Fast 3D joint inversion of gravity and magnetic data based on cross gradient constraint

The gravity and magnetic data can be adopted to interpret the internal structure of the Earth.To improve the calculation efficiency during the inversion process and the accuracy and reliability of the reconstructed physical property models,the triple strategy is adopted in this paper to develop a fast cross-gradient joint inversion for gravity and magnetic data.The cross-gradient constraint contains solving the gradients of the physical property models and performing the cross-product calculation of their gradients.The sparse matrices are first obtained by calculating the gradients of the physical property models derived from the first-order finite difference.Then,the triple method is applied to optimize the storages and the calculations related to the gradients of the physical property models.Therefore,the storage compression amount of the calculations related to the gradients of the physical property models and the cross-gradient constraint are reduced to one-fold of the number of grid cells at least,and the compression ratio increases with the increase of the number of grid cells.The test results from the synthetic data and field data prove that the structural coupling is achieved by using the fast cross-gradient joint inversion method to effectively reduce the multiplicity of solutions and improve the computing efficiency.

A 3D Geological Model Constrained by Gravity and Magnetic Inversion and its Exploration Implications for the World-class Zhuxi Tungsten Deposit, South China

The Zhuxi tungsten deposit in Jiangxi Province,South China,contains a total W reserve of about 2.86 Mt at an average grade of 0.54 wt%WO3,representing the largest W deposit in the world.Numerous studies on the metallogeny of the deposit have included its timing,the ore-controlling structures and sedimentary host rocks and their implications for mineral exploration.However,the deep nappe structural style of Taqian-Fuchun metallogenic belt that hosts the W deposit,and the spatial shape and scale of deeply concealed intrusions and their sedimentary host rocks are still poorly defined,which seriously restricts the discovery of new deposits at depth and in surrounding areas of the W deposit.Modern 3 D geological modeling is an important tool for the exploration of concealed orebodies,especially in brownfield environments.There are obvious density contrast and weak magnetic contrast in the ore-controlling strata and granite at the periphery of the deposit,which lays a physical foundation for solving the 3 D spatial problems of the ore-controlling geological body in the deep part of the study area through gravity and magnetic modeling.Gravity data(1:50000)and aeromagnetic data(1:50000)from the latest geophysical surveys of 2016-2018 have been used,firstly,to carry out a potential field separation to obtain residual anomalies for gravity and magnetic interactive inversion.Then,on the basis of the analysis of the relationship between physical properties and lithology,under the constraints of surface geology and borehole data,human-computer interactive gravity and magnetic inversion for 18 cross-sections were completed.Finally,the 3 D geological model of the Zhuxi tungsten deposit and its periphery have been established through these 18 sections,and the spatial shape of the intrusions and strata with a depth of 5 km underground were obtained,initially realizing―transparency‖for ore-controlling bodies.According the analysis of the geophysical,geochemical,and geological characteristics of the Zhuxi tungsten deposit,we discern three principles for prospecting and prediction in the research area,and propose five new exploration targets in its periphery.

Simultaneous Structure-Coupled Joint Inversion of Gravity and Magnetic Data Based on a Damped Least-Squares Technique

The structure-coupled joint inversion method of gravity and magnetic data is a powerful tool for?developing improved physical property models with high resolution and compatible features;?however, the conventional procedure is inefficient due to the truncated singular values decomposition?(SVD) process at each iteration. To improve the algorithm, a technique using damped leastsquares?is adopted to calculate the structural term of model updates, instead of the truncated SVD. This?produces structural coupled density and magnetization images with high efficiency. A so-called?coupling factor is introduced to regulate the tuning of the desired final structural similarity level.?Synthetic examples show that the joint inversion results are internally consistent and achieve?higher?resolution than separated. The acceptable runtime performance of the damped least squares?technique used in joint inversion indicates that it is more suitable for practical use than the truncated SVD method.

