Lithosphere structure of European sedimentary basins from regional three-dimensional gravity modelling

A three-dimensional (3D) density model, approximated by two regional layers—the sedimentary cover and the crystalline crust (offshore, a sea-water layer was added), has been constructed in 1° averaging for the whole European continent. The crustal model is based on simplified velocity model represented by structure maps for main seismic horizons—the “seismic” basement and the Moho boundary. Laterally varying average density is assumed inside the model layers. Residual gravity anomalies, obtained by subtraction of the crustal gravity effect from the observed field, characterize the density heterogeneities in the upper mantle. Mantle anomalies are shown to correlate with the upper mantle velocity inhomogeneities revealed from seismic tomography data and geothermal data. Considering the type of mantle anomaly, specific features of the evolution and type of isostatic compensation, the sedimentary basins in Europe may be related into some groups: deep sedimentary basins located in the East European Platform and its northern and eastern margins (Peri-Caspian, Dnieper–Donets, Barents Sea Basins, Fore–Ural Trough) with no significant mantle anomalies; basins located on the activated thin crust of Variscan Western Europe and Mediterranean area with negative mantle anomalies of −150 to −200×10⁻⁵ ms⁻² amplitude and the basins associated with suture zones at the western and southern margins of the East European Platform (Polish Trough, South Caspian Basin) characterized by positive mantle anomalies of 50–150×10⁻⁵ ms⁻² magnitude. An analysis of the main features of the lithosphere structure of the basins in Europe and type of the compensation has been carried out.     

Gravity and magnetic 3D inversion of Morro do Engenho complex, Central Brazil

In the Goiás Alkaline Province, in central Brazil, some mafic–ultramafic alkaline bodies crop out, such as Morro do Engenho Complex (ME), whereas other intrusive bodies are completely covered by Quaternary sediments, such as A2 body, and can only be defined by their magnetic and gravity responses. The bodies give rise to strong gravity and magnetic anomalies. We inverted the gravity and magnetic data to estimate 3D density and magnetic intensity contrasts. The gravity inversion reveals that ME and A2 intrusions, totally emplaced in the upper crust, have maximum depths of about 10 and 6 km, respectively. The estimated density contrast distribution indicates a NE–SW preferential direction for ME intrusive body. Since the remnant magnetization is very important in the area, we inverted the magnetic data, testing two hypotheses: only induced field and induced plus remnant magnetizations. The results of the inversion using only induced field could not explain the observed magnetic field, which was about 1000 nT larger than the calculated one. Induced plus remnant field inferred from wandering polar path fitted the observations. 3D magnetic inversions when remnant magnetization was included recover deeper and thinner bodies compared with gravity inversion. The inverted bodies have spherical shapes which are more consistent with the model of magmatic chambers instead of plug intrusions.     

Combining TEM/resistivity joint inversion and magnetic data for groundwater exploration: application to the northeastern part of Greater Cairo,Egypt

This paper presents the combination of two complementary methods, magnetic and joint inversion of resistivity/TEM data, as an effective approach to characterize groundwater reservoirs. Twenty stations of transient electromagnetic (TEM) and vertical electrical soundings (VES) were measured and interpreted using a joint inversion technique to evaluate the subsurface low resistivity zones connected to the groundwater reservoir. A complementary survey including 871 land magnetic stations was carried out at the same area to detect the upper surface of the basaltic sheet, which represents the bottom of the Miocene aquifer in the study area. The geological log from one borehole drilled in the zone was used to partially calibrate the calculated models. The results revealed that the study area consists of five different geological units with ages ranging from Paleogene (Oligocene) to Quaternary. The methodology provides good results at a very low cost when compared with drilling boreholes.     

