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1.
在多孔隙含水层中地面磁共振（surface nuclear magnetic resonance,SNMR）信号呈现多弛豫衰减特性,常规盲源分离方法和单指数拟合方法引起信号严重失真和信息缺失等问题.本文提出了基于稀疏表示的随机噪声背景下多弛豫SNMR信号的提取方法.根据SNMR信号的衰减特征,设计了精确刻画SNMR信号且与随机噪声不相关的离散衰减余弦冗余字典.其次,针对多弛豫SNMR信号稀疏度未知的问题,通过设置合理的残差比阈值控制迭代次数,改进了广义正交匹配追踪（generalized orthogonal matching pursuit,gOMP）算法,使得该方法应用于SNMR信号的提取时,具有更好的自适应性和普适性.再次,鉴于SNMR测量数据为多次独立重复采集的结果,提出了基于数据流的SNMR信号提取策略,在提高算法鲁棒性的同时,保证了信号提取结果的唯一性.最后,通过仿真和实测数据证明了基于gOMP算法的稀疏表示方法可以显著地提升多弛豫SNMR信号的提取质量,降低随机噪声对含水层反演结果的影响,提高SNMR探测能力. 相似文献
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2.5维复电阻率反演及其应用试验 总被引：1，自引：0，他引：1
自复电阻率法被提出以来,其区分矿与非矿方面的潜力就引起了人们的广泛关注,但反演问题一直没有得到较好的解决,已经严重影响和制约了该方法的应用和发展.对此,本文提出并实现了利用多个排列视电阻率和视相位数据的2.5维SIP联合反演方法.算法利用最小二乘原理构建了反演目标函数,为提高反演的稳定性,在反演方程中加入了Occam法的光滑模型约束.通过借助电场的偏导数形式,推导出了灵敏度矩阵的解析表达式,并应用互换定理对其进行精确求取.反演算法充分利用了电磁感应和激电效应中的异常信息,能够同时反演出二维地质断面上所有单元的四种复电阻率参数.最后,利用该反演程序对安徽某地区的SIP实测数据进行了反演成像,通过与已知钻井资料及CSAMT反演结果的对比分析表明,该反演算法具有良好的应用效果. 相似文献
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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。 相似文献
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《地球物理学进展》2020,(4)
井间电磁成像利用低频电磁波对两井或多井之间进行多点扫描,测量出电磁信号经过地层产生的相位变化和幅度衰减情况,反演得到地层电阻率信息.相位反映地层电参数以及位置的敏感性要优于幅度信息.在进行相位测量时,必须保证在同一时刻对发射监测信号和接收信号进行采集.针对发射和接收系统精确同步测量方法展开研究,提出使用GPS时钟接收机在两口井各自地面系统产生同步时钟信号,把同步时钟信号通过电缆传送到井下,井下通过PLL锁相同步电路和频率补偿电路,完成井下工作主频时钟源的建立,达到地面和井下同步目的.在发射地面和接收地面设置有数传电台,相互之间可传送指令,与同步时钟结合起来,实现井下发射监测信号和接收信号的同步采集.经过测试井间电磁成像发射和接收系统同步误差≤±100 ns. 相似文献
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采用非线性最小二乘法结合蒙特卡罗法,实现宽带高频电磁场椭圆极化率数据的精确反演,确定地下层状介质的真实电阻率和介电常数.反演结果表明,对于均匀半空间和二层介质模型,最小二乘法能够很好地实现反演,而对于三层或更多层的介质,首先利用蒙特卡罗法确定拟合初始模型,再进行最小二乘反演,能够避免收敛到局部极小值,提高了反演的稳定性.为了加速正演响应函数的计算和迭代的速度,采用高密度采样的线性滤波算法,大大加快了该精确反演方法的速度.针对如覆盖区地质填图和土壤调查等大面积确定地质体性质的应用,本文还给出了一种近似反演方法（相位矢量图法）,能够快速获取视电阻率和视介电常数,不仅可以为应用提供有用的基础信息,而且可作为精确反演方法的初始模型. 相似文献
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最近开发了一种针对二维大地电磁野外数据进行处理解释的新反演方法。该方法以加入阻抗相位信息的一维大地电磁连续介质曲线对比法为基础 ,把一维反演得到的电阻率和相位的数据集作为二维反演的初始模型 ,使用二维有限单元法做正演模拟。在程序的后继迭代中 ,深度方向上用一维反演修改模型的电阻率和深度值 ,沿测线方向由二维有限元作修改 ,反演结束可得到一个接近真实电性分布的电阻率数据集 ,并绘制成电阻率断面图。对模型的反演实验结果显示 ,该反演方法能够较真实地反映地下电性分布 ,而且避免了偏导数矩阵的计算 ,其原理简单 ,计算速度快 ,表明该反演方案是可行的 相似文献
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在研究大地电磁响应函数频散关系的基础上,构制了一套滤波系数算法,以用于由一套视电阻率资料估算相应的阻抗相位.理论模型和实际大地电磁观测资料的数字试验表明,该法是行之有效的.由频散关系估算的相位值与观测的相位资料的比较,可用于检验观测资料是否满足频散关系.利用经频散关系校正的阻抗相位值,进行大地电磁阻抗的联合反演则可望获得更为可靠的结果.研究了观测频带相互衔接的电偶源频率电磁测深和大地电磁测深视电阻率的一维联合反演问题.对两个实测点两种电磁法的观测资料进行了联合反演试验,与钻井资料对比表明,所获得的电性分层参数是较为可信的.在补充了由频散关系获取的电偶源频率电磁相位资料后,对于两种电磁法的视电阻率————阻抗相位、阻抗实部视电阻率-阻抗虚部视电阻率进行了拟大地电磁反演,获得了相近的反演结果. 相似文献
11.
