Identifying gravity anomalies caused by granitic intrusions in Nanling mineral district,China: a multifractal perspective

Several power‐law relationships of geophysical potential fields have been discussed recently with renewed interests, including field value–distance () and power spectrum–wavenumber () models. The singularity mapping technique based on the density/concentration–area (C–A) power‐law model is applied to act as a high‐pass filter for extracting gravity and magnetic anomalies regardless of the background value and to detect the edges of gravity or magnetic sources with the advantage of scale invariance. This is demonstrated on a synthetic example and a case study from the Nanling mineral district, Southern China. Compared with the analytic signal amplitude and total horizontal gradient methods, the singularity mapping technique provides more distinct and less noisy boundaries of granites than traditional methods. Additionally, it is efficient for enhancing and outlining weak anomalies caused by concealed granitic intrusions, indicating that the singularity method based on multifractal analysis is a potential tool to process gravity and magnetic data.     

多维分形奇异功率谱分析与应用

地球物理信号中普遍含有噪声，消除噪声是地球物理信号处理中的关键技术之一.奇异功率谱分析（SSA）是在状态空间（又称相空间）中研究（系统）动力学、非线性科学与混沌现象的方法.本文在状态空间中通过SSA分解，研究、应用地球物理序列的尺度不变性进行多维分形滤波：通过在状态空间的SSA分解，构造了经验正交函数系（EOF）；在EOF子空间中定义了两种尺度与测度后，发现了两种测度与尺度皆在多个尺度范围内存在尺度不变性；利用这种尺度~测度的尺度不变性，设计、实现了多维分形奇异功率谱(MSSA)滤波模型；处理解释了大洋钻探（ODP）1143A孔岩芯自然反射性（NGR）资料；Fourier功率谱分析结果证明，MSSA能有效地压制噪声，提取有用信号.研究得出，嵌入维数对MSSA基本无影响（小于1/1000），多维分形滤波器（MSSA）能有效压制噪声或提取有用信号.     

Recent research on the Brabant Massif

Summary The Brabant Massif is the southeastern part of the Caledonian fold belt known as the Anglo-Brabant Massif, which extends from East Anglia to central Belgium and is a major constituting part of the Eastern Avalonia microcontinent. Recent research in Lower Palaeozoic stratigraphy and in geophysical interpretation led to a new subcrop map of the Brabant Massif. Palaeogeographical analysis supports the rapid movement of this Massif to lower latitudes and towards Baltica in Ordovician times. Structurally, the Brabant Massif appears as a faulted antiform, flanked longitudinally to the southwest by a magmatic arc of late Ordovician to early Silurian age. An elongated gravity low points to concealed granitic intrusions below a part of the magmatic arc.     

Integrated geophysics for mineral exploration in drift-covered volcanic terrains: examples from northern Vancouver Island,Canada

Known mineral occurrences in northern Vancouver Island are typically hosted in volcanic units of the Bonanza Group. At a local scale, the mineralization is associated with advanced argillic bedrock alteration and is often intimately related to porphyry intrusions. On a larger scale, faults are thought to exert the most significant control on the distribution of mineralized host rocks. Poor exposures and a complex glacial history limit the use of traditional methods of geological mapping and mineral exploration in this region and to date geophysical methods have been under-utilized. Here we present findings from four standard geophysical (gravity, magnetics, electromagnetics and seismic refraction) methods, recently deployed here to elucidate the subsurface geology, as well as to identify new targets for base metal exploration. Results at two different sites show that the integrated interpretation of geophysical data, constrained by physical rock property measurements, yields detailed images of the subsurface at a fraction of the cost of drilling. At one site, east of Rupert Inlet, the final subsurface model shows that the Bonanza Group is not nearly as extensive as previously presumed. An extension of the Holberg Fault is identified some 50 km east of the visibly mapped outcrop and an extensive zone of alteration around the fault is recognized. Furthermore, a number of the methods provide support for the existence of a porphyry dike at this site. At the second site, north of Rupert Inlet, magnetic and electromagnetic data prove effective at mapping alteration and locating shear zones beneath a relatively thin drift cover. Together, these results help outline a strategy for exploration in drift-covered terrains and show that a redirection of exploration effort is warranted in the case of northern Vancouver Island.     

