首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 29 毫秒
1.

Our study interprets large-scale gravity data to delineate concealed banded iron formation (BIF) iron mineralization in India's Rajasthan province. The study area belongs to the Bharatpur, Dausa, and Karauli districts of Rajasthan. We measured 1462 gravity readings to understand the rock types, depth and geometry of the different rock formations in the proposed study area. We also collected representative lithologies from more than 100 locations in the study area and calculated their density values. The measured gravity datasets are investigated via qualitative (e.g., Bouguer anomaly, first derivative and second derivative) and quantitative (radially averaged power spectrum, 3D Euler deconvolution, and 3D inversion) approach. The qualitative methods suggest a general NE–SW orientation of the BIFs, controlled by the general trend of the study area's structural setting. The lithological contact between the Bhilwara and Vindhyan Supergroups is demarcated by a NE–SW trending steep gravity gradient zone. In this area, representative lithologies yield high densities (about 3.746 gm/cc), and the samples identified as BIF represent exploration targets for iron ore. We have also developed our own in-house 3D gravity inversion code in this study. A model space inversion algorithm is converted into a data space using the identity relationship. It makes inversion algorithm very user-friendly on conventional desktop computers. The outcomes from the 3D inversion suggest that the concealed iron ore thickens to the west. This interpretation is also in good correlation with Euler 3D deconvolution of the gravity data.

  相似文献   

2.
Three-dimensional (3D) geological modeling is an important method for understanding geological structures and constructing exploration model with multiple source datasets for potential mineral deposits. The Luanchuan Mo polymetallic district located in Henan Province, China, has a complex geological setting and multiple metallogenic types. In this paper, we build a 3D geological model covering 43 km × 26 km × 5 km (vertical) by gravity and magnetic interactive inversion to delineate geometry, depth, and physical properties of geologic bodies at depths. We reduce the geological uncertainty and verify the reliability of the model through the combination of geophysical interactive inversion and tectonic-geochemistry methods. Several deep targets have also been extracted by combining the metallogenic model in the district with the 3D geologic model and tectono-geochemistry dataset. The research results show that the combination of 3D exploration model constrained by gravity and magnetic data with geologic constraints and tectono-geochemical data can be used to delineate the potential targets quickly and accurately in complex geological settings.  相似文献   

3.
We present some improvements of a gravity inversion method to determine the geometry of the anomalous bodies for priori density contrasts. The 3-D method is based on an exploratory process applied, not for the global model, but for the steps of a growth approach. The (positive and/or negative) anomalous structure is described by successive aggregation of cells, while its corresponding gravity field remains nearly proportional to the observed one. Moreover, a simple (e.g. linear) regional trend can be simultaneously adjusted. The corresponding program is applied to new gravity data on the volcanic island of Faial (Azores archipelago). The inversion approach shows a subsurface anomalous structure for the island, the main feature being an elongated high-density body. The body is interpreted as a compact sheeted dyke swarm, emplaced along Faial-Pico Fracture Zone, a leaky transform structure that forms the current boundary between Eurasian and African plates in the Azores area. The new results in this paper are (1) a Bouguer gravity anomaly map, (2) several improvements in the inversion process (robust process, optimal balance fitness/model magnitude), (3) a new gravimetric method for estimating the mean terrain density, (4) a 3-D model for subsurface mass anomalies in Faial and (5) some interpretative conclusions about a main intrusive complex detected under the island as a wall-like structure extending from a depth of 0.5 to 6 km b.s.l., with a N100°E trend and corresponding to an early fissural volcanic episode controlled by the regional tectonics.  相似文献   

