Joint inversion of gravity and magnetic data for a two-layer model

Based on the synchronous joint gravity and magnetic inversion of single interface by Pilkington and the need of revealing Cenozoic and crystalline basement thickness in the new round of oil-gas exploration, we propose a joint gravity and magnetic inversion methodfor two-layer models by concentrating on the relationship between the change of thicknessI and position of the middle layer and anomaly and discuss the effects of the key parameters. Model tests and application to field data show the validity of this method.

Theoretical Study on Stochastic Modeling of Combined Gravity-Magnetic-Electric-Seismic Inversion and Its Application

As gravity field, magnetic field, electric field and seismic wave field are all physical fields, their object function, reverse function and compound function are certainly infinite continuously differentiable functions which can be expanded into Taylor (Fourier) series within domain of definition and be further reduced into solving stochastic distribution function of series and statistic inference of optimal approximation. This is the basis of combined gravity-magnetic-electric-seismic inversion of stochastic modeling. It is an uncertainty modeling technology of combining gravity-magnetic-electric-seismic inversion built on the basis of separation of field and source gravity-magnetic difference-value (D-value) trend surface, taking distribution-independent fault system as its unit, depths of seismic and electric interfaces of interests as its corresponding bivariate compound reverse function of gravity-magnetic anomalies and using high order polynomial (high order trigonometric function) approximating to its series distribution. The difference from current dominant inversion techniques is that, first, it does not respectively create gravity-seismic, magnetic-seismic deterministic inversion model from theoretical model, but combines gravity-magnetic-electric-seismic stochastic inversion model from stochastic model; second, after the concept of equivalent geological body being introduced, using feature of independent variable of gravity-magnetic field functions, taking density and susceptibility related to gravity-magnetic function as default parameters of model, the deterministic model is established owing to better solution to the contradiction of difficulty in identifying strata and less test analytical data for density and susceptibility in newly explored area; third, under assumption of independent parent distribution, a real modeling by strata, the problem of difficult plane closure arising in profile modeling is avoided. This technology has richer and more detailed fault and strata information than sparse pattern seismic data in newly explored area, successfully inverses and plots structural map of Indosinian discontinuity in Hefei basin with combined gravity-magnetic-electric-seismic inversion. With development of high precision gravity-magnetic and overall geophysical technology, it is certain for introducing new methods of stochastic modeling and computational intelligence and promoting the development of combined gravity-magnetic-electric-seismic inversion to open a new substantial path.

Crustal structure of the western Indian shield: Model based on regional gravity and magnetic data

Regional surface gravity data and global satellite magnetic data have been utilized to generate a preliminary model of the crustal structure along a southwest-northeast profile （Gadra-Fatehpur） through western Rajasthan.The study area represents the western part of the Indian continental landmass which has undergone several major episodes of repeated subduction/collision,plume traces and rifting from Archaean to recent times.The temporal and spatial relationship between the various geotectonic provinces is quite complex,thereby limiting the emergence of a suitable crustal structure model for this region.Exposures of the Malani Igneous Suite （MIS）,a product of bimodal volcanism （～780 Ma）,and considered to be the third largest felsic magmatic province of the world,is evident along the profile and also to the southwest of the study area.The easternmost part of the profile is close to the DAFB （Delhi Aravalli Fold Belt）,a Proterozoic orogenic belt.This study probes the geometry of the different crustal units in terms of density and susceptibility variations in order to decipher the imprints of the major tectonic processes the region has undergone.In order to decipher the crustal geometry of the Gadra-Fatehpur profile,two NW-SE gravity and magnetic profile vertical sections （A-A＇ in the south and B-B＇ in the north） are modelled on the basis of the constraints provided from previous seismic models.The crustal model of the Gadra-Fatehpur profile is composed of alluvium,Tertiary sediments,MIS,Marwar Supergroup,low-density layers （LDLs） and the middle-lower crustal layers,with a distinct change in configuration from the southwest to northeast.The Moho dips from SW to NE,the MIS in the SW gives way to the thick pile of the Marwar Supergroup to the NE.The evolution of MIS has been suggested to have occurred as a consequence of delamination of the upper mantle.LDLs are incorporated in Gadra-Fatehpur model.In the SW,LDL （2550 kg/m3） lies below the MIS in the NE,another LDL （2604 kg/m3） is depicted below the mid-crustal layer.

