用奇异值分解算法编程,反演得到T₂分布,并与SNMR反演软件的结果T₂*对比。结果证明,用SE信号提取T₂时间,可以减少地磁场不均匀对NMR信号的影响,更真实地反映T₂分布,提高信噪比。      

核磁共振双TW测井数据联合反演与流体识别

针对核磁共振测井双TW观测数据分析和流体识别的需要,研究了基于全局搜索的遗传算法和局部搜索的最小二乘法的联合反演算法,实现了核磁共振双TW观测数据处理.首先,研究了饱和油气水岩石物理模型的核磁共振双TW观测模式的测井响应机理;然后,利用全局搜索性能优良的遗传算法, 对核磁共振回波差数据进行了反演,计算出了流体的核磁弛豫性质及其体积;最后,以遗传算法的反演结果为初值, 利用阻尼最小二乘方法对双TW回波串进行更精细的反演,计算出了双TW的T₂分布、孔隙度和流体饱和度.理想模型的合成数据和实际测井资料应用表明,遗传算法与最小二乘方法相结合是一种行之有效的联合反演方法,能较好地实现核磁共振测井双TW观测数据的处理和流体评价.     

基于Backus-Gilbert理论的孔隙介质核磁共振弛豫反演

孔隙介质核磁共振(NMR)弛豫数据的多指数反演在NMR测井和岩心分析中均非常重要.本文基于Backus-Gilbert(BG)理论,提出一种NMR弛豫多指数反演的新方法.从解的非唯一性出发,不仅构造出一种解估计,更重要的是评价各种可接受的解估计,通过引入解估计分辨率和解估计方差对解进行评价,找出最佳折中解.通过算例,比较了新方法与传统正则化方法的效果,结果表明新方法具有明显优越性,在低信噪比条件下解稳定.最后分析了新方法的影响因素.     

Occam's inversion of magnetic resonance sounding on a layered electrically conductive earth

The magnitudes of the initial amplitude of the magnetic resonance sounding (MRS) signals from an aquifer located in a layered electrically conductive earth, are nonlinear functions of water content distribution. Occam's inversion method is adapted to the nonlinear inversion problem. In the case of an electrically conductive medium, the Jacobian matrix is analytically evaluated at the beginning of the inversion. And the uniqueness of the inversion can be partially solved by imposing the flattest and smoothest model constraints on the optimization problem. Synthetic MRS signals from resistive and conductive earth, as well as field data, have been inverted by Occam's method. The results indicate that with the help of Occam's inversion, a true model can be obtained from an initial model of homogeneous water content. Furthermore, for noise-free MRS signals, both the flattest and smoothest models reveal correct water content distributions. When signals are contaminated by noises, the case is different; and the smoothest model might have a lower water content distributing in a larger range than that of the true model, while which might be obtained by utilizing the flattest model Occam's inversion.     

基于印模法的地面磁电阻率数据三维非线性共轭梯度反演

本文提出了能提高异常体分辨能力,同时得到绝对电导率的地面磁电阻率数据三维反演方法.磁电阻率响应用准直流的低频磁场代替;数值模拟由频率域电场满足的Helmholtz方程出发,采用三维交错网格有限差分法;长直导线源作为发射源,其中源的计算包含在背景场中;结合地面磁电阻率数据各分量的特点,选择y分量进行反演研究;反演采用三维非线性共轭梯度反演技术,为了提高异常体的深度分辨能力,进行迭代重构反演;用印模法对初始模型进行重构,采用的是辅模型在浅部,元模型在深部的组合方式.从合成数据和实际数据的反演结果可以得到以下的认识:(1)由频率域麦克斯韦方程组出发,低频磁场数据反演可以直接得到电导率,而不是相对电导率之比;(2)采用印模法组合初始模型,进行迭代重构反演,可以提高地面磁电阻率数据反演对异常体的分辨能力,确定埋深位置,同时不会丧失对于浅部异常体的分辨能力;(3)在结合印模法的地面磁电阻率数据三维反演中,深部异常体的分辨能力受地表不均匀导电体影响较小;(4)确定印模深度可以采用上一次重构反演结束时的模型变化量,通过相邻两次重构反演结束时的模型变化量之差来确定迭代重构是否终止.因为静磁场与重力场在数学上的相似性,本文的反演方法可以被运用到重力场等位场的地面数据的反演中.     