4.
《Basin Research》2018,30(4):799-815
Since the last century, several geological and geophysical studies have been developed in the Santiago Basin to understand its morphology and tectonic evolution. However, some uncertainties regarding sedimentary fill properties and possible density anomalies below the sediments/basement boundary remain. Considering that this is an area densely populated with more than 6 million inhabitants in a highly active seismotectonic environment, the physical properties of the Santiago Basin are important to study the geological and structural evolution of the Andean forearc and to characterize its seismic response and related seismic hazard. Two and three‐dimensional gravimetric models were developed, based on a database of 797 compiled and 883 newly acquired gravity stations. To produce a well‐constrained basement elevation model, a review of 499 wells and 30 transient electromagnetic soundings were used, which contribute with basement depth or minimum sedimentary thickness information. For the 2‐D modelling, a total of 49 gravimetric profiles were processed considering a homogeneous density contrast and independent regional trends. A strong positive gravity anomaly was observed in the centre of the basin, which complicated the modelling process but was carefully addressed with the available constrains. The resulting basement elevation models show complex basement geometry with, at least, eight recognizable depocenters with maximum sedimentary infill of ~ 500 m. The 3‐D density models show alignments in the basement that correlates well with important intrusive units of the Cenozoic and Mesozoic. Along with interpreted fault zones westwards and eastwards of the basin, the observations suggest a structural control of Santiago basin geometry, where recent deformation associated with the Andean contractional deformation front and old structures developed during the Cenozoic extension are superimposed to the variability of river erosion/deposition processes.  相似文献   

5.
Choice of norm for the density distribution of the Earth   总被引：1，自引：0，他引：1  
Summary. The determination of the density distribution of the Earth from gravity data is called the inverse gravimetric problem. A unique solution to this problem may be obtained by introducing a priori data concerning the covariance of density anomalies. This is equivalent to requiring the density to fulfil a minimum norm condition. The generally used norm is the one equal to the integral of the square of the density distribution ( L2 -norm), the use of which implies that blocks of constant density are uncorrelated. It is shown that for harmonic anomalous density distributions this leads to an external gravity field with a power spectrum (degree-variances) which tends too slowly to zero, i.e. implying gravity anomalies much less correlated than actually observed. It is proposed to use a stronger norm, equal to the integral of the square sum of the derivatives of the density distribution. As a consequence of this, base functions which are constant within blocks, are no longer a natural choice when solving the inverse gravimetric problem. Instead a block with a linearly varying density may be used. A formula for the potential of such a block is derived.  相似文献   

6.
Observations of gravity can be aliased by virtue of the logistics involved in collecting these data in the field. For instance, gravity measurements are often made in more accessible lowland areas where there are roads and tracks, thus omitting areas of higher relief in between. The gravimetric determination of the geoid requires mean terrain-corrected free-air anomalies; however, anomalies based only on the observations in lowland regions are not necessarily representative of the true mean value over the topography. A five-stage approach is taken that uses a digital elevation model, which provides a more accurate representation of the topography than the gravity observation elevations, to reduce the unrepresentative sampling in the gravity observations. When using this approach with the Australian digital elevation model, the terrain-corrected free-air anomalies generated from the Australian gravity data base change by between 77.075 and −84.335 mgal (−0.193 mgal mean and 2.687 mgal standard deviation). Subsequent gravimetric geoid computations are used to illustrate the effect of aliasing in the Australian gravity data upon the geoid. The difference between 'aliased' and 'non-aliased' gravimetric geoid solutions varies by between 0.732 and −1.816 m (−0.058 m mean and 0.122 m standard deviation). Based on these conceptual arguments and numerical results, it is recommended that supplementary digital elevation information be included during the estimation of mean gravity anomalies prior to the computation of a gravimetric geoid model.  相似文献   

