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1.
Least squares collocation is a very comprehensive method for gravity field modelling, since it may use known noise characteristics of the data. In many earlier applications the errors affecting the data were considered uncorrelated, mainly due to the difficulty in estimating the systematic character of such kind of errors. In this study, error covariance functions of airborne gravity gradiometer data are estimated by comparing model covariance functions with empirical covariance functions of the gravity gradiometer data. The model covariance functions were estimated from accurate surface gravity data and continuated upward to the height of the airborne measurements using the covariance propagation law. The estimated error covariance functions were modeled as finite ones and used as an additional information for the prediction of gravity anomalies from gravity gradiometer data. The assessment of the prediction results was made by comparing the gravity values predicted from the airborne gradient data and showed up to 25% improvement compared to not using correlated errors.  相似文献   

2.
Spherical Radial Basis Functions (SRBFs) can express the local gravity field model of the Earth if they are parameterized optimally on or below the Bjerhammar sphere. This parameterization is generally defined as the shape of the base functions, their number, center locations, bandwidths, and scale coefficients. The number/location and bandwidths of the base functions are the most important parameters for accurately representing the gravity field; once they are determined, the scale coefficients can then be computed accordingly. In this study, the point-mass kernel, as the simplest shape of SRBFs, is chosen to evaluate the synthesized free-air gravity anomalies over the rough area in Auvergne and GNSS/Leveling points (synthetic height anomalies) are used to validate the results. A two-step automatic approach is proposed to determine the optimum distribution of the base functions. First, the location of the base functions and their bandwidths are found using the genetic algorithm; second, the conjugate gradient least squares method is employed to estimate the scale coefficients. The proposed methodology shows promising results. On the one hand, when using the genetic algorithm, the base functions do not need to be set to a regular grid and they can move according to the roughness of topography. In this way, the models meet the desired accuracy with a low number of base functions. On the other hand, the conjugate gradient method removes the bias between derived quasigeoid heights from the model and from the GNSS/leveling points; this means there is no need for a corrector surface. The numerical test on the area of interest revealed an RMS of 0.48 mGal for the differences between predicted and observed gravity anomalies, and a corresponding 9 cm for the differences in GNSS/leveling points.  相似文献   

3.
The gravitational signal of the upper mantle density structures is investigated in the refined gravity data which are corrected for the gravitational contributions of the crust density structures and the Moho geometry. The gravimetric forward modeling is applied to compute these refined gravity data globally on a 1 × 1 arcdeg grid using the global geopotential model (EGM2008), the global topographic/bathymetric model (DTM2006.0) including the ice-thickness data, and the global crustal model (CRUST2.0). The characteristics of the upper mantle density structures are further analyzed in association with the Moho parameters (i.e., Moho depths and density contrast). The 1 × 1 arcdeg global data of the Moho parameters are estimated by applying the combined least-squares approach based on solving Moritz’s generalization of the Vening–Meinesz inverse problem of isostasy. The refined gravity data exhibit mainly the mantle lithosphere structures attributed to the global mantle convection. A significant correlation found over oceans between the refined gravity data and the Moho density contrast is explained by the increasing density of the oceanic lithosphere with age. Despite the lithosphere structures attributed to the global mantle convection are confirmed also in the refined gravity data over continents, the significant correlation between the refined gravity data and the Moho parameters is in this case absent. Instead, the significant proportion of lateral variations of the Moho density contrast within the continental lithosphere is attributed to the depth-dependant density changes due to pressure and thermal gradient.  相似文献   

