Change search
Refine search result
1234567 101 - 150 of 495
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 101.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Elastic thickness determination based on Vening Meinesz-Moritz and flexural theories of isostasy2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 3, p. 1682-1692Article in journal (Refereed)
    Abstract [en]

    Elastic thickness (Te) is one of mechanical properties of the Earth's lithosphere. The lithosphere is assumed to be a thin elastic shell, which is bended under the topographic, bathymetric and sediment loads on. The flexure of this elastic shell depends on its thickness or Te. Those shells having larger Te flex less. In this paper, a forward computational method is presented based on the Vening Meinesz–Moritz (VMM) and flexural theories of isostasy. Two Moho flexure models are determined using these theories, considering effects of surface and subsurface loads. Different values are selected for Te in the flexural method to see by which one, the closest Moho flexure to that of the VMM is achieved. The effects of topographic/bathymetric, sediments and crustal crystalline masses, and laterally variable upper mantle density, Young's modulus and Poisson's ratio are considered in whole computational process. Our mathematical derivations are based on spherical harmonics, which can be used to estimate Te at any single point, meaning that there is no edge effect in the method. However, the Te map needs to be filtered to remove noise at some points. A median filter with a window size of 5° × 5° and overlap of 4° works well for this purpose. The method is applied to estimate Te over South America using the data of CRUST1.0 and a global gravity model.

  • 102.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    From satellite gradiometry data to the sub-crustal stress due to the mantle convection2014In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 171, no 9, p. 2391-2406Article in journal (Refereed)
    Abstract [en]

    Subcrustal stress induced by mantle convection can be determined by the Earth's gravitational potential. In this study, the spherical harmonic expansion of the simplified Navier–Stokes equation is developed further so satellite gradiometry data (SGD) can be used to determine the subcrustal stress. To do so, we present two methods for producing the stress components or an equivalent function thereof, the so-called S function, from which the stress components can be computed numerically. First, some integral estimators are presented to integrate the SGD and deliver the stress components and/or the S function. Second, integral equations are constructed for inversion of the SGD to the aforementioned quantities. The kernel functions of the integrals of both approaches are plotted and interpreted. The behaviour of the integral kernels is dependent on the signal and noise spectra in the first approach whilst it does not depend on extra information in the second method. It is shown that recovering the stress from the vertical–vertical gradients, using the integral estimators presented, is suitable, but when using the integral equations the vertical–vertical gradients are recommended for recovering the S function and the vertical–horizontal gradients for the stress components. This study is theoretical and numerical results using synthetic or real data are not given.

  • 103.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    From tensor to vector of gravitation2014In: Artificial Satellites, ISSN 2083-6104, Vol. 49, no 2, p. 63-80Article in journal (Refereed)
    Abstract [en]

    Different gravitational force models are used for determining the satellites’ orbits. The satellite gravity gradiometry (SGG) data contain this gravitational information and the satellite accelerations can be determined from them. In this study, we present that amongst the elements of the gravitational tensor in the local north-oriented frame, all of the elements are suitable for this purpose except Txy. Three integral formulae with the same kernel function are presented for recovering the accelerations from the SGG data. The kernel of these integrals is well-behaving which means that the contribution of the far-zone data is not very significant to their integration results; but this contribution is also dependent on the type of the data being integrated. Our numerical studies show that the standard deviations of the differences between the accelerations recovered from Tzz, Txz and Tyz  and those computed by an existing Earth´s gravity model reduce by increasing the cap size of integration. However, their root mean squared errors increase for recovering Tyfrom Tyz.  Larger cap sizes than 5   is recommended for recovering Tx and Tz  but smaller ones for Ty.

  • 104.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Integral Approaches to Determine Sub-Crustal Stress from Terrestrial Gravimetric Data2016In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 173, no 3, p. 805-825Article in journal (Refereed)
    Abstract [en]

    The spherical harmonic expressions of the horizontal sub-crustal stress components induced by the mantle convection are convergent only to low degrees. In this paper, we use the method of stress (S) function with numerical differentiation and present a formula for determining the degree of convergence from the mean Moho depth. We found that for the global mean Moho depth, 23 km, this convergence degree is 622 and for Iran, 35 km, it is 372. Also, three methods are developed and applied for computing the sub-crustal stress, (1) direct integration with a spectral kernel limited up to the degree of convergence, (2) integral inversion with a kernel having closed-form formula without any frequency limit, and (3) solving an integral equation with limited spectral kernel to the convergence degree. The second method has no divergence problem and its kernel function is well behaving so that the system of equations from which the S function is determined is stable, and no regularisation is needed to solve it. It should be noted that for using this method the resolution of the recovery should be higher than 0.5° × 0.5°, otherwise the recovered S function and correspondingly the stress components will have smaller magnitude than those derived from the other two methods. Our numerical studies for stress recovery in Iran and its surrounding areas show that the methods, which use the limited spectral kernels to the convergence degree, deliver consistent results to that of the spherical harmonic expansion.

  • 105.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Integral developments of Vening Meinesz-Moritz formula for local determination of Moho discontinuity and their applications in Iran2014In: Geodynamics Research International Bulletin, E-ISSN 2345-4997, Vol. 2, no 3, p. I-IXArticle in journal (Refereed)
    Abstract [en]

    Global models of the Earth gravity field and topographic/bathymetric data can be used for the gravimetric determination of the Moho discontinuity based on the Vening Meinesz-Moritz theory. In this paper, we mathematically develop this method in such a way that the local data can be used for Moho modelling. Two integral formulae are presented, one for integrating the data and one for their inversion. The kernels of both integrals are well-behaving meaning that the contribution of far-zone quantities being integrated are not very significant in the results. Both of these methods are applied for computing the Moho model of Iran and their results are compared to the Moho model determined based on the global models. Consistency of the computed Moho models from the simulated data and the global models verifies the correctness of both approaches. The presented methods are consistent even for the case of using real data. Numerical results show that the minimum value of the Moho models derived by the simulated data and global models are about 31 km, whilst those derived from the real data are about 3 km smaller. Similarly, the mean value of Moho depths derived from real data is about 1 km smaller than that from the global models.