7.
This paper describes a method for determining Moho depth, lithosphere thinning factor (γ= 1 − 1/β) and the location of the ocean–continent transition at rifted continental margins using 3-D gravity inversion which includes a correction for the large negative lithosphere thermal gravity anomaly within continental margin lithosphere. The lateral density changes caused by the elevated geotherm in thinned continental margin and adjacent ocean basin lithosphere produce a significant lithosphere thermal gravity anomaly which may be in excess of −100 mGal, and for which a correction must be made in order to determine Moho depth accurately from gravity inversion. We describe a method of iteratively calculating the lithosphere thermal gravity anomaly using a lithosphere thermal model to give the present-day temperature field from which we calculate the lithosphere thermal density and gravity anomalies. For continental margin lithosphere, the lithosphere thermal perturbation is calculated from the lithosphere thinning factor (γ= 1 − 1/β) obtained from crustal thinning determined by gravity inversion and breakup age for thermal re-equilibration time. For oceanic lithosphere, the lithosphere thermal model used to predict the lithosphere thermal gravity anomaly may be conditioned using ocean isochrons from plate reconstruction models to provide the age and location of oceanic lithosphere. A correction is made for crustal melt addition due to decompression melting during continental breakup and seafloor spreading. We investigate the sensitivity of the lithosphere thermal gravity anomaly and the predicted Moho depth from gravity inversion at continental rifted margins to the methods used to calculate and condition the lithosphere thermal model using both synthetic models and examples from the North Atlantic.  相似文献   

8.
A new algorithm is presented for the integrated 2-D inversion of seismic traveltime and gravity data. The algorithm adopts the 'maximum likelihood' regularization scheme. We construct a 'probability density function' which includes three kinds of information: information derived from gravity measurements; information derived from the seismic traveltime inversion procedure applied to the model; and information on the physical correlation among the density and the velocity parameters. We assume a linear relation between density and velocity, which can be node-dependent; that is, we can choose different relationships for different parts of the velocity–density grid. In addition, our procedure allows us to consider a covariance matrix related to the error propagation in linking density to velocity. We use seismic data to estimate starting velocity values and the position of boundary nodes. Subsequently, the sequential integrated inversion (SII) optimizes the layer velocities and densities for our models. The procedure is applicable, as an additional step, to any type of seismic tomographic inversion.
We illustrate the method by comparing the velocity models recovered from a standard seismic traveltime inversion with those retrieved using our algorithm. The inversion of synthetic data calculated for a 2-D isotropic, laterally inhomogeneous model shows the stability and accuracy of this procedure, demonstrates the improvements to the recovery of true velocity anomalies, and proves that this technique can efficiently overcome some of the limitations of both gravity and seismic traveltime inversions, when they are used independently.
An interpretation of field data from the 1994 Vesuvius test experiment is also presented. At depths down to 4.5 km, the model retrieved after a SII shows a more detailed structure than the model obtained from an interpretation of seismic traveltime only, and yields additional information for a further study of the area.  相似文献   

9.
According to the theory of isostasy, the Earth has a tendency to deform its surface in order to reach an equilibrium state. The land-uplift phenomenon in the area of the Fennoscandian Shield is thought to be a process of this kind. The geoid, as an equipotential surface of the Earth's gravity field, contains information on how much the Earth's surface departs from the equilibrium state. In order to study the isostatic process through geoidal undulations, the structural effects of the crust on the geoid have to be investigated.
  The structure of the crust of the Fennoscandian Shield has been extensively explored by means of deep seismic sounding (DSS). The data obtained from DSS are used to construct a 3-D seismic-velocity structure model of the area's crust. The velocity model is converted to a 3-D density model using the empirical relationship that holds between seismic velocities and crustal mass densities. Structural effects are then estimated from the 3-D density model.
  The structural effects computed from the crustal model show that the mass deficiency of the crust in Fennoscandia has caused a geoidal depression twice as deep as that observed from the gravimetric geoid. It proves again that the crust has been isostatically compensated by the upper mantle. In other words, an anomalously high-density upper mantle must exist beneath Fennoscandia.  相似文献   

10.
A 3-D density model was created for the Central Balkans area down to a depth of 670  km on the basis of seismic (both artificial sources and earthquakes) and gravity data. This model is based on density columns constructed for the main geological units of the study region. The densities for these columns were obtained using the density variation method. This method makes it possible to extrapolate the density distribution from the well-studied uppermost layers to the deeper levels of the Earth. The constructed 3-D density model was interpreted in relation to the available data on the heat flow and the seismicity of the region. The subdivision of the region by the Maritza fault into two parts—the southern part including the Rhodope massif and the northern part including the structures of Alpine activation of Srednogorie and the Balkanides—was confirmed. The upraised position of the 400  km boundary within the upper mantle, which was established from the density modelling, is assumed to be a sign of development of recent geodynamical processes over the Srednogorie block. From the viewpoint of seismicity prediction, a finding of mantle inhomogeneities orthogonal to the Maritza suture is of great importance.  相似文献   