4.
Gravity and bathymetry data have been extensively used to infer the thermo-mechanical evolution of different segments of the oceanic lithosphere. It is now understood that magmatic fluid processes involved in the accretion of oceanic crust are spatially complex and episodic. The nature of these processes which are in general nonlinear, can be described using fractal analysis of marine geophysical data. Fractal analysis has been carried out for gravity and bathymetry profiles over the aseismic Chagos-Laccadive Ridge and the spreading Carlsberg Ridge. The Iterated Function Systems (IFS) have been used to generate synthetic profiles of known dimension (D) and these are compared with the observed profiles. The D for the data sets are in the range of 1–1.5. The D for gravity profiles is less than those of bathymetry and the D for gravity and bathymetry over spreading ridge is higher than the aseismic ridge. The low fractal dimension indicates that the processes generating them are of low dimensional dynamical systems.  相似文献   

5.
Repeat gravity measurements have been carried out over more than 1000 km of level lines in southern California. The initial surveys over a number of these lines were made in 1962. Repeat gravity survey lines are located primarily in the San Joaquin Valley, in the Los Angeles and Palmdale areas and in the Imperial Valley. Secular changes of gravity have been noted in a number of areas. In areas of subsidence, due to groundwater withdrawal, direct correlation between elevation and gravity change is found. In areas of vertical crustal motion along fault zones, such as in the Palmdale area, gravity changes are also noted. However, these are not, in general, directly related to elevation change. Apparently, subsurface mass redistributions are associated with the vertical motions noted in fault-zone areas before and after earthquakes. Several possible models to explain the gravity observations are being explored.  相似文献   

6.
重力场定义的澄清   总被引：3，自引：1，他引：2  
曾华霖  万天丰 《地学前缘》2004,11(4):595-599
《朗曼现代英文词典》(LongmanModernEnglishDictionary)、《牛津高阶英汉双解词典》(OxfordAdvancedLearner’sEnglish ChineseDictionary)、《韦氏词典》(Merriam Webster’sNewCollegiateDictionary)、作为地球物理专业词汇定义源的《勘探地球物理百科词典》(EncyclopedicDictionaryofExplorationGeo physics)以及中国的一些重力教科书、专著及词典 ,把重力场定义为重力或重力作用存在的空间 ,重力效应可以测量的空间 ,即 :力或重力存在的空间。这个定义不同于国外英文地球物理教科书、专著及中、英、俄文场论教科书对重力场的定义 :空间中的重力。定义重力场为重力存在的空间 ,混淆了“场”本身及“场域”的概念 ,并不是一个标准的物理或地球物理定义 ,是一种误导。物理场是空间中存在的一种物理作用或效应 ,分布于引起它的场源体周围。两个物体之间存在着的相互作用力 ,通过每个物体引起的引力场给予另一个物体。重力场可以定义如下 :地球内部及其附近空间一点处存在的重力作用 ,或单位质量受到的重力。重力场是一种力或力场 ,存在于地球表面及其附近的空间。重力场不是空间 ,重力场的测量应当是在重力场存在的空间或场域中 ,而不是在重力场中进行。  相似文献   

7.
The Eastern Ghats are a prominent topographic feature on the Indian Peninsula, stretching from the southern tip of the peninsula to near Bhubaneswar (20°N, 86°E) along the east coast. The belt is characterised by occurrences of high grade metamorphic rocks such as pyroxene granulites, sillimanite gneisses, charnockites and gabbro-anorthosite masses. The gravity field over the Eastern Ghats is appreciably positive as compared to the surrounding low grade gneissic terrain.Analysis of the gravity field along the coastal and southern granulite terrain comprising the Eastern Ghats shows that a large number of gravity highs are associated with charnockites of basic and intermediate nature as well as gabbro-anorthosite masses. The lows appear to be associated with acid charnockites, syenite masses or granitic intrusives.The boundary between the Eastern Ghats terrain and the adjoining Dharwar/Bastar cratons appears to be a faulted one. The crust underneath the Eastern Ghats is inferred to be of a higher density than that of the Dharwar/Bastar cratons to its west. The gravity field over the Eastern Ghats is compared to that of similar terrains in other parts of the world. It is inferred that the Eastern Ghats are characterised by a crust of higher than normal density.  相似文献   