Semiautomatic and Automatic Cooperative Inversion of Seismic and Magnetotelluric Data

Natural source electromagnetic methods have the potential to recover rock property distributions from the surface to great depths. Unfortunately, results in complex 3D geo-electrical settings can be disappointing, especially where significant near-surface conductivity variations exist. In such settings, unconstrained inversion of magnetotelluric data is inexorably non-unique. We believe that: (1) correctly introduced information from seismic reflection can substantially improve MT inversion, (2) a cooperative inversion approach can be automated, and (3) massively parallel computing can make such a process viable. Nine inversion strategies including baseline unconstrained inversion and new automated/semiautomated cooperative inversion approaches are applied to industry-scale co-located 3D seismic and magnetotelluric data sets. These data sets were acquired in one of the Carlin gold deposit districts in north-central Nevada, USA. In our approach, seismic information feeds directly into the creation of sets of prior conductivity model and covariance coefficient distributions. We demonstrate how statistical analysis of the distribution of selected seismic attributes can be used to automatically extract subvolumes that form the framework for prior model 3D conductivity distribution. Our cooperative inversion strategies result in detailed subsurface conductivity distributions that are consistent with seismic, electrical logs and geochemical analysis of cores. Such 3D conductivity distributions would be expected to provide clues to 3D velocity structures that could feed back into full seismic inversion for an iterative practical and truly cooperative inversion process. We anticipate that, with the aid of parallel computing, cooperative inversion of seismic and magnetotelluric data can be fully automated, and we hold confidence that significant and practical advances in this direction have been accomplished.     

Pore size distributions and hydraulic conductivities of rocks derived from Magnetic Resonance Sounding relaxation data using multi-exponential decay time inversion

In hydrogeology there is a variety of empirical formulae available for determination of hydraulic conductivity of porous media, all based on the analysis of grain size distributions of aquifer materials. Sensitivity of NMR measurements to pore sizes makes it a good indicator of hydraulic conductivity. Analogous to laboratory NMR, Magnetic Resonance Sounding (MRS) relaxation data are of a multi-exponential (ME) nature due to the distribution of different pore sizes in an investigated rock layer. ME relaxation behaviour will also arise due to the superposition of NMR signals which originate from different layers. It has been shown, that both kinds of ME behaviour coexist in MRS and can principally be separated by ME inversion of the field data. Only a few publications exist that have proposed approaches to qualitatively and quantitatively estimate petrophysical parameters such as the hydraulic conductivity from MRS measurements, i.e. MRS porosity and decay times. The so far used relations for the estimation of hydraulic conductivity in hydrogeology and NMR experiments are compared and discussed with respect to their applicability in MRS. Taking into account results from a variety of laboratory NMR and MRS experiments mean rock specific calibration factors are introduced for a data-base-calibrated estimation of hydraulic conductivity when no on-site calibration of MRS is available. Field data have been analysed using conventional and ME inversion using such mean calibration values. The results for conventional and ME inversion agree with estimates obtained from well core analysis for shallow depths but are significantly improved using a ME inversion approach for greater depths.     

Pore size distributions and hydraulic conductivities of rocks derived from Magnetic Resonance Sounding relaxation data using multi-exponential decay time inversion

In hydrogeology there is a variety of empirical formulae available for determination of hydraulic conductivity of porous media, all based on the analysis of grain size distributions of aquifer materials. Sensitivity of NMR measurements to pore sizes makes it a good indicator of hydraulic conductivity. Analogous to laboratory NMR, Magnetic Resonance Sounding (MRS) relaxation data are of a multi-exponential (ME) nature due to the distribution of different pore sizes in an investigated rock layer. ME relaxation behaviour will also arise due to the superposition of NMR signals which originate from different layers. It has been shown, that both kinds of ME behaviour coexist in MRS and can principally be separated by ME inversion of the field data. Only a few publications exist that have proposed approaches to qualitatively and quantitatively estimate petrophysical parameters such as the hydraulic conductivity from MRS measurements, i.e. MRS porosity and decay times. The so far used relations for the estimation of hydraulic conductivity in hydrogeology and NMR experiments are compared and discussed with respect to their applicability in MRS. Taking into account results from a variety of laboratory NMR and MRS experiments mean rock specific calibration factors are introduced for a data-base-calibrated estimation of hydraulic conductivity when no on-site calibration of MRS is available. Field data have been analysed using conventional and ME inversion using such mean calibration values. The results for conventional and ME inversion agree with estimates obtained from well core analysis for shallow depths but are significantly improved using a ME inversion approach for greater depths.     