11.
Magnetotelluric and gravity data have been collected within a ca. 170 km long traverse running from the Pacific coast of Nicaragua in the west to the Nicaraguan Highland in the east. This part of Nicaragua is characterized by sedimentary rocks of the Pacific Coastal Plain, separated from the Tertiary volcanic rocks of the Highland by the NW-SE-trending Nicaraguan Depression. 2-D interpretation of the magnetotelluric (MT) data, collected at 13 stations, indicates four regions of high electrical conductivity in addition to the conductive coastal region. Two of these are associated with conducting sediments and pyroclastics in the upper part of the crust. Two other conductive structures have been defined at depth around 20 km and the one best defined is located below the depression. From the distribution of seismic events, volcanic activity in the depression and the similarity in geophysical characteristics with areas such as the Rio Grande Rift, this conductor is interpreted as a melt layer or a complex of magma chambers. Models of the upper lithosphere, constrained by the MT model, vertical electrical sounding (VES) data, seismic data and densities, have been tested using gravity data. A model that passes this test shows a gradual thickening of the crust eastwards from the Pacific coast. An anomaly centred over the depression is interpreted to have its origin in a thinning of the crust. In this model the melt layer is situated on top of the bulge of the lower lithosphere. A change in the composition of the crust, from the Pacific Coastal Plain to the Highland, is indicated from the change in character of the MT response and from the density distribution in the gravity model. This may support the hypothesis that the Pacific region is an accreted terrane. MT and gravity data indicate a depth to a resistive and high-density basement in the depression of ca. 2 km. On the basis of this, the vertical setting in the depression is estimated to be of the order of 2.5 km.  相似文献   

12.
高超  金凤君  傅娟  刘鹤 《地理科学进展》2013,32(7):1167-1176
南非是中国在非洲的最大投资目的地与经贸合作伙伴, 特别在基础设施建设方面发展迅速。南非人口空间分布研究对开展中南合作具有重要意义。本文通过收集整理南非历年来的全国人口普查资料统计数据, 运用空间自相关模型、不均衡指数、人口重心模型和偏移—分享模型, 分析了南非人口的空间分布格局和演变特征。研究结果表明, 南非人口分布具有显著的“东高西低”的空间格局, 呈现出以工业化程度较高的大都市区和港口城市为中心的“多核心”集聚的分布特征;空间自相关分析得出人口高集聚区主要分布在大都市区和沿海港口城市, 低密度集聚区主要集中分布在中西部地区;从变化趋势上来看, 1996-2011 年间人口分布不平衡趋势不断加强, 人口重心向西北方向移动;偏移—分享分析表明, 南非人口增长速度先后经历了从1996-2001 年快速增长到2001-2007 年缓慢增长, 再到2007-2011 年高速增长的“高—低—高”增长阶段;省域层面上看, 豪登省和西开普省一直是南非人口集聚较强地区。  相似文献   

13.
鼠害是影响草原生态健康的重要因素,了解小型啮齿动物种群密度时空分布特征,对精准的鼠害综合防治具有重要意义。以往对小型啮齿动物时空分布的研究多局限于静态的站点分布或小范围的种群密度时间变化分析,缺乏对较大时空尺度小型啮齿动物种群密度变化的分析。从已发表的文献中收集了天山北坡草地1982—2015年小尺度的有效洞口密度实地调查信息,同时结合环境因子数据,再根据海拔将研究区划分为≤900 m和>900 m 2类,运用Cubist模型和随机森林模型,分析有效洞口密度时空分布。结果表明：（1） 1982—2015年天山北坡海拔≤900 m地区的有效洞口密度总体呈增加趋势,而海拔>900 m的地区总体呈减少趋势。基于Cubist模型构建有效洞口密度与环境因子的模型拟合精度明显优于随机森林模型。（2） 植被状况、气象因子和放牧强度是天山北坡有效洞口密度时空分布主要的环境驱动因素。在天山北坡内海拔≤900 m和>900 m的地区中,有效洞口密度的驱动机制存在着显著差异。（3） 在海拔≤900 m地区,影响有效洞口密度时空分布主要是叶面积指数,而对于海拔>900 m地区为归一化植被指数。这可能是受到大沙鼠（Rhombomys opimus）和黄兔尾鼠（Eolagurus luteus）消耗不同类型植被的影响。  相似文献   