8.
Gravity data were analysed in the Northern Tunisian Atlas (case study of Oued Zarga area) to better understand the organization of its underlying structures. The gravity data analysis included the construction of a gravity anomaly maps and two and a half dimensional gravity model. The qualitative analysis of gravimetric maps served for the foundation of a new structural map of the study region that constitutes a new contribution of the gravimetry in this present work. In addition, the complete Bouguer and residual gravity anomaly maps indicate a negative gravity anomaly over the Triassic evaporitic outcrops of Jebel Guerouaou and prominent NE?CSW-trending features associated with the boundary of the Triassic rocks and surrounded layers. A NW?CSE-trending gravity model that crosses the Triassic evaporitic outcrop at Jebel Guerouaou can be explained by a deep-rooted salt diapir.  相似文献   

9.
Isotropic reproducing kernels for a sphere or spherical shell are derived as weighted product sums o fL 2 orthonormal base functions. For the sphere these functions are products of the surface spherical harmonics and the Jacobi polynomials of degree (0, 2). Reproducing kernels for a sphere are consistent with the covariance function of the outer anomalous gravity potential of the Earth. These reproducing kernels may be used for gravity field modeling which include density (anomaly) data as observations or which aims at predicting such quantities using optimal estimation methods, that is for solving the inverse gravimetric problem.  相似文献   

10.
新疆阿舍勒块状硫化物铜锌矿床,是一个正在勘探的大型富铜矿床,矿体呈厚大透镜状,具层控特征,矿体和围岩之间密度差较大。地面重力方法,理应是最有效的勘探方法,但是,由于阿舍勒矿区地质构造过分复杂,地下不同深度的多种密度体叠合在一起,往往掩盖了矿体所产生的重力异常。为了解决这个疑难问题,通过研究,选择适当的正交函数去进行拟合,实现目标信息分离,将成矿层位的重力异常从实测重力异常值中分离出来,勾绘出了矿区线状构造成矿带的轮廓,揭示了矿区构造;指出找矿方向,圈出１１个重力异常,根据矿区矿石的最低密度大于围岩最高密度的实事,可以说：“凡有矿的地方必然显示出重力异常”。故使用本方法处理重力资料,在阿舍勒这类矿区,可具直接找矿效果。  相似文献   

11.
由于断裂两侧的磁性、密度的纵横向差异在重力、磁力异常上有所表现,因此所获得的重力、磁力数据为深入研究关键的地质课题提供了科学基础,如郯庐断裂带的基底性质、断裂形成特征和岩浆岩分布。利用最新的高精度航空重力和磁力数据以及地面重力数据,绘制了郯庐断裂带地区的1∶50 000重力和磁力异常图,并结合区域地质数据分析了重力和磁力异常特征。分析结果认为:存在连体的郯庐—大别古老构造带,郯庐断裂带南段是元古宙和燕山中期岩浆活动的复合反映带;郯庐断裂带为中元古—新元古代时期南华北陆块与下扬子陆块的界限;磁力、重力异常图对比说明,合肥盆地范围由老到新向东扩展。  相似文献   

12.
Detailed gravity data were analyzed to constrain two controversial geological models of evaporitic structures within the Triassic diapiric zone (Triassic massifs of Jebel Debadib and Ben Gasseur) of the northern Tunisian Atlas. Based on surface observations, two geological models have been used to explain the origin of the Triassic evaporitic bodies: (1) salt dome/diapiric structure or (2) a “salt glacier”. The gravity analysis included the construction of a complete Bouguer gravity anomaly map, horizontal gravity gradient (HGG) map and two and a half-dimensional (2.5D) forward models. The complete Bouguer gravity anomaly map shows a prominent negative anomaly over the Triassic evaporite outcrops. The HGG map showed the location of the lateral density changes along northeast structural trends caused by Triassic/Cretaceous lithological differences. The modeling of the complete Bouguer gravity anomaly data favored the diapiric structure as the origin of the evaporitic bodies. The final gravity model constructed over Jebel Debadib indicates that the Triassic evaporitic bodies are thick and deeply rooted involving a dome/diapiric structure and that the Triassic material has pulled upward the younger sediment cover by halokinesis. Taking in account kinematic models and the regional tectonic events affecting the northern margin of Africa, the above diapirs formed during the reactive to active to passive stages of continental margin evolution with development of sinks. Otherwise, this study shows that modeling of detailed gravity data adds useful constraints on the evolution of salt structures that may have an important impact on petroleum exploration models.  相似文献   