A MATLAB-Based Numerical and GUI Implementation of Cross-Gradients Joint Inversion of Gravity and Magnetic Data

The cross-gradients joint inversion technique has been applied to multiple geophysical data with a significant improvement on compatibility, but its numerical implementation for practical use is rarely discussed in the literature. We present a MATLAB-based three-dimensional cross-gradients joint inversion program with application to gravity and magnetic data. The input and output information was examined with care to create a rational, independent design of a graphical user interface (GUI) and computing kernel. For 3D visualization and data file operations, UBC-GIF tools are invoked using a series of I/O functions. Some key issues regarding the iterative joint inversion algorithm are also discussed: for instance, the forward difference of cross gradients, and matrix pseudo inverse computation. A synthetic example is employed to illustrate the whole process. Joint and separate inversions can be performed flexibly by switching the inversion mode. The resulting density model and susceptibility model demonstrate the correctness of the proposed program.

COMPREHENSIVE INTEGRATION METHODS AND APPLICATION OF GRAVITY-MAGNETIC DATA IMAGE

the technique of image processing and analysis of gravity and magnetic data is one of themost effective ways to extract geological information from gravity and msanetic data. The presentpaper investigates, from an angle of generalized joint inversion, thc methods and procedures ofcomprehensive processing of multi-source geological image , and a specific example in Huai Nan coalfield is given here as well.

THREE-DIMENSIONAL GRAVITY-MAGNETIC CROSS-GRADIENT JOINT INVERSION BASED ON STRUCTURAL COUPLING AND A FAST GRADIENT METHOD

In order to effectively solve the low precision problem of the single gravity density inversion and the magnetic susceptibility inversion,and the limitation of the gravity?magnetic joint inversion method based on the petrophysical parameter constraint,this paper studies the three-dimensional gravity?magnetic cross-gradient joint inversion based on the structural coupling and the fast optimization algorithm.Based on the forward and inversion modeling of three-dimensional gravity density and three-dimensional magnetic susceptibility using the same underground grid,along with cross-gradient coupling as the structural cons train t,we propose a new gravit y?magnetic joint inversion objective function including the data fitting term,the total variation regularization constraint term and the crossgradient term induced by the structural coupling?The depth weighted constraint and the data weighting constraint are included into the objective function,which requires different physical property models to minimize their respective data residuals.At the same time,the cross-gradient term tends to zero,so that the structure of the gravity and magne ic models tends to be consistent.In realization,we address a fast and efficient gradient algorithm to iteratively solve the objective function.We apply this new joint inversion algorithm to the 3D gravity-magnetic model inversion test and compare it with the results of a single inversion algorithm.The experimental tests of synthetic data indicate that the gravity-magnetic cross-gradient joint inversion method can effectively improve the accuracy of the anomaly position and numerical accuracy of the inverted anomaly physical parameters compared with the single physical inversion method.

Three-dimensional lithospheric density distribution of China and surrounding regions

In this paper, we analyze lithospheric density distribution of China and surrounding regions on the basis of 30＇ × 30＇ gravity data and 1°×1 °P-wave velodty data, Firstly, we used the empirical equation be- tween the density and the P-wave velocity difference as the base of the initial model of the Asian lithospheric density. Secondly, we calculated the gravity anomaly, caused by the Moho discontinuity and the sedimentary layer discontinuity, by the Parker formula. Thirdly, the gravity anomaly of the spherical harmonics with 2 40 order for the anomalous body below the lithosphere is calculated based on the model of EGM96. Finally, by using Algebra Reconstruction Techniques （ART）, the inversion of 30＇ ~ 30＇ residual lithospheric Bouguer gravity anomaly caused by the lithosphere yields a rather detailed struc- tural model. The results show that the lithospheric density distribution of China and surrounding regions has a certain connection with the tectonic structure. The density is relatively high in the Philippine Sea plate, Japan Sea, the Indian plate, the Kazakhstan shield and the Western Siberia plain, whereas the Tibetan Plateau has low-density characteristics. The minimum value of density lies in the north of Philippines, in the Taiwan province and in the Ryukyu island arc.