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 Modelling of Gravity and Magnetic Data in a Measured Heat Flux Area to Characterize Geothermal Heat Sources: A Case for Eburru Geothermal Complex, Kenya

Forward modelling of gravity and magnetic data was done simultaneously to show the correlation between gravity and magnetic anomalies on a measured heat flux region. The results were used to characterize the heat source structures in Eburru area. Modelling was done using Oasis montaj geosoft software which is an iteration process where the gravity and magnetic anomalies were calculated and compared to the observed residual anomaly until there was a fit. The start model was constructed based on depths from Euler deconvolution and models constrained using stratigraphy data from the existing wells in the study area. Forward modelling of gravity and magnetic data revealed intrusions within the Earth’s subsurface with depth to the top of the sources ranging from 739 m to 5811 m. The density of the sources ranges between 3.0 g/cm3 and 3.2 g/cm3 while their magnetic susceptibility was zero. This implies that intrusions from the mantle with a magnetic susceptibility of zero have temperatures exceeding the curie temperature of rocks. The density of the intrusions modelled was higher than 2.67 g/cm3, the average crustal density, hence it explains the observed positive gravity anomaly. The results also revealed that areas with high heat flux have shallow heat sources and if the heat sources are deep, then there must be a good heat transfer mechanism to the surface.

Development of 3-D Gravity-Magnetic Models of the Earth’s Crust of Azerbaijan and Adjacent Areas: an Overview

Azerbaijan is located in the Alpine Himalayan collisional zone and is characterized by its complex and variable geological structure. To study Azerbaijan’s deep structure, twelve main regional interpreting profiles were selected for comprehensive 3D combined gravity-magnetic modeling. The development of the initial physical-geological models (PGMs) was based on known surface geology, drilling data, previous seismic, magnetotelluric and thermal data analysis, examination of the richest petrophysical data, as well as quantitative and qualitative gravity/magnetic data examination. The PGMs thus reflect the key structural-formational specifics of Azerbaijan’s geological structure, beginning from the subsurface (hundreds of meters) up to the Moho discontinuity (40 - 60 km). The PGMs revealed common factors controlling ore- and hydrocarbon bearing formations, primarily the boundaries of tectonic blocks, masked faults, and buried uplifts of magmatic rocks. Many of these factors can be used to investigate long-term geodynamic activity at a depth. The article summarizes many years of investigation by exemplifying the most typical PGMs for the Greater and Lesser Caucasus, the Kura depression, and central and northern Azerbaijan.

Depositional and Structural Styles in the Logone Birni Basin (LBB), Northern Cameroon, from 3D Potential Field Modeling: Preliminary Results

A three-dimensional model of the Logone Birni Basin (LBB) is presented by combining gravity and magnetic data and constrained by broad seismic profiles. The 3-D model has revealed the distribution of the stratigraphic formations as well as the top basement variation. Detailed structure of different stratigraphic sequences is presented for the first time for this basin and some of the sequences correlate with established sequences of the neighboring basins. The sediments pill consists of six sedimentary units dating from the Neocomian to the Quartenary. The Makary subbasin or Northern Logone Birni Basin (NLBB) is the deepest part of the basin and may hold good prospect for hydrocarbon generation and accumulation. However, the limited presence of faults and intrusive bodies decreases the possibility of thermal degeneration, contrary to the Central Logone Birni Basin (CLBB) where conditions seem to be fulfilled for possible hydrocarbon generation and maturity. The complexity of the structural pattern of the model is further enhanced by the presences of volcanic bodies, some of which lay directly on basement or interbedded with the sediments layers mainly in the CLBB.

山东齐河-禹城地区三维重磁建模与找矿预测

山东省齐河-禹城矿集区是近年来发现的富铁矿矿集区,矿石品位高、储量大,有望成为中国重要的富铁矿基地。在成岩成矿时代、控矿构造、赋矿层位和找矿规律等矿床学研究方面开展了大量工作,并取得了很多成果。但由于该区处于第四系厚覆盖层区,深部异常信息难以获取,对矿区深部构造样式、隐伏岩体空间形态及规模、主要控矿地层的空间展布等问题还没有得到有效解决,严重制约了矿床规模的进一步扩大。利用重磁约束的三维地质建模技术已成为深部找矿的重要手段,它利用先验信息进行约束反演弥补重磁纵向分辨力的不足,对成矿异常的空间性质进行判断或追踪已知矿带的深部延伸,从而实现找矿预测。笔者基于齐河-禹城地区1:50 000重力数据和1:25 000航磁数据,进行位场分离获得反演基础数据,在地质、钻孔和物性资料的约束下,完成了19条剖面的交互式反演,利用剖面反演结果建立了研究区三维地质模型,获取了地下约3 km深度的地层、岩体、构造的空间信息,并以此为基础结合成矿规律和控矿因素提出5处找矿新靶区。