  • 106.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Local recovery of lithospheric stress tensor from GOCE gravitational tensor2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 209, no 1, p. 317-333Article in journal (Refereed)
    Abstract [en]

    The sub-lithospheric stress due to mantle convection can be computed from gravity data and propagated through the lithosphere by solving the boundary-value problem of elasticity for the Earth's lithosphere. In this case, a full tensor of stress can be computed at any point inside this elastic layer. Here, we present mathematical foundations for recovering such a tensor from gravitational tensor measured at satellite altitudes. The mathematical relations will be much simpler in this way than the case of using gravity data as no derivative of spherical harmonics or Legendre polynomials is involved in the expressions. Here, new relations between the spherical harmonic coefficients of the stress and gravitational tensor elements are presented. Thereafter integral equations are established from them to recover the elements of stress tensor from those of the gravitational tensor. The integrals have no closed-form kernels, but they are easy to invert and their spatial truncation errors are reducible. The integral equations are used to invert the real data of the gravity field and steady-state ocean circulation explorer (GOCE) mission, in November 2009, over the South American plate and its surroundings to recover the stress tensor at a depth of 35 km. The recovered stress fields are in good agreement with the tectonic and geological features of the area.

  • 107.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Non-singular expressions for vector and gradient tensor of gravitation in a geocentric spherical frame2008In: Computers & Geosciences, ISSN 0098-3004, E-ISSN 1873-7803, Vol. 34, no 12, p. 1762-1768Article in journal (Refereed)
    Abstract [en]

    The traditional expressions of the gravitational vector (GV) and the gravitational gradient tensor (GGT) have complicated forms depending on the first- and the second-order derivatives of associated Legendre functions (ALF), and also singular terms when approaching the poles. This article presents alternative expressions for the GV and GGT, which are independent of the derivatives, and are also non-singular. By using such expressions, it suffices to compute the ALF to two additional degrees and orders, instead of computing the first and the second derivatives of all the ALF. Therefore, the formulas are suitable for computer programming. Matlab software as well as an output of a numerical computation around the North Pole is also presented based on the derived formulas.

  • 108.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering. University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Numerical aspects of EGM08-based geoid computations in Fennoscandia regarding the applied reference surface and error propagation2013In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 96, p. 28-32Article in journal (Refereed)
    Abstract [en]

    So far the recent Earth's gravity model, EGM08, has been successfully applied for different geophysical and geodetic purposes. In this paper, we show that the computation of geoid and gravity anomaly on the reference ellipsoid is of essential importance but error propagation of EGM08 on this surface is not successful due to downward continuation of the errors. Also we illustrate that some artefacts appear in the computed geoid and gravity anomaly to lower degree and order than 2190. This means that the role of higher degree harmonics than 2160 is to remove these artefacts from the results. Consequently, EGM08 must be always used to degree and order 2190 to avoid the numerical problems. © 2013 Elsevier B.V.

  • 109.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    On the approximations in formulation of the Vening Meinesz-Moritz theory of isostasy2017In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 210, no 1, p. 500-508Article in journal (Refereed)
    Abstract [en]

    Different approximations are used in Moho modelling based on isostatic theories. The well-known approximation is considering a plate shell model for isostatic equilibrium, which is an oversimplified assumption for the Earth’s crust. Considering a spherical shellmodel, as used in the Vening Meinesz-Moritz (VMM) theory, is a more realistic assumption, but it suffers from different types of mathematical approximations. In this paper, the idea is to investigate such approximations and present their magnitudes and locations all over the globe. Furthermore, we show that the mathematical model of Moho depth according to the VMM principle can be simplified to that of the plate shell model after four approximations. Linearisation of the binomial term involving the topographic/bathymetric heights is sufficient as long as their spherical harmonic expansion is limited to degree and order 180. The impact of the higher order terms is less than 2 km. The Taylor expansion of the binomial term involving the Moho depth (T) up to second order with the assumption of T-2 = TT0, T-0 is the mean compensation depth, improves this approximation further by up to 4 km over continents. This approximation has a significant role in Moho modelling over continents; otherwise, loss of frequency occurs in the Moho solution. On the other hand, the linear approximation performs better over oceans and considering higher order terms creates unrealistic frequencies reaching to a magnitude of 5 km in the Moho solution. Involving gravity data according to the VMM principle influences the Moho depth significantly up to 15 km in some areas.

  • 110.
    Eshagh, Mehdi
    Royal Institute of Technology Division of Geodesy .
    On the estimation of variance in unstable condition adjustment models2011In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, ISSN 1217-8977, Vol. 46, no 1, p. 71-83Article in journal (Refereed)
    Abstract [en]

    Estimation of variance in an ordinary adjustment model is straightforward, but if the model becomes unstable or ill-conditioned its solution and the variance of the solution will be very sensitive to the errors of observations. This sensitivity can be controlled by stabilizing methods but the results will be distorted due to stabilization. In this paper, stabilizing an unstable condition model using Tikhonov regularization, the estimations of variance of unit weight and variance components are investigated. It will be theoretically proved that the estimator of variance or variance components has not the minimum variance property when the model is stabilized, but unbiased estimation of variance is possible. A simple numerical example is provided to show the performance of the theory.

    |

  • 111.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    On the relation between Moho and sub-crustal stress induced by mantle convection2015In: Journal of Geophysics and Engineering, ISSN 1742-2132, E-ISSN 1742-2140, Vol. 12, no 1, p. 1-11Article in journal (Refereed)
    Abstract [en]

    The sub-crustal stress components due to mantle convection have a direct relation with the spherical harmonic coefficients of the Earth's disturbing potential like those of the Moho model, developed by the Vening–Meinesz–Moritz theory. In this paper, the relation between the stress components and the global and local models of Moho is mathematically developed in three different ways. Here, we present the S function (S) with a numerical differentiation approach to generate the stress components and we show that its spherical harmonic series is convergent to a degree of about 600 based on a mean global Moho depth of 23 km. An integral approach is developed for integration of a local Moho model for the stress recovery, but the kernels of this integral are not likely to be convergent and should be generated by their spectral forms to a limited degree. Another method is developed based on integral inversion, which is free of any mathematical problem and suitable for recovering S from an existing model of Moho. Our numerical presentation shows that the stress has a good agreement with the tectonic boundaries and the places at which the curvature of the Moho surface changes.