13.
Important areas of the earth are still not covered by accurate gravity measurements. The gravity field may be determined by using different techniques but airborne gravity surveying is becoming the most powerful tool available today.One of the main problems in airborne gravity is the separation of the vertical accelerations acting on the airborne platform from the natural gravity signal. With the advances in DGPS techniques new prospects arise for gravity field recovery which are of great importance for geodesy, geophysics oceanography and satellite navigation. Furthermore, airborne gravimetric measurements depend not only on the determination of the position but also on the attitude of the aircraft. Inertial systems can provide attitude as well as information on short-term accelerations, which are more problematic for the gravimeter. A proper integration of these systems may allow a further improvement of the whole technique where the quality of both the accelerometers and the gyros is the key sensing element. In the scope of the MAST III Project AGMASCO, an airborne geoid mapping system was successfully implemented in different aeroplanes. The characteristics of the aeroplane and the flight parameters play a major role in airborne measurements.Within AGMASCO the airborne system was applied both in a close and an open ocean (Skagerrak, Fram Strait and Azores) areas. The system proved to be a powerful tool in a variety of conditions. The results obtained showed that an accuracy better than 2mGal over 5 to 6 kilometres can be achieved.This was proven by comparison of the airborne data with ground truth and satellite data. This accuracy makes the system interesting for use in various applications including geophysical exploitation.Different hardware installations were experienced and the methods validated. Recovery of the gravity values directly from measurements with the Lacoste & Romberg air/sea gravimeter and from measurements with the inertial sensors was analysed. The potential of these sensors to recover gravity and the experience gained within this project are reported here.  相似文献   

14.
The upper part of the lithosphere has been actively involved in various exogenic and endogenic processes which have left their imprint on the gravity field on the Indian Peninsula and the Himalaya. Analysis of the gravity field over the Dharwar craton shows that the greenstone belts of this craton have been formed as a result of development of deep fractures in the earth's crust during Archaean times. Precambrian mountain ranges such as the Aravallies, Vindhyans, Satpura and Eastern Ghats are located peripheral to Archaean cratons. Most of these mountain belts are characterized by gravity highs suggesting that the underlying crust is of higher than normal density. These mountain ranges with the exception of the Eastern Ghats do not appear to be locally compensated. Regional compensation seems to prevail over all these areas. Eastern Ghats ranges are also underlain by a crust of higher than normal density relative to the Dharwar and Bastar cratons and exist with a sharp contact with the cratons in the West. Isostatic compensation in the Eastern Ghats appears to have been achieved by thickening of the underlying crust. The Himalaya has attained a fairly high degree of isostatic compensation.  相似文献   