辽宁抚顺地区铁矿地球物理模型——以毛公堡铁矿为例

辽宁抚顺地区铁矿十分丰富.它们与著名的鞍本铁矿同属辽东吉南成矿带.成矿地层均为鞍山群,属沉积变质型铁矿,受变质岩系层状控制明显.鞍山-本溪地区为鞍山群的茨沟岩组、樱桃园岩组;抚顺-清源地区为鞍山群的石棚岩组、通什村组.它们具有相似的岩性组分和物性特征.通过开展“深部盲矿体勘查技术研究”科研项目,以抚顺毛公堡铁矿为例,总结分析抚顺地区铁矿的重磁电物性特征,建立了地球物理模型.研究认为,应当打破传统的以磁测为唯一手段的磁铁矿勘查工作模式,开展重、磁、电联合勘查是可取的、有效的.在水平定位技术方面,重力方法有其独特的优势;在垂向定位技术方面可以开展电(磁)测深工作.磁铁矿的极化作用是明显的,应符合低阻高极化.铁矿重磁电特性是高磁、高密度、高极化、低电阻,呈“三高一低”的物性特征,而且品位越高极化率越高.电阻率应不高于2000Ωm,极化率应不低于6%,磁化率不低于100000×10^(-6)SI,密度高于3.00×10^(3) kg/m^(3).

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。

胶西北焦家—仓上地区深部地球物理探测综合应用

近几年,为配合深部找矿工作,在胶东地区开展了大量的物探工作,取得了明显的探测效果。焦家—仓上地区虽开展过大量钻探工作,并对地壳深部进行揭露,但覆盖面小,无法全面掌握地质信息。在前人成果的基础上,进一步开展了高精度重力剖面、高精度磁法剖面、MT重磁电联合反演、CSAMT、二维地震等综合物探工作,通过对所得结果互相验证并结合钻探揭露,较全面地提取了焦家—仓上地区深部地质、构造等信息,构建了三维地质模型,确定了深部找矿靶区,为下一步深部找矿工作提供了方向及可靠依据。

庐枞火山岩盆地及其外围重、磁场特征

为探测长江中下游成矿带庐江-枞阳白垩纪火山岩盆地的深部构造和地壳结构,2007年初在庐枞火山岩盆地进行了以深反射地震剖面探测为主的,新一轮重力、磁力和大地电磁剖面测量。作者在前人研究的基础上对庐枞火山岩盆地及其外围重、磁场特征进行了研究。作者首先分析长江中下游地区重、磁场的分布特征,然后以区域重、磁场特征为背景来认识庐枞地区重、磁场的分布特征。研究各类地质体的物性参数是开展地球物理解释的前提,文中收集并分析了前人对庐枞地区的岩石物性的较为系统研究成果。为了提取重、磁异常的特征,文中对重、磁异常进行了位场转换和图像处理。利用新的深反射地震剖面探测和大地电磁剖面研究成果,采用定性和定量解释方法对庐枞地区重、磁场的分布特征进行了研究并提出新的认识。庐枞火山岩盆地深部存在隐伏的磁性强的中碱性岩类是产生区域磁异常的主要原因。庐枞火山岩盆地下部火成岩所侵入的地层向盆地东南方向延伸,盆地的西北边界向东南方向倾斜。而在罗河断裂带以西没有火成岩存在。亦即庐枞火山岩盆地是一个沿北东向罗河断裂向东发育的非对称火山盆地。另外,在庐枞火山岩盆地西部边缘罗河深部存在切穿莫霍面的断裂带沿北东向延展数十千米。

羌塘坳陷石油地质走廊剖面重磁异常处理模拟及地质解释

利用"重磁视深度滤波"方法对羌塘石油地质走廊南、北两条剖面上的实测重磁数据进行处理解释,提取两条剖面位场异常中含有的地质-地球物理断面的初始特征,然后使用最新的"重磁模拟解释系统(GM-SYS)",以初始特征为模拟初值,以地震、地电和地质资料为约束,进行重磁剖面模拟反演,获得了羌塘盆地南、北两条剖面精细的地质结构解释断面图和栅状图。结合剖面域内的其它资料对走廊域内几个重要地质问题进行了初步的分析和解释,对羌塘盆地石油地质走廊域内的地层、基底、断裂及火山岩分布有了进一步的认识,为整个羌塘盆地区域地质解释及油气远景评价提供了新的解释依据。