Inversion of resistivity in Magnetic Resonance Sounding 总被引：3，自引：0，他引：3
Magnetic Resonance Sounding (MRS, or Surface Nuclear Magnetic Resonance - SNMR) is used for groundwater exploration and aquifer characterization. Since this is an electromagnetic method, the excitation magnetic field depends on the resistivity of the subsurface. Therefore, the resistivity has to be taken into account in the inversion: either as a priori information or as an inversion parameter during the inversion process, as introduced in the presented paper. Studies with synthetic data show that water content and resistivity can be resolved for a low resistive aquifer even using only the amplitude of the MRS signal. However, the inversion result can be significantly improved using amplitude and phase of the MRS signal. The successful implementation of the inversion for field data shows that the resistivities derived from MRS are comparable to those from conventional geoelectric methods such as DC resistivity and transient electromagnetic. By having information about both the resistivity and the water content, MRS inversions give information about the quality of the water in the aquifer. This is of utmost interest in hydrogeological studies as this specific information cannot be determined solely by geoelectric measurements, due to the nonunique dependence of resistivity on water content and salinity. 相似文献
12.
在常规测井约束反演的的基础上,开展神经网络特征参数反演,将波阻抗等地震属性转化为与含水性更为密切的孔隙度、视电阻率数据体,使地震反演的地质属性与测井上的地质属性达到最优的相关性,从而实现应用三维地震对煤层顶板富水进行评价的目的。由于煤层顶板富水区的特殊性质,它同样也是地震后的易破坏层,因而对它的探明从抗震角度以及震害预测角度都是有价值的。以淮北某采区为例,通过孔隙度及电阻率的神经网络反演对研究区10#煤层顶板的富水性进行预测。反演结果表明采区北部发育一个强富水陷落柱,与钻孔揭示结果吻合。采区西部10#煤层顶板与第四系含水层呈不整合接触关系,神经网络反演结果预测为强富水区,同样与井下工程揭示富水特征吻合。利用多属性融合的神经网络反演可有效预测煤层顶板的富水特征,为煤矿安全生产以及抗震提供重要保障。 相似文献
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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. 相似文献
14.
Yves Albouy Pierre Andrieux Gérard Rakotondrasoa Michel Ritz Marc Descloitres Jean-Lambert Join Eddy Rasolomanana 《Ground water》2001,39(1):87-97
Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt water-bearing formations. Interpretation models for these aquifers typically contain four layers: a highly resistive unsaturated zone; a surficial fresh water aquifer of intermediate resistivity; an underlying conductive, salt water saturated aquifer; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated: direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thicknesses of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently. 相似文献
15.
Weng Aihua 《Journal of Applied Geophysics》2010,70(1):84-92
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. 相似文献
16.
To minimize the number of solutions in 3D resistivity inversion, an inherent problem in inversion, the amount of data considered have to be large and prior constraints need to be applied. Geological and geophysical data regarding the extent of a geological anomaly are important prior information. We propose the use of shape constraints in 3D electrical resistivity inversion, Three weighted orthogonal vectors （a normal and two tangent vectors） were used to control the resistivity differences at the boundaries of the anomaly. The spatial shape of the anomaly and the constraints on the boundaries of the anomaly are thus established. We incorporated the spatial shape constraints in the objective function of the 3D resistivity inversion and constructed the 3D resistivity inversion equation with spatial shape constraints. Subsequently, we used numerical modeling based on prior spatial shape data to constrain the direction vectors and weights of the 3D resistivity inversion. We established a reasonable range between the direction vectors and weights, and verified the feasibility and effectiveness of using spatial shape prior constraints in reducing excessive structures and the number of solutions. We applied the prior spatially shape-constrained inversion method to locate the aquifer at the Guangzhou subway. The spatial shape constraints were taken from ground penetrating radar data. The inversion results for the location and shape of the aquifer agree well with drilling data, and the number of inversion solutions is significantly reduced. 相似文献
17.