高精度重磁方法寻找隐伏矽卡岩型铁矿

位于华北克拉通东部的鲁中莱芜地区是矽卡岩型富铁矿重要产区,矿床主要产于矿山岩体与中奥陶统碳酸盐岩地层的接触带中。本文利用最新重磁测量成果,对矿山岩体外围西部石家泉—刘家庙一带重磁异常特征进行了细致研究,结合区内已知铁矿重磁场特征,圈定了深部找矿靶区。在成矿有利地段布设了大比例尺重磁剖面,以已知钻孔为约束条件,利用2.5D重磁联合反演技术,对重磁异常进行了定性及定量解释。解释结果为后续钻孔位置布设及深度预测提供了依据,该钻孔揭露了15.8 m厚的富铁矿,找矿效果显著,为今后本区进一步寻找矽卡岩型铁矿提供了重要指示依据。

磁偶极子单自由度定位河床冲刷监测方法研究

为解决传统磁偶极子定位方法依磁力梯度张量反演磁偶极子位置,所需测量矩阵复杂的弊端,从磁偶极子空间分布模式出发,仅以单自由度的磁偶极子在某点处所产生的磁场三分量来定位磁偶极子。依据磁偶极子空间分布模式,推导单自由度磁偶极子的反演方程,实现了磁偶极子单自由度定位。在此基础上,基于推导的反演方程,将磁偶极子单自由度定位方法与重力驱动河床冲刷监测方法相结合,提出了一种具有普适性的河床冲刷监测方法,该方法可以通过单自由度反演公式反映河床冲刷情况。

加权空间内约束型反演方法研究

笔者等分析了正则化项对反演效果的影响,为了使正则化项在反演过程中起到更好的约束作用,将传统的加权空间正则化约束改为原始空间解的L2正则化,强化了正则化系数在反演过程中的作用,提升了反演精度。1加权空间内约束型反演重磁物性反演能够直观地获得地下的密度和磁化率结构.

重磁井震联合正反演技术在二连盆地深层目标勘探中的应用

二连盆地具有丰富的油气资源,前白垩系作为盆地的主力生油层系,其含油气性早已被证实。但该盆地高力罕凹陷及周缘整体勘探程度较低,早期完成的少量二维地震仅局限在几个独立的凹陷内,无法连片,缺少对该区整体地质结构的认识,油气勘探至今未能取得显著效果。前期研究发现,二连盆地前白垩系地层分布面积广,烃源岩有机质丰度高,成熟度高,处于成熟凝析油湿气阶段。前白垩系地层钻探程度低,探索前白垩系含油气性,实现二连盆地新层系勘探突破,拓展二连盆地战略接替新层系。近年来,高精度重磁勘探通过重磁井震联合正反演等针对性处理解释技术,较好恢复了该盆地前白垩纪盆地原貌,有望开启研究二连盆地深层地质结构的新篇章。

Distribution of Helium Resources in Weihe Basin,Central China:Insight from 3D Magnetic Inversion

There is promising indication of helium-rich natural gas in Weihe Basin.However,the lack of a detailed investigation on the origin and the spatial distribution of helium source rocks(mainly Yanshanian granites)limits the helium potentiality evaluation in Weihe Basin(WB).We performed three-dimensional(3 D)inversions of magnetic data in Weihe Basin and its adjacent areas to figure out the crustal thermal structure and the temporal-spatial distribution of deep granite in the basin.Based on this,we have proposed a geological model of helium accumulation and predicted the potential area of helium distribution.The results of 3 D magnetic inversion indicate that the granites in the deep Weihe Basin are mostly located at the central and southern parts of the basin,which are connected spatially with the granites in the North Qinling Orogenic Belt.These granites were all derived from largescale intra-continental orogeny in the Qinling area during the mid-Mesozoic,providing a good material basis for crust-derived helium in the basin.The local uplift of the Curie surface makes the thermal fluid more actively,which contributes to helium accumulation.The faults developed in the WB are the migration pathway of crust-derived helium and the upward migration of the mantle-derived helium.The wells with high percentage helium are mostly located near the Weihe fault and the areas on the south of it.The Wugong-Xi’an-Lantian area in the central and the Lintong-Weinan-Tongguan area in the eastern Weihe Basin are the most promising helium distribution areas.Furthermore,the region from the north of Taibai Mountain to Baoji City in the western Weihe Basin may also be another potential area of Helium resource.