  • 112.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    On the reliability and error calibration of some recent Earth's gravity models of GOCE with respect to EGM082013In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 48, no 2, p. 199-208Article in journal (Refereed)
    Abstract [en]

    The Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission is dedicated to recover spherical harmonic coefficients of the Earth's gravity field to degree and order of about 250 using its satellite gradiometric data. Since these data are contaminated with coloured noise, therefore, their inversion will not be straightforward. Unsuccessful modelling of this noise will lead to biases in the harmonic coefficients presented in the Earth's gravity models (EGMs). In this study, five of the recent EGMs of GOCE such as two direct, two time-wise and one space-wise solution are used to degree and order 240 and their reliability is investigated with respect to EGM08 which is assumed as a reliable EGM. The detected unreliable coefficients and their errors are replaced by the corresponding ones from EGM08 as a combination strategy. A condition adjustment model is organised for each two corresponding coefficients of GOCE EGMs and EGM08; and errors of the GOCE EGMs are calibrated based on a scaling factor, obtained from a posteriori variance factor. When the factor is less than 2.5 it will be multiplied to the error otherwise the error of EGM08 coefficient will be considered as the calibrated one. At the end, a simple geoid estimator is presented which considers the EGMs and their errors and its outcomes are compared with the corresponding geoid heights derived from the Global Positioning System (GPS) and the levelling data (GPS/levelling data), over Fennoscandia. This comparison shows that some of the combined-calibrated GOCE EGMs are closer to the GPS/levelling data than the original ones.

  • 113.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    On Vening Meinesz-Moritz and flexural theories of isostasy and their comparison over Tibet Plateau2016In: Journal of Geodetic Science, ISSN 2081-9919, E-ISSN 2081-9943, Vol. 6, p. 139-151Article in journal (Refereed)
    Abstract [en]

    Gravity and topographic/bathymetric data are used for gravimetric modelling of Moho discontinuity by hydrostatic or flexural theories of the isostasy. Here, two hydrostatic models, based on the Vening Meinesz-Moritz (VMM) principle, and two based on the loading theories and flexural isostasy are compared over Tibet Plateau. It is shown that the Moho models generated based on the VMM theory and flexural isostasy have very good agreements if the mean compensation depth and the mean elastic thickness are selected properly. However, the model computed based on the flexural isostasy is smoother. A more rigorous flexural model, which considers the membrane stress and curvature of the lithosphere, is used to model the Moho surface over the study area. It is shown that the difference between the Moho models, derived by considering and ignoring these parameters, is not significant. By combination of the flexural and VMM hydrostatic models new mathematical formulae for crustal gravity anomalies are provided and it is shown that the crustal gravity anomalies produced by them are also equivalent.

  • 114.
    Eshagh, Mehdi
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering. Department of Geodesy, K.N.Toosi University of Technology, Tehran.
    Spectral combination of spherical gradiometric boundary-value problems: a theoretical study2012In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 169, p. 2201-2215Article in journal (Refereed)
    Abstract [en]

    The Earth’s gravity potential can be determined from its second-order partial derivatives using the spherical gradiometric boundary-value problems which have three integral solutions. The problem of merging these solutions by spectral combination is the main subject of this paper. Integral estimators of biased- and unbiased-types are presented for recovering the disturbing gravity potential from gravity gradients. It is shown that only kernels of the biased-type integral estimators are suitable for simultaneous downward continuation and combination of gravity gradients. Numerical results show insignificant practical difference between the biased and unbiased estimators at sea level and the contribution of far-zone gravity gradients remains significant for integration. These contributions depend on the noise level of the gravity gradients at higher levels than sea. In the cases of combining the gravity gradients, contaminated with Gaussian noise, at sea and 250 km levels the errors of the estimated geoid heights are about 10 and 3 times smaller than those obtained by each integral

  • 115.
    Eshagh, Mehdi
    * Islamic Azad University, Shahre-Rey Branch, Tehran, Iran .
    Step-variable numerical orbit determination of a low earth Orbiting Satellite2005In: Jounal of the Earth and Space Physics, Vol. 31, no 1, p. 1-12Article in journal (Refereed)
  • 116.
    Eshagh, Mehdi
    Royal Institute of Technology (KTH), Division of Geodesy and Geoinformatics.
    The effect of spatial truncation error on integral inversion of satellite gravity gradiometry data2011In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, ISSN 0273-1177, Vol. 47, no 7, p. 1238-1247Article in journal (Refereed)
    Abstract [en]

    The satellite gravity gradiometry (SGG) data can be used for local modelling of the Earth's gravity field. In this study, the SGG data in the local north-oriented and orbital frames are inverted to the gravity anomaly at sea level using the second-order partial derivatives of the extended Stokes formula. The emphasis is on the spatial truncation error and the kernel behaviour of the integral formulas in the aforementioned frames. The paper will show that only the diagonal elements of gravitational tensor at satellite level are suitable for recovering the gravity anomaly at sea level. Numerical studies show that the gravity anomaly can be recovered in Fennoscandia with an accuracy of about 6 mGal directly from on-orbit SGG data.

  • 117.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Ashargie, Andenet
    Bedada, Tulu B.
    Regional recovery of gravity anomaly from the inversion of diagonal components of GOCE gravitational tensor: A Case Study in Ethiopia, Artificial Satellites2018In: Artificial Satellites : he Journal of Space Research Centre of Polish Academy of Sciences, E-ISSN 2083-6104, Vol. 53, no 2, p. 55-74Article in journal (Refereed)
    Abstract [en]

    The tensor of gravitation is traceless as the gravitational field of the Earth is harmonic outside the Earth's surface. Therefore, summation of the 2nd-order horizontal derivatives on its diagonal components should be equal to the radial one but with the opposite sign. The gravity field can be recovered locally from either of them, or even their combination. Here, we use the in-orbit diagonal components of the gravitational tensor measured by the gravity field and steady stateocean circulation explorer (GOCE) mission for recovering gravity anomaly with a resolution of 1°×1° at sea level in Ethiopia. In order to solve the system of equations, derived after discretisation of integral equations, the Tikhonov regularisation is applied and the bias of thi sregularisation is estimated and removed from the estimated gravity anomalies. The errors of the anomalies are estimated and their significance of recovery from these diagonal components is investigated. Statistically, the difference between the recovered anomalies from each scenario isnot significant comparing to their errors. However, their joint inversion of the diagonal components improved the solution by about 1 mGal. Furthermore, the inversion processes arebetter stabilised when using errors of the input data compared with its exclusion, but at the penalty of degradation in accuracy of the estimates.