14.
A global particle swarm optimization (GPSO) technique is developed and applied to the inversion of residual gravity anomalies caused by buried bodies with simple geometry (spheres, horizontal, and vertical cylinders). Inversion parameters, such as density contrast of geometries, radius of body, depth of body, location of anomaly, and shape factor, were optimized. The GPSO algorithm was tested on noise-free synthetic data, synthetic data with 10% Gaussian noise, and five field examples from different parts of the world. The present study shows that the GPSO method is able to determine all the model parameters accurately even when shape factor is allowed to change in the optimization problem. However, the shape was fixed a priori in order to obtain the most consistent appraisal of various model parameters. For synthetic data without noise or with 10% Gaussian noise, estimates of different parameters were very close to the actual model parameters. For the field examples, the inversion results showed excellent agreement with results from previous studies that used other inverse techniques. The computation time for the GPSO procedure is very short (less than 1 s) for a swarm size of less than 50. The advantage of the GPSO method is that it is extremely fast and does not require assumptions about the shape of the source of the residual gravity anomaly.  相似文献   

15.
大盐滩地区盐湖企业面临水资源和盐矿资源双重危机,利用重力方法探寻新资源具有重要的实际意义。本文对该区布格重力异常进行三维聚焦反演,获得了大盐滩地区三维剩余密度结构。通过研究发现,大盐滩地区浅地表高低度异常,表征着赋存较为丰富的盐矿资源,但地层中水资源较为贫瘠,浅表水资源主要赋存在山前冲积扇及断裂带内。中深层位不仅具有多个含水层,而且出现多处低密度异常区(T1～T4),可能为深部卤水富集体的反映,是探寻新卤水资源的有利靶区。依此建议在阿尔金山山前冲积带及断裂带内寻找新的淡水资源,并在成矿有利区,开展深部卤水资源勘查工作。  相似文献   

16.
20 magnetotelluric (MT) soundings were collected on the Isle of Skye, Scotland to provide a high-resolution three-dimensional (3-D) electrical resistivity model of a volcanic province within the framework of a project jointly interpreting gravity, seismic, geological and MT data. The full 3-D inversion of the MT data jointly interpreted with gravity data reveals upper crustal structure. The main features of the model are interpreted in conjunction with previous geological mapping and borehole data. Our model extends to 13 km depth, several kilometres below the top of the Lewisian basement. The top of the Lewisian basement is at approximately 7–8 km depth and the topography of its surface was controlled by Precambrian rifting, during which a 4.5 km thick sequence of Torridonian sediments was deposited. The Mesozoic sediments above, which can reach up to 2.2 km thick, have small-scale depocentres and are covered by up to 600 m of Tertiary lava flows. The interpretation of the resistivity model shows that 3-D MT inversion is an appropriate tool to image sedimentary structures beneath extrusive basalt units, where conventional seismic reflection methods may fail.  相似文献   

17.
To solve the problem of handling numerous data from drillings and geophysical surveys for specific information of subsurface geological formations, the use of databases and geographic information systems (GIS) is demonstrated. Geological formations are in 3D and commercial GIS packages are normally in 2D. This problem is solved here by a 1D database of the layering of the drillings and export of query results from the database to a 2D GIS. The queries will ask for grain size and altitude for the actual sedimentary layers. To include point data from ground penetrating radar (GPR) data survey profiles in a 2D GIS, a stepwise technique of dynamic segmentation has been developed. The points are digitized along sedimentary boundaries from 2D GPR profiles. The concept was applied on a small area of glacial sediments in Telemark, Norway. Results from the processes were clustering of points with properties connected to specific geological formations. The clustering subsurface geological formations were moraine ridges, a diamict layer, gravel and coarse sand above and below the diamict layer, till above bedrock, and glaciomarine deposits of gravel and sand. Information about extension of these geological formations is useful and essential for modelling of sedimentary environments and for aquifer modelling.  相似文献   

