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Gravity Anomalies (gravity + anomaly)

Distribution by Scientific Domains

Earth and Environmental Science	100%

Selected Abstracts

Iterative generalized cross-validation for fusing heteroscedastic data of inverse ill-posed problems

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2009
Peiliang Xu
SUMMARY The method of generalized cross-validation (GCV) has been widely used to determine the regularization parameter, because the criterion minimizes the average predicted residuals of measured data and depends solely on data. The data-driven advantage is valid only if the variance,covariance matrix of the data can be represented as the product of a given positive definite matrix and a scalar unknown noise variance. In practice, important geophysical inverse ill-posed problems have often been solved by combining different types of data. The stochastic model of measurements in this case contains a number of different unknown variance components. Although the weighting factors, or equivalently the variance components, have been shown to significantly affect joint inversion results of geophysical ill-posed problems, they have been either assumed to be known or empirically chosen. No solid statistical foundation is available yet to correctly determine the weighting factors of different types of data in joint geophysical inversion. We extend the GCV method to accommodate both the regularization parameter and the variance components. The extended version of GCV essentially consists of two steps, one to estimate the variance components by fixing the regularization parameter and the other to determine the regularization parameter by using the GCV method and by fixing the variance components. We simulate two examples: a purely mathematical integral equation of the first kind modified from the first example of Phillips (1962) and a typical geophysical example of downward continuation to recover the gravity anomalies on the surface of the Earth from satellite measurements. Based on the two simulated examples, we extensively compare the iterative GCV method with existing methods, which have shown that the method works well to correctly recover the unknown variance components and determine the regularization parameter. In other words, our method lets data speak for themselves, decide the correct weighting factors of different types of geophysical data, and determine the regularization parameter. In addition, we derive an unbiased estimator of the noise variance by correcting the biases of the regularized residuals. A simplified formula to save the time of computation is also given. The two new estimators of the noise variance are compared with six existing methods through numerical simulations. The simulation results have shown that the two new estimators perform as well as Wahba's estimator for highly ill-posed problems and outperform any existing methods for moderately ill-posed problems. [source]

The high-resolution gravimetric geoid of Iberia: IGG2005

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2005
V. Corchete
SUMMARY It is well known that orthometric heights can be obtained without levelling by using ellipsoidal and geoidal heights. For engineering purposes, these orthometric heights must be determined with high accuracy. For this reason, the determination of a high-resolution geoid is necessary. In Iberia, since the publication of the most recent geoid (IBERGEO95), a new geopotential model has become available (EIGEN-CG01C, released on 2004 October 29) and a new high-resolution digital terrain model (SRTM 90M obtained from the Shuttle Radar Topography Mission) has been developed for the Earth. Logically, these new data represent improvements that must be included in a new geoid of Iberia. With this goal in mind, we have carried out a new gravimetric geoid determination in which these new data are included. The computation of the geoid uses the Stokes integral in convolution form, which has been shown as an efficient method to reach the proposed objective. The terrain correction has been applied to the gridded gravity anomalies to obtain the corresponding reduced anomalies. The indirect effect has also been taken into account. Thus, a new geoid is provided as grid data distributed for Iberia from 35° to 44° latitude and ,10° to 4° longitude (extending to 9°× 14°) in a 361 × 561 regular grid with a mesh size of 1.5,× 1.5, and 202 521 points in the GRS80 reference system. This calculated geoid and previous geoids that exist for this study area (IBERGEO95, EGM96, EGG97 and EIGEN-CG01C) are compared to the geoid undulations corresponding to 16 points of the European Vertical Reference Network (EUVN) on Iberia. The new geoid shows an improvement in precision and reliability, fitting the geoidal heights of these EUVN points with more accuracy than the other previous geoids. [source]

Gravity-enhanced representation of measured geoid undulations using equivalent sources