  • 118.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Divison of Natural Sciences, Surveying and Mechanical Engineering.
    Bagherbandi, Mohammad
    University of Gävle, Department of Industrial Development.
    Combined Moho Estimators2014In: Geodynamics Research International Bulletin, E-ISSN 2345-4997, Vol. 1, no 3, p. 1-11Article in journal (Other academic)
    Abstract [en]

    In this study, we develop three estimators to optimally combine seismic and gravimetric models of Moho surface. The first estimator combines them by their special harmonic coefficients; the second one uses the spherical harmonic coefficients of the seismic model and use integral formula for the gravimetric one. The kernel of the integral terms of this estimator shows that a cap size of 20 is required for the integration, but since this integral is presented to combine the low frequencies of the gravimetric model, a low resolution model is enough for the integration. The third estimator uses the gravity anomaly and converts its low frequencies to those of the gravimetric Moho model, meanwhile combining them with those of seismic one. This integral requires an integration domain of 30 for the gravity anomalies but since the maximum degree of this kernel is limited to a specific degree, the use of its spectral form is recommended. The kernel of the integral involving the gravity anomalies, developed for recovering high frequencies of Moho, is written in a closed-from formula and its singularity is investigated. This kernel is well-behaving and decreases fast, meaning that it is suitable for recovering the high frequencies of Moho surface.

  • 119.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering. K N Toosi University of Technology, Department of Geodesy, Tehran, Iran.
    Bagherbandi, Mohammad
    University of Gävle, Department of Industrial Development, IT and Land Management .
    Quality description for gravimetric and seismic Moho models of Fennoscandia through a combined adjustment2012In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 47, no 4, p. 388-401Article in journal (Refereed)
    Abstract [en]

    The gravimetric model of the Moho discontinuity is usually derived based on isostatic adjustment theories considering floating crust on the viscous mantle. In computation of such a model some a priori information about the density contrast between the crust and mantle and the mean Moho depth are required. Due to our poor knowledge about them they are assumed unrealistically constant. In this paper, our idea is to improve a computed gravimetric Moho model, by the Vening Meinesz-Moritz theory, using the seismic model in Fennoscandia and estimate the error of each model through a combined adjustment with variance component estimation process. Corrective surfaces of bi-linear, bi-quadratic, bi-cubic and multi-quadric radial based function are used to model the discrepancies between the models and estimating the errors of the models. Numerical studies show that in the case of using the bi-linear surface negative variance components were come out, the bi-quadratic can model the difference better and delivers errors of 2.7 km and 1.5 km for the gravimetric and seismic models, respectively. These errors are 2.1 km and 1.6 km in the case of using the bi-cubic surface and 1 km and 1.5 km when the multi-quadric radial base function is used. The combined gravimetric models will be computed based on the estimated errors and each corrective surface.

  • 120.
    Eshagh, Mehdi
    et al.
    Islamic Azad University, Shahr-e-Rey branch, Tehran.
    Bagherbandi, Mohammad
    Royal Institute of Technology (KTH), Stockholm.
    Smoothing impact of isostatic crustal thickness models on local integral inversion of satellite gravity gradiometry data,2011In: Acta Geophysica, ISSN 1895-7455, Vol. 59, no 5, p. 891-906Article in journal (Refereed)
    Abstract [en]

    The effects of topographic masses on satellite gradiometric data are

    large and in order to reduce the magnitude of these effects some compensation

    mechanisms should be considered. Herewe use the isostatic hypotheses

    of Airy–Heiskanen and the recent Vening Meinesz–Moritz for compensating

    these effects and to smooth the data prior to their downward continuation

    to gravity anomaly. The second-order partial derivatives of extended

    Stokes’ formula are used for the continuations over a topographically rough

    territory like Persia. The inversions are performed and compared based on

    two schemes of the remove-compute-restore technique and direct downward

    continuation. Numerical results show that the topographic-isostatic effect

    based onVening Meinesz–Mortiz’s hypothesis smoothes the data better than

    that based on Airy–Heiskanen’s hypothesis. Also the quality of inversions

    of the smoothed data by this mechanism is twice better than that of the nonsmoothed

    ones.

  • 121.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering. University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Ebadi, Sahar
    K.N.Toosi University of Technology, Tehran, Iran.
    A strategy to calibrate errors of Earth gravity models2014In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 103, no April, p. 215-220Article in journal (Refereed)
    Abstract [en]

    In this paper, three independent Earth gravity models (EGMs) ofGO_CONS_GCF_2_TIM_R4, AIUB-GRACE03S and ULux_CHAMP2013s are combined to degree and order 120. The geoid models of these EGMs are computed and compared with the Global Positioning System (GPS) and levelling data over Fennoscandia. We found that the simple mean of these geoid models is closer to the GPS/levelling data than their weighted mean. This means that errors of the EGMs are not properly estimated as they are used in the weighted mean solution. We develop a method based on solving a nonlinear condition adjustment model to calibrate the errors so that the result of weighted mean becomes the same as that of the simple mean. Numerical results show slight changes in the errors of GRACE03S but large ones in those of GO_CONS_GCF_2_TIM_R4 and ULux_CHAMP2013s. Furthermore, the weighted mean solution considering the calibrated errors and some additional constraints is better than GOCO03S to degree and order 120 over Fennoscandia.