18.
We study the tectonic setting and lithospheric structure of the greater Barents Sea region by investigating its isostatic state and its gravity field. 3-D forward density modelling utilizing available information from seismic data and boreholes shows an apparent shift between the level of observed and modelled gravity anomalies. This difference cannot be solely explained by changes in crustal density. Furthermore, isostatic calculations show that the present crustal thickness of 35–37 km in the Eastern Barents Sea is greater than required to isostatically balance the deep basins of the area (>19 km). To isostatically compensate the missing masses from the thick crust and deep basins and to adequately explain the gravity field, high-density material (3300–3350 kg m−3) in the lithospheric mantle below the Eastern Barents Sea is needed. The distribution of mantle densities shows a regional division between the Western and Eastern Barents and Kara Seas. In addition, a band of high-densities is observed in the lower crust along the transition zone from the Eastern to Western Barents Sea. The distribution of high-density material in the crust and mantle suggests a connection to the Neoproterozoic Timanide orogen and argues against the presence of a Caledonian suture in the Eastern Barents Sea. Furthermore, the results indicate that the basins of the Western Barents Sea are mainly affected by rifting, while the Eastern Barents Sea basins are located on a stable continental platform.  相似文献   

19.
We developed an inversion method to estimate the stress fields related to earthquake generation (seismogenic stress fields) from the centroid moment tensors (CMT) of seismic events by using Akaike's Bayesian information criterion (ABIC). On the idea that the occurrence of an earthquake releases some part of the seismogenic stress field around its hypocentre, we define the CMT of a seismic event by a weighted volume integral of the true but unknown seismogenic stress field. Representing each component of the seismogenic stress field by the superposition of a finite number of 3-D basis functions (tri-cubic B-splines), we obtain a set of linear observation equations to be solved for the expansion coefficients (model parameters). We introduce prior constraint on the roughness of the seismogenic stress field and combine it with observed data to construct a Bayesian model with hierarchic, highly flexible structure controlled by hyper-parameters. The optimum values of the hyper-parameters are objectively determined form observed data by using ABIC. Given the optimum values of the hyper-parameters, we can obtain the best estimates of model parameters by using a maximum likelihood algorithm. We tested the validity of the inversion method through numerical experiments on two synthetic CMT data sets, assuming the distribution of fault orientations to be aligned with the maximum shear stress plane in one case and to be random in the other case. Then we applied the inversion method to actual CMT data in northeast Japan, and obtained the pattern of the seismogenic stress field consistent with geophysical and geological observations.  相似文献   

20.
An analysis of the field scaling power spectrum yields useful information about the source distribution, but it is uncertain whether deterministic, random, fractal or mixed approaches have to be used for the interpretation. To this end, the scaling properties of potential field spectra are analysed for a number of different source models of geological interest. Besides the models of Naidu (purely random sources) and Spector and Grant (gross block statistical ensembles) we consider other types of density and magnetization distributions with spectral exponents in the fractal range, such as a single homogeneous body with a random white source distribution. Spectral slopes in the fractal range are obtained.
We also study the effects of important natural sources, such as salt domes and sedimentary basins, representing them with simple Gaussians or combinations of Gaussian signals. The same spectral slopes as for gravity signals generated by 3-D fractal source distributions are found for them. Hence the power law decay of the field is not a characteristic only of fractal source models.
If a 3-D fractal source distribution is assumed a priori , a way of verifying the goodness of the model is to examine the whitened field at source level. The probability that the whitened field derives from a random white population is estimated for synthetic and real anomalies by applying the usual statistical tests.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号