地球物理信号能量(密度)多维分形及应用

地球物理信号代表的地质地球物理过程在多种尺度上和尺度之间表现为自相似性（self-affinity)或尺度无关性（Scale Invariant)，称为地球物理信号的分形性质，多个分形地球物理信号叠加在一起表现为多维分形特征，研究多维分形地球物理信号的能量或能量密度特征，可以进行时间或空间地球物理信号的校正、奇异性研究分析，或进行不同地球物理动力学过程的分解，本文描述了地球物理时间（空间）信号的多维分形过程和功率谱密度（能量密度）与波数以及重磁场能谱密度及面积（能量）与能谱密度的多维分形关系，并用地球物理测井与重磁资料作了试算。     

一种综合地学数据的矿产资源潜力制图方法

研究适应信息化时代特征的矿产资源潜力制图新技术、新方法对推动矿产资源评价理论与技术的发展具有重要的意义.笔者把GIS技术、图像分类算法和空间统计学理论进行有机集成,在空间统计学的空间结构分析技术和遥感图像纹理分类算法的基础上,提出了一种以综合地学数据（地质、地球物理、地球化学和遥感图像数据等）为基本数据源的矿产资源潜力自动制图方法.该方法的技术流程为：①数据准备,即对地球物理和地球化学勘探数据进行预处理,生成一个物化遥综合图像文件;②图像空间结构性分析和纹理图像生成,以综合地学图像为研究对象,用空间统计学的结构分析技术研究地学数据综合图像的空间结构性,生成纹理图像;③纹理图像多元分类,用实验变差函数纹理分类方法对研究区进行多元分类,生成分类专题图;④分类后处理,用叠置分析修正空间分类结果,生成区域矿产资源潜力分布图.     

Gravity evidence for a mafic intrusion beneath a mineralized zone in the Bondy gneiss complex, Grenville Province, Quebec — Exploration implications

A ground gravity survey over the Bondy gneiss complex and its mineralized iron oxide- and copper-rich hydrothermal system(s) in the Grenville Province of SW Quebec was undertaken to aid mineral exploration in mapping subsurface intrusions. Several kilometric-scale positive Bouguer anomalies were identified that coincide with outcropping mafic and intermediate intrusive rocks of the post peak-metamorphic, 1.17-1.16 Ga mafic to intermediate Chevreuil suite intrusions and a 1.09-1.07 Ga Rolleau ultramafic stock. An additional 4 × 3 kilometre positive gravity anomaly indicates a mafic body underlies part of the metamorphosed hydrothermal system in the area of magnetite, pyrite, pyrrhotite, and chalcopyrite mineralization. Advanced argilic alteration associated with sulphide enrichment here is however indicative of an epithermal system with a felsic intrusion fluid source. As a felsic intrusion cannot explain the positive Bouguer gravity anomaly both felsic and mafic bodies must be present beneath the mineralized zone. Our preferred interpretation based on integrating gravity data and 2D forward gravity modelling with the results of field and geochemical studies is that this anomaly corresponds to a ca. 500 m deep mafic 1.17-1.16 Ga Chevreuil suite pluton that may have provided the source for hydrothermal fluids associated with late ductile shear- and fault-related mineralization or remobilization of early mineralization associated with a felsic pluton into late structures. This interpretation is compatible with gabbro xenoliths in the 1.07 Ga Rivard lamprophyre dyke on the NW margin of the gravity anomaly that bear significant similarities with those of the Chevreuil intrusive suite. The presence of both early felsic and late mafic intrusions beneath a group of three mineral occurrences in the Bondy gneiss complex strengthens their prospectivity in comparison to other mineral occurrences in the area. That early, pre-metamorphic mineralization was upgraded late in the tectonothermal evolution during a subsequent igneous and deformation event agrees with interpretations of other IOCG-style deposits in the Lac Manitou area of the eastern Grenville Province.     

Geological and geophysical data integration for delimitation of mineralized areas in a supergene manganese deposits

Geophysical methods are widely used in mineral exploration. This paper discusses the results of geological and geophysical studies in supergene manganese deposits of southern Brazil. Mineralized zones as described in geological surveys were characterized as of low resistivity (20Ω.m) and high chargeability (30ms), pattern found also in oxides and sulfite mineral deposits. Pseudo-3D modeling of geophysical data allowed mapping at several depths. A relationship between high chargeability and low resistivity may define a pattern for high grade gonditic manganese ore. Large areas of high chargeability and high resistivity may result in accumulation of manganese and iron hydroxides, due to weathering of the gonditic ore, dissolution, percolation and precipitation.     