重力资料聚焦反演成像改进及其应用

聚焦反演在实际应用中面临着两个问题需要及时解决: 成像的趋肤效应和异常体的过度聚合。利用指数深度加权函数来约束聚焦反演可以克服趋肤效应。结果表明,深度加权可以很好地限制过多异常源在地表的聚合。对于异常体的过度聚合,本文提出了异常源外扩方法,该方法将超出限制的异常源进行向异常体外的迭代式扩充。实验表明,异常体外扩方法成像效果优于常规的异常源截除方法。     

Lemniscate representation for inversion of gravity and magnetic data through nonlocal optimization

A method of representing surfaces and volumes by a set of geometric points and a small set of auxiliary parameters, based on generalization of Bernoulli’s notion of lemniscates is introduced. It provides for easy generation and modification of surfaces and volumes, which could be connected, disjoint or even with very irregular boundaries. This allows solving geophysical inversion problems, without constraining the anomalous volumes to some ideal or simple forms. This is illustrated by the example of joint inversion of gravity and magnetic data sets attributable to two-dimensional anomalous bodies. A nonlocal optimization algorithm calledsegmented Hamming scan is used for inversion. Comparison with nonlinear least-squares algorithm shows the advantages of the chosen approach. The concepts ofdesideratal andprocedural detours are illustrated.     

Bernstein preprocessing for nonlinear least squares inversion of gravity and magnetic data

Nonlinear, nonlocal and adaptive optimization algorithms, now readily available, as applied to parameter estimation problems, require that the data to be inverted should not be very noisy. If they are so, the algorithm tends to fit them, rather than smoothening the noise component out. Here, use of Bernstein polynomials is proposed to prefilter noise out, before inversion with the help of a sophisticated optimization algorithm. Their properties are described. Inversion of gravity and magnetic data for basement depth estimation, singly and jointly, and without and after Bernstein-preprocessing is conducted to illustrate that the inversion of Bernstein-preprocessed gravity data alone may be slightly superior to the joint inversion of gravity and magnetic data.     

Using a genetic algorithm for 3-D inversion of gravity data in Fuerteventura (Canary Islands)

The use of genetic algorithms in geophysical inverse problems is a relatively recent development and offers many advantages in dealing with the non-linearity inherent in such applications. We have implemented a genetic algorithm to efficiently invert a set of gravity data. Employing several fixed density contrasts, this algorithm determines the geometry of the sources of the anomaly gravity field in a 3-D context. The genetic algorithms, based on Darwins theory of evolution, seek the optimum solution from an initial population of models, working with a set of parameters by means of modifications in successive iterations or generations. This searching method traditionally consists of three operators (selection, crossover and mutation) acting on each generation, but we have added a further one, which smoothes the obtained models. In this way, we have designed an efficient inversion gravity method, confirmed by both a synthetic example and a real data set from the island of Fuerteventura. In the latter case, we identify crustal structures related to the origin and evolution of the island. The results show a clear correlation between the sources of gravity field in the model and the three volcanic complexes recognized in Fuerteventura by other geological studies.     

北秦岭华阳川地区重磁三维反演及岩浆岩特征研究

华山岩体、老牛山杂岩体等中酸性花岗岩体和华阳川韧性剪切带在北秦岭华阳川地区的成矿过程中起着重要的作用,既是构造演化的记录,又是构造—岩浆—成矿的重要指示。本文在系统研究华阳川地区地面和航空重力、磁测数据以及地面电法剖面等多种地球物理资料的基础上,完成物性统计分析、航空和地面数据融合处理,通过先验信息约束的重磁2.5D交互式反演和三维反演计算,构建了华阳川地区区域三维地质—地球物理模型,为区域地质背景和深部地质构造的研究提供地球物理依据;推断解释了华山岩体、老牛山岩体和华阳川韧性剪切带的三维形态和空间分布,探讨了华阳川韧性剪切带、老牛山岩体和华山岩体的深部接触关系,对大岩体外围接触带的铀、钼等矿床的深部找矿具有一定指导意义。     

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。     

磁法约束线性反演在福建大田-漳平地区的应用

通过约束反演的方法对磁法数据进行了反演解释,从而对采集得到的物探数据进行有效约束。在地质研究的基础上,以大田-漳平地区的野外勘探数据为例进行分析,并对数据用化探异常进行了对比约束,线性约束反演的结果很好地反映出场源体的特征,取得了预期的应用效果。     

重磁三维可视化反演系统的设计与实现

重磁三维可视化反演解释系统一直是国内外重磁勘探领域的研究重点之一。介绍了基于Visual C++与OpenGL环境研发的重磁三维可视化反演系统,详细阐述了系统的设计思想与实现的关键技术,包括可视化技术、面向对象技术、图形拾取技术、碰撞检测技术及反演约束技术等。系统具有人机交互几何反演与最优化物性自动反演两种功能,可满足目标异常、区域模拟和盆地建模。     