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2004
Fernando Guspí
SUMMARY Direct Global Positioning System measurement of geoid undulations on accurately levelled stations, usually tens of kilometres apart, can be interpolated by taking advantage of denser surveys of free-air gravity anomalies covering the same area. Using either a spherical or a planar earth model, a two-layer equivalent source is constructed, with the deepest masses located under the geoid stations and the shallower ones under the gravity stations, in such a way that the effect of the masses fits simultaneously, with different precisions, the anomalous potential related to the geoid and its vertical gradient or gravity anomaly. This poses a linear Bayesian problem, whose associated system of equations can be solved directly or by iterative procedures. The ability of the described method to predict the geoid elevation over the gravity stations is assessed in a synthetic example; and in the application to a real case, a gravity-enhanced geoid is mapped for an area of Buenos Aires province, Argentina, where local features are put in evidence. [source]

Lithosphere structure of Europe and Northern Atlantic from regional three-dimensional gravity modelling

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2002
T. P. Yegorova
Summary Large-scale 3D gravity modelling using data averaged on a 1° grid has been performed for the whole European continent and part of the Northern Atlantic. The model consists of two regional layers of variable thickness,the sediments and the crystalline crust, bounded by reliable seismic horizons,the ,seismic' basement and the Moho surface. Inner heterogeneity of the model layers was taken into account in the form of lateral variation of average density depending on the type of geotectonic unit. Density parametrization of the layers was made using correlation functions between velocity and density. For sediments, sediment consolidation with depth was taken into account. Offshore a sea water layer was included in the model. As a result of the modelling, gravity effects of the whole model and its layers were calculated. Along with the gravity modelling an estimation of isostatic equilibrium state has been carried out for the whole model as well as for its separate units. Residual gravity anomalies, obtained by subtracting the gravity effect of the crust from the observed field, reach some hundred mGal (10,5 m s,2) in amplitude; they are mainly caused by density heterogeneities in the upper mantle. A mantle origin of the residual anomalies is substantiated by their correlation with the upper-mantle heterogeneities revealed by both seismological and geothermal studies. Regarding the character of the mantle gravity anomalies, type of isostatic compensation, crustal structure, age and supposed type of endogenic regime, a classification of main geotectonic units of the continent was made. As a result of the modelling a clear division of the continent into two large blocks,Precambrian East-European platform (EEP) and Variscan Western Europe,has been confirmed by their specific mantle gravity anomalies (0 ÷ 50 × 10,5 m s,2 and ,100 ÷,150 × 10,5 m s,2 correspondingly). This division coincides with the Tornquist,Teisseyre Zone (TTZ), marked by a gradient zone of mantle anomalies. In the central part of the EEP (over the Russian plate) an extensive positive mantle anomaly, probably indicating a core of ancient consolidation of the EEP, has been distinguished. To the west and to the east of this anomaly positive mantle anomalies occur, which coincide with a deep suture zone (TTZ) and an orogenic belt (the Urals). Positive mantle anomalies of the Alps, the Adriatic plate and the Calabrian Arc, correlating well with both high-velocity domains in the upper mantle and reduced temperatures at the subcrustal layer, are caused by thickened lithosphere below these structures. Negative mantle anomalies, revealed in the Western Mediterranean Basin and in the Pannonian Basin, are the result of thermal expansion of the asthenosphere shallowing to near-Moho depths below these basins. [source]

Three-dimensional VP and VP,/VS models of the upper crust in the Friuli area (northeastern Italy)