Using the Digital Models of Gravity Anomalies for Zoning of the Earth’s Gravity Field

This paper addresses the questions associated with using the models in regioning the Earth’s gravity field depending on its roughness and type of anomalies. A formalized technique for zoning based on the analysis of the intensity and variability of gravity anomalies is suggested. The suggested technique includes preprocessing the initial gravity anomalies aimed at eliminating the noise component; partitioning the studied region into the elementary segments; calculating the primary characteristics of the complexity and roughness (the intensity and variability of gravity anomalies) for each segment; and, based on these characteristics, classifying each segment into a particular category of complexity. The proposed system of classification of segments relies on the use of three classes of intensity and three classes of variability of gravity anomalies and four categories of complexity of the regions, which are derived from these classes. As a result of applying the technique, the mapped territory is subdivided into the regions that are uniform in terms of their geophysical properties. The developed technique is used for comparing the quality of the different digital models of gravity anomalies, including the Russian RGM model (A.P. Karpinsky Russian Geological Research Institute (VSEGEI)) and the WGM model (Bureau Gravimétrique International (BGI)) in the region of the Mongol–Okhotsk orogenic belt and Sikhote–Alin fold system. The results of the study can be used for zoning of the other geophysical fields and for planning the locations of the new survey networks for increasing the accuracy of the initial data used for compiling the maps.     

Review paper: Exploration geophysics for intrusion‐hosted rare metals

Igneous intrusions, notably carbonatitic–alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare‐earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene‐rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite‐bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska. The magnetic method is probably the most successful geophysical technique for locating carbonatitic–alkalic host intrusions, which are typically characterized by intense positive, circular to sub‐circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic–alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare‐metal mineralization and are proposed to have a similar origin. Discovery of carbonatitic–alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare‐earth metal deposits, which includes the Nechalacho deposit, were found by follow‐up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec–Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.     

南海海岛海山的重磁响应特征

南海的海岛、海山等地貌单元的地球物理研究对于南海成因、海岛利用、资源问题和我国海防建设均具有重要意义.过去我国的南海海洋实际测量资料覆盖面小,且多数为测线调查,海底地形测量精度和重磁等测量精度较低,因此,一直无法得到精度较高的研究成果.本文利用半个多世纪我国在南海历年的多波束、重力、磁力等船载海洋实际地球物理调查资料,加上少数卫星、航空测量成果,得到能够覆盖南海全部海域的多波束、重力、磁力实际测量的地球物理基础数据.追溯南海周边的地磁台站与当年调查时间匹配的日变数据,重新校正历年磁力测量成果,并利用"十一五"863国家海洋高科技计划的处理、拼合技术,获得了南海海底地形、重力、磁力三方互为印证的可靠地球物理成果,为海岛海山的地球物理研究奠定基础.研究发现,南海海岛海山按其地球物理性质并结合现有的岩石物性资料,可以分为三大类:1)南海大部分海岛海山为空间重力异常值高、正磁力ΔZ_⊥异常值也高的高密度高磁性的双高海山,以基性喷出岩(玄武岩)为主;2)空间重力异常值高、磁力ΔZ_⊥异常值低的海山,以花岗岩、变质岩为主;3)空间重力异常值高、部分磁力ΔZ_⊥异常值高部分低的海山,可能是花岗岩、变质岩海山的部分区域出现火山喷发形成的. 海山的分布有规律,与南海的成因与南海块体的分异状态有关.     