重磁2.5D/3D互相约束反演技术在三维地质填图中的应用研究

三维地质填图为我国启动的新一轮地质调查项目,为提高重磁资料在三维地质填图中的应用效果,笔者提出重磁资料2.5D/3D相互约束重磁反演技术方案:利用重磁资料2.5D剖面反演结果、3D物性反演结果作为彼此反演约束条件,并通过了理论模型试验。试验结果表明,该技术方案使反演结果中物性参数、空间位置更接近理论模型体。通过对本溪—临江地区思山岭铁矿磁异常及酸性岩体重力异常进行反演实践——估算磁异常铁矿资源量、研究酸性侵入岩深部展布形态的效果良好,可为大面积三维填图提供有效途径。     

Sub-surface geometry of Ar Rika and Ruwah faults from gravity and magnetic surveys

Ten global positioning system (GPS)–gravity profiles were conducted to provide sub-surface geometry of two sections of the Najd Fault System (NFS) Ruwah and Ar Rika faults, six in the Afif and four in the Al Muwayh area about 500 and 650 km west of Riyadh, respectively. GPS surveys were collected in differential GPS (DGPS) mode, allowing a large area to be covered in limited time. DGPS is utilized for the advantages of accuracy, economy, and speed. Output DGPS location coordinates were used in free-air and Bouguer reductions; terrain corrections were applied using a 3-arcsecond digital elevation model; finally, isostatic and decompensative corrections were applied. Integration of the resulting decompensative isostatic residual anomalies and aeromagnetic map has mapped the NFS very accurately. Modeling the gravity field crossing the Ruwah fault zone revealed that it is associated with low gravity anomalies probably due to a complex of lower density crushed rocks and modeled the geometry of the subsurface structure of Ar Rika fault as an inclined fault with reverse movement that would imply a compression component (post-dated the shearing) parallel to the plane of the cross-section.     

The crustal structure of the Guayana Shield, Venezuela, from seismic refraction and gravity data

We present results from a seismic refraction experiment on the northern margin of the Guayana Shield performed during June 1998, along nine profiles of up to 320 km length, using the daily blasts of the Cerro Bolívar mines as energy source, as well as from gravimetric measurements. Clear Moho arrivals can be observed on the main E–W profile on the shield, whereas the profiles entering the Oriental Basin to the north are more noisy. The crustal thickness of the shield is unusually high with up to 46 km on the Archean segment in the west and 43 km on the Proterozoic segment in the east. A 20 km thick upper crust with P-wave velocities between 6.0 and 6.3 km/s can be separated from a lower crust with velocities ranging from 6.5 to 7.2 km/s. A lower crustal low velocity zone with a velocity reduction to 6.3 km/s is observed between 25 and 25 km depth. The average crustal velocity is 6.5 km/s. The changes in the Bouguer Anomaly, positive (30 mGal) in the west and negative (−20 mGal) in the east, cannot be explained by the observed seismic crustal features alone. Lateral variations in the crust or in the upper mantle must be responsible for these observations.     

重磁异常数据三维人机联作模拟

在研究三角形多面体模型重、磁异常三维正演和反演技术的基础上,吸取人机交互正演模拟的优点,实现了三角形多面体模型重、磁异常数据三维人机联作反演。通过研究三角形多面体模型节点偏导数的计算方法,对目标函数进行线性化处理,形成了计算机自动迭代修改模型体的技术。利用计算机图形技术,在三维空间显示重、磁场和模型体,开发了模型的交互修改技术,使数据解释过程中,可以结合已知信息及人的推断和经验,完成重、磁异常数据的三维模拟,减少了数据解释结果的不确定性。     

Geodynamic evolution of the northern Molucca Sea area (Eastern Indonesia) constrained by 3-D gravity field inversion