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2000
G. F. Gentile
3-D images of P velocity and P - to S -velocity ratio have been produced for the upper crust of the Friuli area (northeastern Italy) using local earthquake tomography. The data consist of 2565 P and 930 S arrival times of high quality. The best-fitting VP and VP,/VS 1-D models were computed before the 3-D inversion. VP was measured on two rock samples representative of the investigated upper layers of the Friuli crust. The tomographic VP model was used for modelling the gravity anomalies, by converting the velocity values into densities along three vertical cross-sections. The computed gravity anomalies were optimized with respect to the observed gravity anomalies. The crust investigated is characterized by sharp lateral and deep VP and VP,/VS anomalies that are associated with the complex geological structure. High VP,/VS values are associated with highly fractured zones related to the main faulting pattern. The relocated seismicity is generally associated with sharp variations in the VP,/VS anomalies. The VP images show a high-velocity body below 6 km depth in the central part of the Friuli area, marked also by strong VP,/VS heterogeneities, and this is interpreted as a tectonic wedge. Comparison with the distribution of earthquakes supports the hypothesis that the tectonic wedge controls most of the seismicity and can be considered to be the main seismogenic zone in the Friuli area. [source]

Short note: Source geometry identification by simultaneous use of structural index and shape factor

GEOPHYSICAL PROSPECTING, Issue 1 2001
Lopamudra Roy
A cross-plot of the shape factors and the structural indices, determined from gravity anomalies over various idealized sources, namely horizontal/vertical lines and vertical ribbons with various strike lengths and depth extents, forms a closed loop. Different segments of this loop, termed the source geometry identification loop (SGIL), correspond to different source geometries. Combined use of the structural index and the shape factor determined from an isolated gravity anomaly reduces the ambiguity in characterizing the source geometry. A simulated example and three field examples, namely a Cuban chromite anomaly, an Indian example over manganese ore and a sulphide ore from Quebec, have been analysed by the proposed method in order to identify their respective source geometries. [source]

Uplift at lithospheric swells,I: seismic and gravity constraints on the crust and uppermost mantle structure of the Cape Verde mid-plate swell

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2010
D. J. Wilson
SUMMARY Wide-angle seismic data have been used to determine the velocity and density structure of the crust and uppermost mantle beneath the Cape Verdes mid-plate swell. Seismic modelling reveals a ,standard' oceanic crust, ,8 km in thickness, with no direct evidence for low-density bodies at the base of the crust. Gravity anomaly modelling within the constraints and resolution provided by the seismic model, does not preclude, however, a layer of crustal underplate up to 3 km thick beneath the swell crest. The modelling shows that while the seismically constrained crustal structure accounts for the short-wavelength free-air gravity anomaly, it fails to fully explain the long-wavelength anomaly. The main discrepancy is over the swell crest where the gravity anomaly, after correcting for crustal structure, is higher by about 30 mGal than it is over its flanks. The higher gravity can be explained if the top 100 km of the mantle beneath the swell crest is less dense than its surroundings by 30 kg m,3. The lack of evidence for low densities and velocities in the uppermost mantle, and high densities and velocities in the lower crust, suggests that neither a depleted swell root or crustal underplate are the origin of the observed shallower-than-predicted bathymetry and that, instead, the swell is most likely supported by dynamic uplift associated with an anomalously low density asthenospheric mantle. [source]

Gravity-enhanced representation of measured geoid undulations using equivalent sources

Gravity evidence for a larger Limpopo Belt in southern Africa and geodynamic implications

GEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2002
R. T. Ranganai
Summary The Limpopo Belt of southern Africa is a Neoarchean orogenic belt located between two older Archean provinces, the Zimbabwe craton to the north and the Kaapvaal craton to the south. Previous studies considered the Limpopo Belt to be a linearly trending east-northeast belt with a width of ,250 km and ,600 km long. We provide evidence from gravity data constrained by seismic and geochronologic data suggesting that the Limpopo Belt is much larger than previously assumed and includes the Shashe Belt in Botswana, thus defining a southward convex orogenic arc sandwiched between the two cratons. The 2 Ga Magondi orogenic belt truncates the Limpopo,Shahse Belt to the west. The northern marginal, central and southern marginal tectonic zones define a single gravity anomaly on upward continued maps, indicating that they had the same exhumation history. This interpretation requires a tectonic model involving convergence between the Kaapvaal and Zimbabwe cratons during a Neoarchean orogeny that preserved the thick cratonic keel that has been imaged in tomographic models. [source]