原状地层水电阻率是重要的储层参数,也是进行精细储层评价的基础.基于泥浆侵入数值模拟与侵入过程中井周岩石物理特征分析,确定了薄膜电位的产生位置,针对储层高、低侵等不同侵入特征,提出了可适用于包括存在"低阻环"等不同侵入特征时储层电阻率分布的"五参数"地层模型,基于几何因子理论与有限元方法,建立了阵列感应与自然电位测井联合反演算法,实现了地层电阻率参数反演,重构了地层径向电阻率剖面,进而精确求取了地层水电阻率.通过对实际资料处理表明:反演算法稳定可靠,"五参数"模型能很好地表征储层电阻率分布形态,重构储层电阻率剖面,确定薄膜电位产生位置;基于阵列感应与自然电位的联合反演,能精确计算原状地层水电阻率,为储层评价与流体性质识别提供依据. 相似文献
18.
Marc Descloitres Olivier Ribolzi Yann Le Troquer Jean Pierre Thibaux 《Journal of Applied Geophysics》2008,64(3-4):83-98
Herbaceous vegetation in the Sahel grows almost exclusively on sandy soils which preferentially retain water through infiltration and storage. The hydrological functioning of these sandy soils during rain cycles is unknown. One way to tackle this issue is to spatialize variations in water content but these are difficult to measure in the vadose zone. We investigated the use of Electrical Resistivity Tomography (ERT) as a technique for spatializing resistivity in a non-destructive manner in order to improve our knowledge of relevant hydrological processes. To achieve this, two approaches were examined. First, we focused on a possible link between water tension (which is much easier to measure in the field by point measurements than water content), and resistivity (spatialized with ERT). Second, because ERT is affected by solution non-uniqueness and reconstruction smoothing, we improved the accuracy of ERT inversion by comparing calculated solutions with in-situ resistivity measurements. We studied a natural microdune during a controlled field experiment with artificial sprinkling which reproduced typical rainfall cycles. We recorded temperature, water tension and resistivity within the microdune and applied surface ERT before and after the 3 rainfall cycles. Soil samples were collected after the experiment to determine soil physical characteristics. An experimental relationship between water tension and water content was also investigated. Our results showed that the raw relationship between calculated ERT resistivity and water tension measurements in sand is highly scattered because of significant spatial variations in porosity. An improved correlation was achieved by using resistivity ratio and water tension differences. The slope of the relationship depends on the soil solution conductivity, as predicted by Archie's law when salted water was used for the rain simulation. We found that determining the variations in electrical resistivity is a sensitive method for spatializing the differences in water tension which are directly linked with infiltration and evaporation/drainage processes in the vadose zone. However, three factors complicate the use of this approach. Firstly, the relation between water tension and water content is generally non-linear and dependent on the water content range. This could limit the use of our site-specific relations for spatializing water content with ERT through tension. Secondly, to achieve the necessary optimization of ERT inversion, we used destructive resistivity measurements in the soil, which renders ERT less attractive. Thirdly, we found that the calculated resistivity is not always accurate because of the smoothing involved in surface ERT data inversion. We conclude that further developments are needed into ERT image reconstruction before water tension (and water content) can be spatialized in heterogeneous sandy soils with the accuracy needed to routinely study their hydrological functioning. 相似文献
19.
The Wadi El Natrun area is characterized by a very complicated geological and hydrogeological system. 45 vertical electrical soundings (Schlumberger array) were measured in the study area to elucidate the peculiarity of this unique regime, specifically the nature of waterless area. 2D and 3D resistivity inversion based on the finite element technique and regularization method were applied on the data set. 2D and 3D model resolution was investigated through the use of the Depth and Volume of Investigation Indexes. A very good matching was found between the zones of high resistivity, the waterless area, and the non-productive wells. The low resistivity zones (corresponding to Lower Pliocene clay) were also identified. The middle resistivity fresh water aquifer zones were recognized. Available results can assist in the aquifer management by selecting the most productive zone of groundwater. 相似文献
20.
The surface nuclear magnetic resonance (SNMR) method has been tested at a site in Haldensleben, northern Germany, to assess the suitability of this new method for groundwater exploration and environmental investigations. More information is obtained by SNMR, particularly with respect to aquifer parameters, than with other geophysical techniques. SNMR measurements were carried out at three borehole locations, together with 2D and 1D direct current geoelectrics, as well as ground-penetrating radar, and well logging (induction log, gamma-ray log and pulsed neutron-gamma log). Permeabilities were calculated from the grain-size distributions of core material determined in the laboratory. It is demonstrated that the SNMR method is able to detect groundwater and the results are in good agreement with other geophysical and hydrogeological data. Using the SNMR method, the water content of the unsaturated and saturated zones (i.e. porosity of an aquifer) can be reliably determined. This information and resistivity data permit in situ determination of other aquifer parameters. Comparison of the SNMR results with borehole data clearly shows that the water content determined by SNMR is the free or mobile water in the pores. The permeabilities estimated from the SNMR decay times are similar to those derived from sieve analysis of core material. Thus, the combination of SNMR with geoelectric methods promises to be a powerful tool for studying aquifer properties. 相似文献