航空重(磁)多参量梯度探测与反演技术研究进展

航空重磁梯度、张量梯度测量能有效地减弱环境噪声的干扰,更高分辨率地描述地质体的分布并突出浅部地质体,随着测量装备的成熟,已经在油气与矿产资源勘查中得到应用。本文系统地总结了航空重(磁)多参量梯度探测技术的研究进展,为后续相关研究提供借鉴。基于航空重磁多参量梯度技术的研究背景,总结了国内外装备的进展。针对航空重磁多参量梯度数据的特点,介绍了其数据处理流程;针对航空重磁多参量梯度的分辨率特征,总结了现今重磁多参量梯度的高分辨率反演以及联合反演方法;基于航空重磁多参量梯度探测技术在矿产和油气资源勘查中的应用特征,分析了该项技术在深部资源勘查等方面的应用前景,明确了其在识别地下构造、寻找矿产资源、探测地下小型目标体等方面的应用进展。

新疆阿舍勒矿集区三维地质—地球物理建模研究

阿舍勒矿集区位于我国新疆哈巴河县北部约20 km处,研究区内已发现多个多金属矿床,如阿舍勒铜锌矿、萨尔朔克金多金属矿、喀英德铜矿等,是我国重要的VMS型矿集区。近年来,随着研究工作的不断深入,认为在阿舍勒盆地深部或周边区域,仍有较大的成矿潜力(张志欣等,2014;杨富全等,2015),而深部和周边区域新的矿床的发现.

基于岩石物性引导的地球物理联合反演研究

多元地球物理勘探资料的联合处理和综合解释是深部地热资源勘探评价工作必不可缺的环节,联合反演和后反演地质分异技术是目前深部资源勘探领域的2个研究热点。为融合多元地球物理场信息,降低单一地球物理场反演的多解性,本文利用岩石物性先验信息作为引导,以地震解释地层结构信息建立结构模型,采用高斯混合模型约束地层的地球物理参数,将重力、磁法、大地电磁进行正则化联合反演,实现了多物性结构的耦合,最终形成了重、磁、电、震联合反演软件,基于多物性联合反演的结果,利用Arrhenius定律,通过电阻率实现对典型干热岩场地温度场的预测。通过对直立六面体理论模型正演结果进行联合反演计算,联合反演结果相对于单独反演,对异常体的空间形态刻画以及物性数值恢复具有较好的效果,而且能够充分融合地质、岩石物性和地球物理等多元数据,更加符合实际规律。

重磁联合反演在隐伏矿产勘查中的应用

在研究区针对1∶1万磁测圈定的带状高磁异常,通过重力、磁测综合剖面测量对异常源进行多参数综合分析定性,利用重磁联合反演方法对隐伏目标地质体定量计算。减少了地球物理解释的多解性,提高地质解释合理性及反演计算的可靠性。可以有效合理的评价隐伏矿产资源信息,并指导深部地质工程。

基于重磁资料在山东齐河——禹城探获矽卡岩型富铁矿:对超深覆盖区找矿的启示

【研究目的】山东齐河—禹城矿集区内分布近南北向且具有一定同源特征的重磁异常带,前期陆续在北部李屯、南部大张重磁异常内揭露到厚大的矽卡岩型富铁矿床,铁矿勘查工作取得较大进展,但矿集区中部潘店异常区被超深的新生界覆盖,异常规模小、强度弱,深部铁矿体的地球物理信息被极大弱化,且矿致磁异常又一定程度被大规模的磁性岩体异常所掩盖,识别和提取难度较大,因此一直未取得找矿突破。【研究方法】本文充分结合相邻矿区重磁勘查的成功找矿经验,由已知到未知,对潘店重磁异常进行平面剖析和剖面定量反演,实现了研究区构造格架划分、成矿地质体圈定、异常剖析研究及2.5D联合反演的全流程解译。【研究结果】基于高精度重磁资料的综合研究,获取了超深覆盖层下的深部找矿信息,最终在齐河—禹城矿集区中部实现了矽卡岩型富铁矿的找矿新突破,使矿集区内3处重磁异常得以紧密关联,并证实了大张、潘店、李屯三处铁矿床处于同一近南北向的中性岩浆岩带内,开辟了齐河—禹城矿集区内又一处重要的找矿新空间。【结论】本次找矿成果为后续进一步开展超深覆盖层下的深部找矿工作提供了重要参考和指示,同时表明齐河—禹城矿集区内具有广阔的矽卡岩型富铁矿找矿前景。