  • 122.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Ebadi, Sahar
    Department of Geodesy, K.N.Toosi University of Technology.
    Geoid modelling based on EGM08 and the recent Earth gravity models of GOCE2013In: Earth Science Informatics, ISSN 1865-0473, Vol. 6, no 3, p. 113-125Article in journal (Refereed)
  • 123.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Ebadi, Sahar
    University of Tehran, Department of Surveying Engineering, Iran.
    Tenzer, Robert
    University of West Bohemia, New Technologies for the Information Society (NTIS), Czech Republic.
    Isostatic GOCE Moho model for Iran2017In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 138, p. 12-24Article in journal (Refereed)
    Abstract [en]

    One of the major issues associated with a regional Moho recovery from the gravity or gravity-gradient data is the optimal choice of the mean compensation depth (i.e., the mean Moho depth) for a certain area of study, typically for orogens characterised by large Moho depth variations. In case of selecting a small value of the mean compensation depth, the pattern of deep Moho structure might not be reproduced realistically. Moreover, the definition of the mean compensation depth in existing isostatic models affects only low-degrees of the Moho spectrum. To overcome this problem, in this study we reformulate the Sjöberg and Jeffrey’s methods of solving the Vening-Meinesz isostatic problem so that the mean compensation depth contributes to the whole Moho spectrum. Both solutions are then defined for the vertical gravity gradient, allowing estimating the Moho depth from the GOCE satellite gravity-gradiometry data. Moreover, gravimetric solutions provide realistic results only when a priori information on the crust and upper mantle structure is known (usually from seismic surveys) with a relatively good accuracy. To investigate this aspect, we formulate our gravimetric solutions for a variable Moho density contrast to account for variable density of the uppermost mantle below the Moho interface, while taking into consideration also density variations within the sediments and consolidated crust down to the Moho interface. The developed theoretical models are applied to estimate the Moho depth from GOCE data at the regional study area of the Iranian tectonic block, including also parts of surrounding tectonic features. Our results indicate that the regional Moho depth differences between Sjöberg and Jeffrey’s solutions, reaching up to about 3 km, are caused by a smoothing effect of Sjöberg’s method. The validation of our results further shows a relatively good agreement with regional seismic studies over most of the continental crust, but large discrepancies are detected under the Oman Sea and the Makran subduction zone. We explain these discrepancies by a low quality of seismic data offshore.

  • 124.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Ghorbannia, Morteza
    K.N. Toosi University of Technology, Department of Geodesy, Tehran.
    The effect of spatial truncation error on the variance of gravity anomalies derived from inversion of satellite orbital and gradiometric data2014In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 54, no 2, p. 261-271Article in journal (Refereed)
    Abstract [en]

    The spatial truncation error (STE) is a significant systematic error in the integral inversion of satellite gradiometric and orbital data to gravity anomalies at sea level. In order to reduce the effect of STE, a larger area than the desired one is considered in the inversion process, but the anomalies located in its central part are selected as the final results. The STE influences the variance of the results as well because the residual vector, which is contaminated with STE, is used for its estimation. The situation is even more complicated in variance component estimation because of its iterative nature. In this paper, we present a strategy to reduce the effect of STE on the a posteriori variance factor and the variance components for inversion of satellite orbital and gradiometric data to gravity anomalies at sea level. The idea is to define two windowing matrices for reducing this error from the estimated residuals and anomalies. Our simulation studies over Fennoscandia show that the differences between the 0.5°×0.5°0.5°×0.5° gravity anomalies obtained from orbital data and an existing gravity model have standard deviation (STD) and root mean squared error (RMSE) of 10.9 and 12.1 mGal, respectively, and those obtained from gradiometric data have 7.9 and 10.1 in the same units. In the case that they are combined using windowed variance components the STD and RMSE become 6.1 and 8.4 mGal. Also, the mean value of the estimated RMSE after using the windowed variances is in agreement with the RMSE of the differences between the estimated anomalies and those obtained from the gravity model.

  • 125.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Ghorbannia, Morteza
    Department of Geodesy, K.N.Toosi University of Technology.
    The use of Gaussian equations of motions of a satellite for local gravity anomaly recovery2013In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 52, no 1, p. 30-38Article in journal (Refereed)
    Abstract [en]

    The orbital elements of a low Earth orbiting satellite and their velocities can be used for local determination of gravity anomaly. The important issue is to find direct relations among the anomalies and these parameters. Here, a primary theoretical study is presented for this purpose. The Gaussian equations of motion of a satellite are used to develop integral formulas for recovering the gravity anomalies. The behaviour of kernels of the integrals are investigated for a two-month simulated orbit similar to that of the Gravity field and steady-state ocean circulation explorer (GOCE) mission over Fennoscandia. Numerical investigations show that the integral formulas have neither isotropic nor well-behaved kernels. In such a case, gravity anomaly recovery is not successful due to large spatial truncation error of the integral formulas. Reformulation of the problem by combining the orbital elements and their velocities leads to an integral with a well-behaved kernel which is suitable for our purpose. Also based on these combinations some general relations among the orbital elements and their velocities are obtained which can be used for validation of orbital parameters and their velocities

  • 126.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Hussain, Matloob
    University West, Department of Engineering Science. Earth Sciences department, Quaid-i-Azam University, Islamabad, Pakistan.
    Relationship amongst gravity gradients, deflection of vertical, Moho deflection and the stresses derived by mantle convections: a case study over Indo-Pak and surroundings2015In: Geodynamics research international bulletin, E-ISSN 2345-4997, Vol. 3, no 4, p. I-XIII, article id 12Article in journal (Refereed)
    Abstract [en]

    All  quantities,  which are  measured  in  the  gravity  field  of  the  Earth are affected by  the  field,  therefore,  there should  be correlations  amongst  them.  Here,  we  focus  on  some  gravimetrically-determined  quantities  like  deflections  of  vertical, deflections of Moho, vertical-horizontal gravity gradients and the shear sub-lithospheric stress components due to mantle convection. We show that how these quantities are related to each other mathematically so that one of them can be written in term of another. This somehow proves the presence of the mentioned correlations theoretically. Also, we generate the maps of these quantities over the Indo-Pak and surrounding areas and show how similar they are. Thereafter, they are explained and  interpreted  geologically.  Our  investigations  show  that  the  maps  of  these  quantities  are  in  good  agreements  with topographic and geological features. The map of the vertical-horizontal gravity gradients shows more detailed information of the  gravity field due to  signal amplification at high  degrees,  that of Moho deflection shows sub-surface features due to reduction of the effect of topographic masses. The map of the shear sub-lithospheric stress components is much smoother than the gradients, as expected, and has good agreement with the collisional and subduction zones as well. 