苏北大陆科学钻探靶区重磁异常反演解释

对江苏东海县毛北地区重、磁观测异常进行了处理和特征分析 ,并根据科学钻探主要目标体榴辉岩与其围岩有达 0 .9× 1 0 3kg/m3的明显密度差 ,进行了等效密度反演成像与综合地震约束反演计算 .给出了该地区重力异常模型 。

准噶尔盆地北部基底结构与属性问题探讨

准噶尔盆地的基底结构与属性一直是地学界关注的焦点问题之一.横跨准噶尔盆地北部,走向近东西的克拉玛依—喀姆斯特地震剖面提供了该盆地北部详细的地壳与上地幔顶部的速度结构与构造,特别是基底顶界面的速度.沿剖面发现了数条走向近南北的"H"型超壳断裂,它们没有明显的断差,断裂处反射系数明显降低,介质的Q值减小,推测具"开裂"性质;利用盆地内1:20万重磁数据完成了重磁联合反演,获得了沿剖面的地壳与上地幔顶部的二维密度结构与二维磁性结构.根据在一定深度范围内介质的速度-密度-岩性之间的关系,确定了盆地北部基底岩性分布.结果表明,准噶尔盆地北部的基底多处为基性和超基性物质,推测为深部(上地幔)物质沿超壳断裂进入地壳内部并对地壳物质进行改造的结果.这一推断得到盆地内部高磁性、高重力异常的支持,也与盆地具有较高的地壳平均速度相一致.综合其他地球物理与地质资料综合分析,给出了综合地质解释剖面,建立了准噶尔盆地北部基底结构与属性的动力学模型.

1∶5万重磁勘查在安徽宣城覆盖区地质找矿中的应用探索

安徽省宣城地区是长江中下游成矿带的重要组成部分,地表以第四系覆盖下(红层覆盖、推覆构造下)为主要特点。本文以最新获取的1∶5万高精度重磁资料为基础,通过对研究区进行平面解译,获得了区内地质构造、控矿层位、侵入岩体的分布等平面属性;以研究区重磁二度半定量反演计算成果为基础,探索建立了全区准三维立体结构,掌握了区内主要控矿断裂、岩浆岩、矿源层等要素的几何空间展布规律及成矿有利部位等重要找矿空间属性;以研究区内已知典型矿床为切入点,总结了其地球物理场特征、成矿条件、控矿要素等成矿因素,探索构建了目标区地质-地球物理找矿模型,划分了成矿远景区,并圈定了重点找矿靶区。朱桥成矿远景区魏墩靶区钻探验证见矿实例表明,通过1∶5万高精度重磁资料的处理与研究,进行位场平面-三维勘查、地质-地球物理找矿模型构建的方式发挥了地球物理找矿的优势,提高了找矿效果。本文研究成果可为中国东部覆盖区找矿勘查工作提供参考和指示。

复杂形体重、磁异常正演问题综述

总结了复杂形体重、磁异常各种正演方法,并对各种方法所具备的优点和存在的缺点进行了分析说明。据此讨论了复杂形体重、磁异常正演方法将来的发展方向。

地质力学磁力模型试验相似材料磁力特性研究

地质力学磁力模型试验是利用电磁力(场)模拟重力(场)的原理来研究地质力学工程问题的试验方法。在模型中,时间t、几何尺度l缩小n倍、重力加速度g扩大n倍,而材料的其他基本物理力学参数的相似比均为1,从理论上说可以采用原型材料进行地质力学磁力模型试验,这在模型试验的操作上十分方便。磁性材料的磁力特性是磁力模型试验方法的关键。磁粉、土体、水在电磁场中表现出不同的电磁和磁力特性。磁粉材料的电磁特性由磁回滞曲线表示。磁性材料在电磁场中的磁力与磁场强度的梯度成正比。A-200磁粉在磁感应强度为563.2mT,磁粉含量为40%时的最大磁力倍数为20.92,并发生于第二次加磁;C-180磁粉在磁感应强度为569.6mT,磁粉含量为30%时的最大磁力倍数为23.94,也发生于第二次加磁,这2种磁粉均可考虑作为磁力模型试验的模型材料。