15.
An improved hybrid gravimetric geoid model for Egypt, EGY-HGM2016, has been recently computed implementing the least-squares collocation (LSC) method through the remove-compute-restore (RCR) procedure. The computation of EGY-HGM2016 involves different datasets in terms of gravity anomalies determined from the GOCE (gravity field and steady-state ocean circulation explorer)-based global geopotential model (SPW-R4) up to d/o 200 and EGM2008 from d/o 201 to 720 combined with terrestrial gravity datasets in terms of 2140 gravity field anomalies and about 121,480 marine surface gravity anomalies. In addition, orthometric heights from 17 GPS/levelling measurements have been considered during the modelling process to improve the determination of the hybrid gravimetric geoid over the Egyptian region. The EGY-HGM2016 model estimated over Egypt provides geoid heights that are ranging from 7.677 to 21.095 m with a standard deviation (st. dev.) of about 2.534 m in the northwest of the country excluding the involvement of the orthometric heights from GPS/levelling measurements. When the later dataset is considered during the implementation of LSC process, hybrid residual height anomalies ranging from ?1.5 to +0.9 m, with a mean of 0.22 m and a st. dev. of 0.17 m, are obtained. Comparison of the predicted hybrid gravimetric geoid with the corresponding ones obtained from EGM2008, GOCE-based SPW R4 model, and GPS/levelling reveals considerable improvements of our EGY-HGM2016 model over Egypt.  相似文献   

16.
The structure of the Glueckstadt Graben has been investigated by use of 3D gravity backstripping technique and by 2D gravity and magnetic modelling. Subtracting the gravity effects of the Meso-Cenozoic sediments together with Permian salt reveals a positive residual anomaly within the Glueckstadt Graben. This anomaly includes two local maxima over the Westholstein and Eastholstein Troughs. The 2D gravity models point to the presence of a high-density body within the lower crust of the Glueckstadt Graben. In addition, the results of 2D magnetic modelling indicate that the central part of the high-density body is overlain by an area with high susceptibility. Most probable, the formation of this high-density body is a result of complex poly-phase tectonic history of the study area. Finally, the results of gravity modelling indicate that Permian salt is not homogeneous. 3D gravity analysis and, especially, 2D gravity modelling have distinguished the differences in degree of salt saturation in salt-rich bodies, and elucidate the proportion of Rotliegend salt.  相似文献   

17.
The distinctive nature of the structure of the Earth's gravity field over high seismicity areas, as observed in some limited scale studies, indicates that it is possible to use these associative patterns to outline areas of high seismicity or high earthquake hazard potential from a knowledge of the finer structure of the Earth's gravity field. The global scale investigation of such relationships as a function of the geophysical characteristics of various tectonic provinces, and the parameterisation of such relationships by hypocentral depth and earthquake intensity/energy data, become even more attractive because of the availability of (1) satellite-determined gravity models which provide global information on the long wavelength components of the gravity field, (2) satellite altimetry data which provide oceanwide information on the detailed geoidal structure, (3) surface gravity data which provide information on the short wavelength components in the areas of surface gravity coverage, and (4) bathymetric and topographic data which, though still somewhat limited in spite of their recent extensions of coverage, are available in more and more areas and provide information on the tectonic and morphological environments of an area to enable its gravity data conversion to some standard environments for direct comparisons of underlying structures. Additionally, the rapidly mounting geological and geophysical evidence of considerable intraplate tectonic activity, not so fashionable until recently, makes the study of these correlative patterns even more attractive and productive from a scientific viewpoint. This paper presents the major elements of the theoretical formulation for conducting such investigations.  相似文献   