    Relationship amongst gravity gradients, deflection of vertical, Moho deflection and the stresses derived by mantle convections-a case study over Indo-Pak and surroundings. Available from: https://www.researchgate.net/publication/292538682_Relationship_amongst_gravity_gradients_deflection_of_vertical_Moho_deflection_and_the_stresses_derived_by_mantle_convections-a_case_study_over_Indo-Pak_and_surroundings [accessed Feb 1, 2016].

  • 127.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Hussain, Matloob
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering. Quaid-i-Azam University, Department of Earth Sciences, Islamabad 45320, Pakistan.
    Tenzer, Robert
    Wuhan University, The Key Laboratory of Geospace Environment and Geodesy, Wuhan 430079, China.
    Romeshkani, Mohsen
    University of Tehran, School of Surveying and Geospatial Engineering, College of Engineering, Tehran 14395-515, Iran.
    Moho density contrast in central Eurasia from GOCE gravity gradients2016In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 8, no 5, p. 1-18, article id 418Article in journal (Refereed)
    Abstract [en]

    Seismic data are primarily used in studies of the Earth's inner structure. Since large partsof the world are not yet sufficiently covered by seismic surveys, products from the Earth's satellite observation systems have more often been used for this purpose in recent years. In this study we use the gravity-gradient data derived from the Gravity field and steady-state Ocean Circulation Explorer (GOCE), the elevation data from the Shuttle Radar Topography Mission (SRTM) and other global datasets to determine the Moho density contrast at the study area which comprises most of the Eurasian plate (including parts of surrounding continental and oceanic tectonic plates). A regional Moho recovery is realized by solving the Vening Meinesz-Moritz's (VMM) inverse problem of isostasy and a seismic crustal model is applied to constrain the gravimetric solution. Our results reveal that the Moho density contrast reaches minima along the mid-oceanic rift zones and maxima under the continental crust. This spatial pattern closely agrees with that seen in the CRUST1.0 seismic crustal model as well as in the KTH1.0 gravimetric-seismic Moho model. However, these results differ considerably from some previously published gravimetric studies. In particular, we demonstrate thatt here is no significant spatial correlation between the Moho density contrast and Moho deepening under major orogens of Himalaya and Tibet. In fact, the Moho density contrast under most of the continental crustal structure is typically much more uniform.

  • 128.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Hussain, Mutloob
    Department of Earth Sciences, Quad-i-Azam University, Islamabad 45320, Pakistan.
    An approach to Moho discontinuity recovery from on-orbit GOCE data with application over Indo-Pak region2016In: Tectonophysics, ISSN 0040-1951, E-ISSN 1879-3266, Vol. Part B, p. 253-262Article in journal (Refereed)
    Abstract [en]

    In this research, a modified form of Vening Meinesz-Moritz (VMM) theory of isostasy for the second-order radial derivative of gravitational potential, measured from the Gravity field and steady-state Ocean Circulation Explorer (GOCE), is developed for local Moho depth recovery. An integral equation is organised for inverting the GOCE data to compute a Moho model in combination with topographic/bathymetric heights of SRTM30, sediment and consolidated crystalline basement and the laterally-varying density contrast model of CRUST1.0. A Moho model from EGM2008 to degree and order 180 is also computed based on the same principle for the purpose of comparison. In addition, we compare both of them with the 3 available seismic Moho models; two global and one regional over the Indo-Pak region. Numerical results show that our GOCE-based Moho model is closer to the all seismic models than that of EGM2008. The model is closest to the regional one with a standard deviation of 5.5 km and a root mean squares error of 7.8 km, which is 2.3 km smaller than the corresponding one based on EGM2008.

  • 129.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Hussain, Mutloob
    Quaid-i-Azam University, Department of Earth Sciences, 45320 Islamabad, Pakistan.
    Tiampo, Kristy F.
    University of Colorado at Boulder, Department of Geological Sciences and CIRES, USA.
    Towards sub-lithospheric stress determination from seismic Moho, topographic heights and GOCE data2016In: Journal of Asian Earth Sciences, ISSN 1367-9120, E-ISSN 1878-5786, Vol. 129, p. 1-12Article in journal (Refereed)
    Abstract [en]

    Sub-lithospheric stresses can be estimated by analysis of gravity field measurements. Depending on the measured gravimetric quantity, different methods can be employed to estimate those sub-lithospheric stresses. Here, we further develop the Runcorn's theory for estimation of mantle stresses (1967) such that a Moho model and full topographic information are used to recover the function from which the stress can be computed by taking derivatives northwards and eastwards. We develop new integral equations for such a purpose and recover this function by solving those integral equations locally over the Indo-Pak (India-Pakistan) region from (1) a gravimetric Moho model computed from the SRTM (Shuttle Radar Topography Mission) and the Earth gravity model EGM2008, (2) SRTM and the seismic Moho model of CRUST1.0 and (3) data and measurements of the GOCE (Gravity field and steady-state Ocean Circulation Explorer) mission. Finally, we perform a joint inversion of seismic and GOCE data for the same purpose. The numerical results show that the use of a seismic Moho model recovers information about the stress field which is not seen in the results derived from a gravimetric Moho model. A combination of the seismic Moho model, SRTM and GOCE yields a better stress field than that of either the seismic and/or gravimetric data alone. The magnitudes of the sub-lithospheric stress are computed from the shear stress components over the area and good agreement is seen between the recovered combined stress field, the regional tectonic boundaries and the seismicity of the World Stress Map 2008 database.

  • 130.
    Eshagh, Mehdi
    et al.
    Royal Institute of Technology, Division of Geodesy, Stockholm, Sweden .
    Lars E., Sjöberg
    Royal Institute of Technology, Division of Geodesy, Stockholm, Sweden .
    Impact of topography and atmosphere over Iran on validation and inversion of GOCE gradiometric data2008In: Journal of the Earth and Space Physics, ISSN 1025-8647, Vol. 34, no 3, p. 15-30Article in journal (Refereed)
    Abstract [en]

    The dedicated satellite mission GOCE will sense various small mass variations along its path around the Earth. Here we study the effect of the Earth's topography and atmosphere on GOCE data. The effects depend on the magnitude of topographic height, and they will therefore vary by region. As the effect of the atmosphere and topography must be removed from the total gravity anomaly prior to geoid determinations, these effects should also be removed to simplify the downward continuation of the GOCE data to the sea level. The main goal of this article is to investigate the direct topographic and atmospheric effects in a rough region like Iran. Maps of the direct effects and their statistics are presented and discussed. Numerical results show maximum direct topographic and atmospheric effects on the GOCE data can reach 2.64 E and 5.53 mE, respectively, when the satellite flies over Iran. The indirect effect of the atmospheric and topographic masses are also formulated and presented.