NVESTIGATION OF TECTONICS BY GRAVITY DETAILING*

Mapping of fault patterns is an important part of geophysical exploration. A computerized digital template analysis method is described which tests gravity maps for the effects of faults by comparing measured gravity data with calculated master curves. The interpreted gravity data are incorporated in a tectonic map using geological symbols and units, for ready use by the geologists. Tectonics can be investigated by gravity detailing if the smallest undulations on the Bouguer map are taken into consideration. Up to now, residual and derivative gravity maps have explained gravity effects by assuming spherical bodies which are almost unknown in geology. The method discussed here uses tectonic elements, such as fault-blocks and dikes as a basis of interpretation instead of the spherical bodies of the conventional interpretation methods. Gravity data can be easily and relatively cheaply obtained in the early phases of exploration by area wide spot coverage following lines of easy access such as roads, etc. Seismic studies, by contrast, have the disadvantage of being executed along predetermined profile lines. Only after sufficient detailing do seismic profiles permit-if at all -an areal mapping of faults. Thus a tectonic map is obtained only at the end of a geophysical survey instead of being available prior to the planning of costly seismic profiles. The use of gravimetric data and their interpretation by the suggested method provides tectonic detail maps in the early phases of geophysical studies. In addition, this article discusses a general geophysical interpretation method, using the investigation of faults by gravity as an example. Applications of this method for different gravity and magnetics problems as well as for combined interpretations are outlined. Detailed case histories will be published in later articles.     

Role of Geological and Geophysical Data in Modeling a Southwestern Alluvial Basin

A parameter-estimation technique based on existing hydrological, geophysical, and geological data was developed to approximate transmissivity values for use in a ground-water flow model of the Animas Valley, southwest New Mexico. Complete Bouguer gravity anomaly maps together with seismic-refraction profiles, geologic maps, geologic, geophysical, and drillers' logs, water levels, and pumping-test data provide insight into the transmissivity of bolson deposits throughout the basin. The transmissivity distribution was primarily based on reported pumping and specific-capacity tests in conjunction with complete Bouguer gravity anomaly maps and well log data. Reported transmissivity values were characterized by gravity values and well log data. In grid blocks lacking pumping and specific-capacity tests, transmissivity values were assigned based on the relationship of gravity values and well log data within the grid block to gravity values and well log data within other grid blocks for which transmissivity values are available. A two-dimensional, finite-difference, ground-water flow computer code was used to evaluate the effectiveness of the parameter-estimation technique. Although the trial-and-error method of calibration was employed, the actual computer time necessary for model calibration was minimal. The conceptually straightforward approach for parameter estimation utilizing existing hydrological, geophysical, and geological data provides realistic parameter estimates.     

Detailed investigation of preserved maar structures by combined geophysical surveys

The detection of completely preserved maar structures is important not only for underground mapping but also for paleoclimate research because laminated maar lake sediments may contain a very detailed archive of climate history. Objective evidence for the existence of such structures can only be provided by geophysics and boreholes. The combination of gravity and magnetic ground surveys appears to be an excellent tool to detect and identify buried maar structures. Their prominent properties are an almost circular gravity minimum corresponding to a crater filled with limnic sediments of low density, and a magnetic anomaly caused by a pyroclastic or basaltic body in the diatreme which indicates the volcanic character. Seismic measurements provide the most detailed information about the internal structure of the maar sediments. Zones of low seismic reflectivity and very low density represent sediments of the late maar-lake period. The early lake period is indicated by debris flow deposits and turbidites represented by seismic reflectors. The seismic sections clearly reveal the bowl-like structure of the maar. Outside this bowl-like structure, there are only a few reflections, which represent the basement. Taking into account the shape of the gravity anomaly, seismic information allows geometrical modelling of the maar structure. Optimal drilling sites can be selected based on the results of geophysical surveying. Comparing the results of combined geophysical surveys above two maar structures of different ages yields a marked similarity in their geophysical pattern.Editorial responsibility: J McPhie     

A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.     

中国东部海区编图及基本地学特征

随着我国海洋区域地质调查事业的发展,编制新的海区地质地球物理图件显得非常必要.中国东部海区1∶100万地质地球物理系列图包括空间重力异常图、布格重力异常图、磁力异常图、表层沉积物分布图和区域构造图.本研究通过利用数字化编图技术对新旧资料进行同化和标准化处理,获得了大范围、基于新资料的图件成果.此外,在编图成果的基础上,...     