大斜度井和水平井井地三维电阻率数值模拟和联合反演

井地电阻率成像法利用井套管作电流源向井下供入大功率直流电流,在地表测量由地下介质的电性变化形成的电位分布,通过反演可得到地下介质的电阻率分布.针对大斜度井和水平井开展井地三维电阻率数值模拟和反演研究,对油田注水及压裂效果监测具有重要意义.基于井地电阻率成像法原理,采用有限差分法和不完全切勒斯基共轭梯度法进行了三维正演模拟,研究了大斜度井和水平井的井地电位响应特征.提出了采用层状约束阻尼最小二乘法由浅到深地进行大斜度井和水平井的多层联合反演,并对实际水平井井地电位各个层段数据进行了三维反演.模拟结果表明,倾斜线源和水平线源会对地面电位响应产生明显影响,在反演中需要考虑线源形态.实际水平井井地电位反演成像表明,考虑倾斜线源或者水平线源的联合反演得到了准确的水平井三维注水层成像图,得出注水层的真电阻率分布,能够判断注水运移方向.     

A Small‐Diameter NMR Logging Tool for Groundwater Investigations

A small‐diameter nuclear magnetic resonance (NMR) logging tool has been developed and field tested at various sites in the United States and Australia. A novel design approach has produced relatively inexpensive, small‐diameter probes that can be run in open or PVC‐cased boreholes as small as 2 inches in diameter. The complete system, including surface electronics and various downhole probes, has been successfully tested in small‐diameter monitoring wells in a range of hydrogeological settings. A variant of the probe that can be deployed by a direct‐push machine has also been developed and tested in the field. The new NMR logging tool provides reliable, direct, and high‐resolution information that is of importance for groundwater studies. Specifically, the technology provides direct measurement of total water content (total porosity in the saturated zone or moisture content in the unsaturated zone), and estimates of relative pore‐size distribution (bound vs. mobile water content) and hydraulic conductivity. The NMR measurements show good agreement with ancillary data from lithologic logs, geophysical logs, and hydrogeologic measurements, and provide valuable information for groundwater investigations.     

基于不等式约束的三维电阻率探测混合反演方法

三维电阻率探测的线性反演和非线性反演中均存在着多解性的固有难题.电阻率线性反演方法的效率较高,但反演结果对初始模型的依赖性较强,易陷入局部极小;而非线性反演方法不依赖初始模型,但搜索效率极低,尚未见到关于三维电阻率非线性反演的文献.针对上述问题,融合线性与非线性反演方法的互补优势,提出了最小二乘法(线性方法)与改进遗传算法(非线性方法)相结合的混合反演方法的概念和思想.首先,提出了将介质电阻率变化范围作为不等式约束引入反演方程的思路,以实现压制多解性、提高可靠性的目标.提出了宽松不等式约束和基于钻孔推断的局部严格不等式约束的获取及定义方法.在此基础上,分别提出了基于不等式约束的最小二乘线性反演方法和遗传算法非线性反演方法.其次,对于遗传算法在变异搜索方向控制、初始群体产生等方面进行了改进,优化了其搜索方向和初始群体多样性.然后,提出了混合反演方法及其实现方案,利用改进遗传算法进行第一阶段反演,发挥其对初始模型的依赖程度低的优势,搜索到最优解附近的空间,输出当前最优个体;利用最小二乘法进行第二阶段反演,将遗传算法得到的当前最优个体作为初始模型,在最优解附近空间执行高效率的局部线性搜索,最终实现地电结构的三维成像.最后,开展了合成数据与实际工程算例验证,与传统最小二乘方法进行了对比,发现混合反演方法在压制多解性、摆脱初始模型依赖和提高反演效果方面有较好效果.     

分层多指数磁共振弛豫信号反演方法研究

磁共振测深技术传统反演方法包括平滑反演和分块反演,通过分别获取初始振幅和平均弛豫时间构建地层含水量及有效孔隙度.然而,这些方法局限于单指数拟合方式,损失了大部分有效采集信息,受限于多孔地质环境解释,并在某些情况下无法刻画含水层清晰分界面.针对上述问题,本文建立了基于MRS全数据的多指数反演方法,依据全部采集时间下的有效信息,通过弛豫时间e指数分解,推导出新的磁共振正演核函数,结合泛函极小值方程,直接反演建立含水量,弛豫时间及地层深度三个重要参数关系,适用于复杂地质环境解释.为得到快速稳定的反演结果和更清晰的含水层分界面,本文借鉴分块反演思想,进一步构建了新的反演目标函数,利用基于不等式约束的空间信赖域算法进行优化,最终实现了一种基于分层反演与多指数结合的磁共振弛豫信号反演方法.模型数据以及实测算例表明该方法的效果和优势,并具备较高的计算效率,本研究为地面磁共振反演提供了一种新的思路与方法.