In order to better understand the tectonic framework of the Northern Molucca Sea area, we inverted satellite and sea-surface gravity data into an iterative scheme including a priori seismological and geological data. The resulting 3-D density model images the various tectonic units from the surface down to 40 km. We proceed to various tests to assess the stability and robustness of our inversion. In particular, we performed an offset and average smoothing method to properly refine our results. The resulting model shows a striking vertical regularity of the structures through the different layers, whereas the density contrasts appear strongly uneven in the horizontal direction.The density model emphasizes the complexity of the upper lithospheric structure in the northern Molucca Sea, which is clearly dominated by the interaction between ophiolitic ridges, sedimentary wedges and rigid blocks of the Philippine Sea Plate. It also provides new, hard information that can be used in discussion of the evolution of the region.Large density variations are concentrated in the central part of northern Molucca Sea and dominate the upper lithospheric. North–south trending density structures along the Central Ridge and west dipping thrust faults on the western side of the region are clearly imaged. In the eastern part of the region, we distinguish several blocks, especially the Snellius Plateau which seems to be split into two parts. We interpret this as an oceanic plateau associated with thicker crust that previously belonged to the Philippine Sea Plate. This crust is now trapped between the Molucca Sea complex collision zone and the Philippine Trench, due to the development of a new subduction zone in its eastern side.     

新元古代扬子—华夏陆块拼合带：来自华南重磁异常的启示

扬子和华夏两大陆块是构成华南大陆的最基本单元, 但它们的拼合带位置（尤其是西延位置）是人们长期争论的焦点。本文以国家专项“深部探测技术与实验研究”探测得到的深反射地震剖面以及华南大陆的地质资料为约束, 对重、 磁异常作地壳构造融合解译, 追踪扬子—华夏陆块拼合带。由于华南大陆广泛发育具有剩磁多期、 多类的火山—侵入岩, 本文将磁异常换算为受剩磁影响小的解析信号作构造解译, 减小剩磁造成的地质解释偏差。研究认为, 扬子与华夏两陆块的拼合带东界为鹰潭—萍乡—衡阳—贺州—北海一线, 而西界为宜昌—张家界—铜仁—都匀—百色一线, 鹰潭以东和以北的拼合带位置因为受大规模岩浆岩的干扰而不清。     

Magnetic anomalies of two-dimensional bodies and algorithms for magnetic inversion of dykes and basement topographies

Generalized equations for the anomalies in any component of the earth’s magnetic field due to two-dimensional bodies of arbitrary magnetization are derived in terms of a new parameter, called the direction of measurement. Schemes for inverting the magnetic anomalies of arbitrarily magnetized dykes and basement topographies are then developed and the relevant computer software is presented. In both the schemes, the initial values of the parameters are calculated by the computer, so that the input merely consists of the anomalies and their distances. The differences in the observed and calculated anomalies are solved iteratively for the errors in initial values of the parameters.     

基于重力大数据的中国大陆区域地质构造解析

本文从重力异常角度对中国大陆区域地质构造的深部结构进行了研究。基于卫星重力大数据,利用改进后的位场分离方法获取了从地表到上地幔顶部不同深度的重力异常场。分析了各个碰撞-俯冲带、造山带及盆地等地区上地幔顶部（参考深度约为72～76 km）和中地壳（参考深度约为12～16 km）深度的异常特征。结合地质构造、地球化学和地球物理学证据,重点讨论了重力异常产生的原因及其与中国大陆主要构造之间的关系,获得了中国大陆在环太平洋动力学体系和特提斯动力学体系这两大动力作用下不同地区差异性的壳幔响应。     

Damped least-squares inversion of confined aquifer pumping data based on singular value decomposition

An iterative method for calculating the transmissivity and storage coefficient from pumping test data for a confined aquifer is presented. The method optimizes the fit between the measured and the theoretical data (computed using the Theis equation) in the leastsquare sense. Unlike the existing schemes, this method employs the Levenberg-Marquardt method and the singular value decomposition technique resulting in a stable and rapidly convergent data inversion algorithm. The inverse procedure is initialized by an automatically created starting model derived using a novel technique that operates on the timederivative of the drawdown curve. An important feature of the algorithm is that all the computations are done in logarithmic space which effectively linearizes the pmblem. The proposed method has several advantages over the conventional iterative inversion algorithms because of the linearizing parameterizations at both the forward and inverse stages of the problem. Detailed derivations of the basic equations are provided to guide the potential users as well as applications to field data to demonstrate the usefulness of the proposed algorithm.