18.
Based on the volume magnetic susceptibility and specific gravity measurements and mineral and lithologic identification results for 540 samples,the rock type,density,and magnetic susceptibility of rocks from northern Borneo were analyzed,and the applicability of gravity and magnetic data to the lithologic identification of the Mesozoic strata in the southern South China Sea was assessed accordingly.The results show that there are 3 types and 25 subtypes of rocks in northern Borneo,mainly intermediate-mafic igneous rocks and exogenous clastic sedimentary rocks,with small amounts of endogenous sedimentary rocks,felsic igneous rocks,and metamorphic rocks.The rocks that are very strongly-strongly magnetic and have high-medium densities are mostly igneous rocks,tuffaceous sandstones,and their metamorphic equivalents.The rocks that are weakly magnetic-non-magnetic and have medium-very low densities are mostly conglomerates,sandstones,siltstones,mudstones,and coal.The rocks that are weakly magnetic-diamagnetic and have highmedium densities are mostly limestones and siliceous rocks.The Cenozoic rocks are characterized by low densities and medium susceptibilities;the Mesozoic rocks are characterized by medium densities and medium-high susceptibilities;and the pre-Mesozoic rocks are characterized by high densities and low magnetism.Based on these results and the distribution characteristics of the various rock types,it was found that the pre-Mesozoic rocks produce weak regional gravity anomalies;the Mesozoic sedimentary rocks produce negative regional gravity anomalies;whereas the Mesozoic igneous rocks produce positive regional gravity anomalies;and the Cenozoic igneous rocks produce positive regional gravity anomalies.The regional high magnetic anomalies in the southern part of the South China Sea originate from the Mesozoic mafic igneous rocks and their metamorphic equivalents;and the regional medium magnetic anomalies may be produced by the felsic igneous rocks and their metamorphic equivalents.Accordingly,the identification of the Mesozoic lithology in the southern South China Sea shows that the Mesozoic sedimentary rocks are distributed over a large area of the southern South China Sea.Thus,it is concluded that the Mesozoic strata in this area have the potential for oil and gas exploration.  相似文献   

19.
This report considers the problem of the propagation of nonlinear surface gravity waves over shallow waters with regard to dispersion. By means of the semigraphical method proposed, the transformation of the profile of nonlinear surface gravity waves is described. The profiles of surface gravity waves are presented for the stages of crest sharpening and front steepening, as well as the wave train disintegration caused by dispersion.  相似文献   

20.
The western Pacific hosts major subduction systems such as Izu–Bonin–Mariana and Tonga–Kermadec, but also less conspicuous systems such as Yap, Mussau and Hjort trenches which constitute the young, incomplete, or ultraslow-member in the evolutionary spectrum of subduction zones. We used satellite-derived gravity data to compare well-developed and immature subduction systems. It is shown that at spatial resolution > 10–20 km or so, the satellite data have accuracy comparable to ship-board gravity measurements over intra-oceanic subduction zones. In the isostatic residual gravity anomaly map, the width of non-isostatically-compensated region of the mature subduction zones is much wider than that of immature ones. More importantly, when the gravitational attraction due to seafloor is removed, a large difference exists between the mature and immature subduction zones in the overriding plate side. Mature subduction zones exhibit broad low gravity anomalies of ~ 200–250 mGal centered at distances of 150–200 km from the trench which are not found over immature subduction zones. The cause of the broad low gravity anomalies over mature subduction zones is debatable due to lack of information on the deep crust and upper mantle structure and property. We discuss the following four causes: (1) serpentinization of the upper mantle beneath the forearc; (2) presence of partial melt in the mantle wedge caused by release of volatiles from the slab, frictional heating and distributed by mantle circulation; (3) difference in density structure between the overriding and subducting plates caused by difference in age and thermal structures with and without compositional stratification between crust and mantle; and (4) anomalous thickness of the arc not explained by isostasy. Our analysis suggests that serpentinization cannot explain the observed gravity anomaly which appears ~ 150–200 km from the trench. Although the extent and distribution of partial melt within the mantle wedge remain in question, to our best estimate, partial melting contributes little (< 50 mGal) to the total negative gravity anomaly. The difference in density structure reflecting temperature difference can only explain less than half of the low gravity anomaly. The sinking of lighter crustal material produces a large negative anomaly in the forearc but its location does not match the observed gravity anomaly. It appears that one cannot explain the total difference in gravity anomaly without invoking anomalous thickness of the arc. Although we could not identify the sole or combination of factors that give rise to the low gravity anomaly in mature subduction zones, the comparison of gravity anomalies between mature and immature subduction zones is likely to provide an important constraint for understanding the evolution and structure of subduction zones as more complementary evidences become available.  相似文献   

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