  • 131.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering. Royal Institute of Technology, Division of Geodesy, Stockholm, Sweden .
    Lars E:, Sjöberg
    Royal Institute of Technology, Division of Geodesy, Stockholm, Sweden .
    The modified best quadratic unbiased non-negative estimator (MBQUNE) of variance components2008In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 52, no 3, p. 305-320Article in journal (Refereed)
    Abstract [en]

    Estimated variance components may come out as negative numbers without physical meaning. One way out of this problem is to use non-negative methods. Different approaches have been presented for the solution. Sjöberg presented a method of Best Quadratic Unbiased Non-Negative Estimator (BQUNE) in the Gauss-Helmert model. This estimator does not exist in the general case. Here we present the Modified BQUNE (MBQUNE) obtained by a simple transformation from the misclosures used in the BQUE to residuals. In the Gauss-Markov adjustment model the BQUNE and MBQUNE are identical, and they differ in condition and Gauss-Helmert models only by a simple transformation. If the observations are composed of independent/disjunctive groups the MBQUNE exists in any adjustment model and it carries all the properties of the BQUNE (when it exists). The presented variance component models are tested numerically in some simple examples. It is shown that the MBQUNE works well for disjunctive groups of observations.

  • 132.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering. Division of Geodesy, Royal Institute of Technology, Stockholm, Sweden.
    Lars E., Sjöberg
    Division of Geodesy, Royal Institute of Technology, Stockholm, Sweden.
    Ramin, Kiamehr
    Department of Geodesy and Geomatics, Zanjan University, Zanjan, Iran.
    Evaluation of robust techniques in suppressing the impact of outliers in a deformation monitoring network – A case study on the Tehran Milad tower network2007In: Acta Geodaetica et Geophysica Hungarica, ISSN 1217-8977, E-ISSN 1587-1037, Vol. 42, no 4, p. 449-463Article in journal (Refereed)
    Abstract [en]

    The problem of handling outliers in a deformation monitoring network is of special importance, because the existence of outliers may lead to false deformation parameters. One of the approaches to detect the outliers is to use robust estimators. In this case the network points are computed by such a robust method, implying that the adjustment result is resisting systematic observation errors, and, in particular, it is insensitive to gross errors and even blunders. Since there are different approaches to robust estimation, the resulting estimated networks may differ. In this article, different robust estimation methods, such as the M-estimation of Huber, the “Danish”, and the L 1-norm estimation methods, are reviewed and compared with the standard least squares method to view their potentials to detect outliers in the Tehran Milad tower deformation network. The numerical studies show that the L 1-norm is able to detect and down-weight the outliers best, so it is selected as the favourable approach, but there is a lack of uniqueness. For comparison, Baarda’s method “data snooping” can achieve similar results when the outlier magnitude of an outlier is large enough to be detected; but robust methods are faster than the sequential data snooping process.

  • 133.
    Eshagh, Mehdi
    et al.
    Royal Institute of Technology (KTH),Division of Geodesy and Geoinformatics.
    Lemoine, Jean-Michel
    Department of Space Geodesy (GRGS), French Space Agency (CNES), Toulouse, France.
    Gegout, Pascal
    Department of Space Geodesy (GRGS), French Space Agency (CNES), Toulouse, France.
    Biancale, Richard
    Department of Space Geodesy (GRGS), French Space Agency (CNES), Toulouse, France.
    On regularized time varying gravity field models based on GRACE data and their comparisons with hydrological models2013In: Acta Geophysica, ISSN 1895-6572, Vol. 61, no 1, p. 1-17Article in journal (Refereed)
    Abstract [en]

    Determination of spherical harmonic coefficients of the Earth's gravity field is often an ill-posed problem and leads to solving an ill-conditioned system of equations. Inversion of such a system is critical, as small errors of data will yield large variations in the result. Regularization is a method to solve such an unstable system of equations. In this study, direct methods of Tikhonov, truncated and damped singular value decomposition and iterative methods of ν, algebraic reconstruction technique, range restricted generalized minimum residual and conjugate gradient are used to solve the normal equations constructed based on range rate data of the gravity field and climate experiment (GRACE) for specific periods. Numerical studies show that the Tikhonov regularization and damped singular value decomposition methods for which the regularization parameter is estimated using quasioptimal criterion deliver the smoothest solutions. Each regularized solution is compared to the global land data assimilation system (GLDAS) hydrological model. The Tikhonov regularization with L-curve delivers a solution with high correlation with this model and a relatively small standard deviation over oceans. Among iterative methods, conjugate gradient is the most suited one for the same reasons and it has the shortest computation time

  • 134.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering. Royal Institute of Technology, Stockholm, Sweden.
    Mehdi, Najafi-Alamdari
    KN Toosi University of Technology,Tehran, Iran.
    Perturbations in orbital elements of a low earth orbiting satellite2007In: Journal of the Earth and Space Physics, Vol. 33, no 1, p. 1-12Article in journal (Refereed)
    Abstract [en]

    The main point of this paper is to evaluate the perturbations in orbital elements of a low Earth orbiting satellite. The outcome of a numerical orbit integration process is the position and velocity vectors of satellite in an inertial coordinate system. The velocity and position vectors are converted into the corresponding orbital elements. Perturbations in a satellite motion affect the orbital elements in the sense of Keplerian motion. In this paper after introducing the perturbing forces acting on a satellite, the method of converting the position and velocity into the orbital elements is presented, and finally the perturbations in orbital elements of the low Earth orbiting satellite of CHAMP are evaluated. The numerical results show that, disregarding the geopotential perturbing forces, the air drag is the most predominant among other perturbing forces: rotational deformation, solar radiation, third body effect, solid Earth tide, ocean tide, and general relativity arranged by their magnitude respectively.