基于Gramian约束与高斯-牛顿算法的重磁三维联合反演

为了降低重力与磁法单独反演中的非唯一性问题,本文实现了一种基于Gramian约束的重磁三维联合反演算法.使用基于模型参数或其空间梯度的Gramian算子,将其添加到目标函数中,约束剩余密度与剩余磁化率模型参数或其梯度更具相关性.采用高斯-牛顿法对目标函数进行最优化求解,对求解方程进行了简化,节省内存并提高了计算效率.反演过程中使用对数法约束模型参数的上下界,使反演结果更符合真实地质情况.提出了新的模型更新步长计算策略,同时考虑了数据拟合与联合约束的影响,使联合反演更加稳定.通过三个数值模型验证了算法的可靠性,并进一步将该算法应用于加拿大McFaulds湖实测航空重磁数据.对比了反演结果切片并展示了交会图,联合反演得到的剩余密度与剩余磁化率的物性、梯度分布都比单独反演的相关性更强.联合反演提供了一个既能拟合数据又在一定程度上满足耦合条件的反演解.验证了所开发的联合反演算法在实际数据上的有效性与适用性.

基于曲波压缩的重磁异常三维反演及其应用

重磁异常三维反演在构建地下三维空间精细结构研究中作用显著,然而反演核函数矩阵规模较大,对计算机内存需求高,限制了三维反演的广泛应用。为了减小核函数的内存需求,提出了基于曲波压缩的重磁异常三维反演方法,即通过曲波变换,核矩阵的能量主要保留在小部分稀疏的曲波系数中,可实现高效的压缩反演。相比小波变换,曲波变换具有更优越的信号稀疏表征能力,因此在相同压缩率条件下曲波变换可以获得更高的压缩精度。文中对比研究了小波压缩与曲波压缩在重磁异常三维反演中的应用效果。考虑相同计算精度,曲波压缩可以将核函数压缩率提高100%,降低50%的内存需求。理论模型试验中反演核函数大小为(100×100)×(100×100×50)(双精度存储量约37 GB),利用文中方法仅需4%的压缩比(即核函数的内存需求仅为1.48 GB),即可实现与传统小波压缩10%的压缩比类似的反演效果。将文中方法应用于中国西部尕林格矿区的磁异常三维反演,曲波压缩反演得到的三维磁化强度模型较清晰地揭示了矿体的埋藏深度、空间分布、磁性强弱等特征,为后期钻探验证、储量评估、盲矿探测等提供了可靠依据。

基于重磁异常的北京副中心地质构造特征和地热控制作用研究

重磁勘探对于分析地热系统特征和成因十分重要。北京城市副中心当前地热开发程度较低,为给地热资源可持续利用提供依据,对近期完成的1:50000高精度区域重力测量资料,开展了场源边缘检测、2.5D剖面反演和三维反演计算。利用归一化Theta图垂向导数和一阶水平方向导数获得了精度更高的断裂位置,研究了断裂汇交特征;使用频率域Parker-Oldenburg方法反演获得了两条不同方向剖面的基岩起伏形态;使用信赖域反演算法获得了5 km以浅地层三维视密度异常特征。在此基础上结合1:100000航磁和钻孔资料分析,综合研究了副中心地质构造特征及其地热控制作用,得到结论如下:(1)研究区断裂构造以近NE向和NW向为主。NE向断裂延伸较远、梯度带宽,但被NW向断裂切割改造迹象明显。(2)太古界结晶基底差异性隆升使区内储层和盖层厚度变化大。研究区南部隆起区蓟县系储层厚度变薄或缺失。新生界盖层在隆起区厚度为200~800 m,在断陷盆地区达2000 m以上。(3)南苑—通县断裂和夏垫断裂是区内良好导热构造;NW向台湖断裂、张家湾断裂与NE向燕郊断裂、姚辛庄断裂是主要导水通道,与南苑—通县断裂和夏垫断裂交汇形成区内地热资源的主要补给径流通道。区内地热异常受不同级次构造控制在赋存特征上呈现较大差异。