Structural Investigations of Afghanistan Deduced from Remote Sensing and Potential Field Data

This study integrates potential gravity and magnetic field data with remotely sensed images and geological data in an effort to understand the subsurface major geological structures in Afghanistan. Integrated analysis of Landsat SRTM data was applied for extraction of geological lineaments. The potential field data were analyzed using gradient interpretation techniques, such as analytic signal (AS), tilt derivative (TDR), horizontal gradient of the tilt derivative (HG-TDR), Euler Deconvolution (ED) and power spectrum methods, and results were correlated with known geological structures.The analysis of remote sensing data and potential field data reveals the regional geological structural characteristics of Afghanistan. The power spectrum analysis of magnetic and gravity data suggests shallow basement rocks at around 1 to 1.5 km depth. The results of TDR of potential field data are in agreement with the location of the major regional fault structures and also the location of the basins and swells, except in the Helmand region (SW Afghanistan) where many high potential field anomalies are observed and attributed to batholiths and near-surface volcanic rocks intrusions.A high-resolution airborne geophysical survey in the data sparse region of eastern Afghanistan is recommended in order to have a complete image of the potential field anomalies.     

金属矿产勘查技术发展现状与思考

近年来，以寻找金属矿产为目的发愤卢业的勘查新技术可概括为：（1）以寻找隐伏矿为目标的覆盖区地球物理勘查技术，其中包括“三维地震”和“高光谱遥感”技术；（2）地球化学勘查新技术，其中包括“深穿透地球化学技术”、“地球化学信息的综合解释和地球化学填图技术”；（3）“找矿信息提取和综合”技术等。限于篇幅，本文仅对地球化学勘查新技术的研究现状进行了分析，同时对下列问题进行了思考：（1）在一个地区开展地球化学找矿工作首先要确立合适的取样介质，取样介质的立应以查明成矿成晕元素的形成机制和赋存状态为基础；（2）加强不同景观条件下不同矿种和不同矿床类型的异常特征和异常模式研究，建立区分矿异常和非矿异常的模式识别模型；（3）加强不同景观地质条件下，不同勘查尺度的地球化学填图技术研究，以实现迅速掌握全局，逐步缩小靶区的找矿战略；（4）加强地球化学信息与地质、地球物理和遥感信息相结合的信息综合技术的研究，应用综合信息定量圈定和评价找矿靶区，减少矿产勘查的不确定性。     

利用地球物理方法探测火山沉积型硼矿——在西藏阿里地区的应用

火山沉积型硼矿形成于新近纪特定的火山+湖相沉积环境条件下,含硼地层具有含硼湖相沉积岩与凝灰岩、安山岩等火山岩互层的多旋回二元沉积结构的特点.由于硼酸盐矿物的沉积发生多在沉积后期,通常与凝灰岩、粘土、灰岩等互层沉积,因此形成的硼矿层具有低密度、低电阻率的特征.针对硼矿层的这种特征,采用综合地球物理技术进行探测.在西藏雄巴地区,首先进行了1:5万高精度重力勘探,获得了研究区的布格重力异常,通过对重力资料的处理分析,获取了研究区的断裂构造发育和重力异常变化,推断了地下地层岩性空间变化特点并圈定出火山沉积型硼矿的发育有利区;在此基础上对有利区域进行了1:5000音频大地电磁测深,获得了研究区地下地层电性结构特征.通过对研究区电法数据的震电拟合及重力和大地电磁资料联合反演并与研究区地质资料相结合,进一步分析了该区地层构造、断裂体系、沉积单元与火山沉积型硼矿的形成关系,并对该区火山沉积型硼矿的分布进行了预测,最终钻探验证结果与预测结果基本一致.     

用于塔里木地壳构造成像的重力场谱矩方法研究

现代地壳探测取得的各种地球物理场中包含有大量地壳构造信息,如果提取这些信息,可以在数学物理普适规律指导下计算出关于地壳构造的图件.由于区域重力场的高阶谱矩包含了场轮廓面的几何信息,可用于地质构造要素的分析计算,为地质构造要素选取和定位的计算机自动制图提供理论依据.由地面布格重力场的二阶谱矩的统计不变量计算,可导出表征地壳变形带的脊形化系数,用于计算机自动制作区域地壳变形带分布图.脊形化系数强弱区的边界代表了活动的和稳定的构造单元的边界.在计算脊形化系数的基础上,还可进一步计算其二阶谱矩的统计不变量并作进一步增强处理,计算构造单元的边界系数,对地壳构造单元边界作连续和精细的追踪.文中介绍了脊形化系数和边界系数的计算方法,以及用在塔里木盆地进行地壳构造分析的结果.圈定了塔里木盆地深层满中高密度扰动带和满南、满北低密度扰动带,为深层油气勘探提供重要基础资料.