  • 135.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Mohsen, Romeshkani
    Islamic Azad University, Qazvin Branch Iran.
    Determination of sub-lithospheric stress due to mantle convection using GOCE gradiometric data over Iran2015In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 122, p. 11-17, article id 2807Article in journal (Refereed)
    Abstract [en]

    Sub-lithospheric stress due to mantle convection can be determined from gravimetric data based on Runcorn’s theory. In this paper, the satellite gradiometric data of the recent European satellite mission, the Gravity field and steady-state Ocean Circulation Explorer (GOCE) is used to determine the sub- lithospheric stress locally in Iran. The method of S function (SF) with numerical differentiation is developed further and an integral equation connecting satellite gradiometric data to SF is presented. The integral equation will be used to invert the real gradiometric data of GOCE to recover the SF. Later on, the sub-lithospheric shear stresses, which are the northwards and eastwards derivatives of the SF, are computed numerically. Our numerical results show that the mean square error of the recovered SF is smaller than the values of the SF meaning that the recovery process is successful. Also, the recovered stress has a good agreement with the tectonic boundaries and active seismic points of the world stress map (WSM) database. This stress reaches amplitude of 100 MPa in the territory.

  • 136.
    Eshagh, Mehdi
    et al.
    Royal Institute of Technology (KTH).
    Najafi Alamdari, M
    K.N.Toosi University of Technology, P.O.Box 15875-4416,Tehran, Iran .
    Comparison of different methods of orbit integration of a low Earth orbiting satellite2006In: Journal of the Earth and Space Physics, Vol. 32, no 3, p. 41-57Article in journal (Refereed)
  • 137. Eshagh, Mehdi
    et al.
    Najafi-Alamdari, Mehdi
    Department of Geodesy, K.N.Toosi University of Technology.
    Perturbations in orbital elements of a low Earth orbiting (LEO) satellite2007In: Journal of the Earth and Space Physics, Vol. 33, no 1, p. 1-12Article in journal (Refereed)
  • 138. Eshagh, Mehdi
    et al.
    Najafi-Alamdari, Mehdi
    Department of Geodesy, K.N.Toosi University.
    The effects of Solid Tide on an elastic and unelastic Earth2006In: Journal of the Earth and Space Physics, Vol. 32, no 3, p. 1-9Article in journal (Refereed)
  • 139.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Pitonak, Martin
    University of West Bohemia, NTIS - The New Technologies for the Information Society, Faculty of Applied Sciences, Pilsen, Czech Republic.
    Tenzer, Robert
    Hong Kong Polytechnic University, Department of Land Surveying and Geo-Informatics, Hong Kong.
    Lithospheric elastic thickness estimates in central Eurasia2019In: Terrestrial, Atmospheric and Oceanic Science, ISSN 1017-0839, E-ISSN 2223-8964, Vol. 30, no 1, p. 73-84Article in journal (Refereed)
    Abstract [en]

    We estimate the elastic thickness of a continental lithosphere by using two approaches that combine the Vening Meinesz-Moritz (VMM) regional isostatic principle with isostatic flexure models formulated based on solving flexural differential equations for a thin elastic shell with and without considering a shell curvature. To model the response of the lithosphere on a load more realistically, we also consider lithospheric density heterogeneities. Resulting expressions describe a functional relation between gravity field quantities and mechanical properties of the lithosphere, namely Young’s modulus and Poisson’s ratio that are computed from seismic velocity models in prior of estimating the lithospheric elastic thickness. Our numerical study in central Eurasia reveals that both results have a similar spatial pattern, despite exhibiting also some large localized differences due to disregarding the shell curvature. Results show that cratonic formations of North China and Tarim Cratons, Turan Platform as well as parts of Siberian Craton are characterized by the maximum lithospheric elastic thickness. Indian Craton, on the other hand, is not clearly manifested. Minima of the elastic thickness typically correspond with locations of active continental tectonic margins, major orogens (Tibet, Himalaya and parts of Central Asian Orogenic Belt) and an extended continental crust. These findings generally support the hypothesis that tectonically active zones and orogens have a relatively small lithospheric strength, resulting in a significant respond of the lithosphere on various tectonic loads, compared to a large lithospheric strength of cratonic formations.

  • 140.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Pitoňák, Martin
    University of West Bohemia, NTIS-The New Technologies for the Information Society, Faculty of Applied Sciences, Pilsen, Czech Republic.
    Elastic Thickness Determination from on-orbit GOCE Data and CRUST1.02019In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 176, no 2, p. 685-696Article in journal (Refereed)
    Abstract [en]

    Elastic thickness (Te) is a parameter representing the lithospheric strength with respect to the loading. Those places, having large values of elastic thickness, flexes less. In this paper, the on-orbit measured gravitational gradients of the Gravity field and steady-state Ocean Circulation Explorer (GOCE) mission are used for determining the elastic thickness over Africa. A forward computational method is developed based on the Vening Meinesz-Moritz (VMM) and flexural theories of isostasy to find a mathematical relation between the second-order derivative of the Earth’s gravity field measured by the GOCE satellite and mechanical properties of the lithosphere. The loading of topography and bathymetry, sediments and crystalline masses are computed from CRUST1.0, in addition to estimates of laterally-variable density of the upper mantle, Young’s modulus and Poisson’s ratio. The second-order radial derivatives of the gravitational potential are synthesised from the crustal model and different a priori values of elastic thickness to find which one matches the GOCE on-orbit gradient. This method is developed in terms of spherical harmonics and performed at any point along the GOCE orbit without using any planar approximation. Our map of Te over Africa shows that the intra-continental hotspots and volcanoes, such as Ahaggar, Tibesti, Darfur, Cameroon volcanic line and Libya are connected by corridors of low Te. The high values of Te are mainly associated with the cratonic areas of Congo, Chad and the Western African basin.

  • 141.
    Eshagh, Mehdi
    et al.
    Royal Institute of Technology (KTH), Division of Geodesy and Geoinformatics, .
    Romeshkani, Mohsen
    Department of Geodesy, K.N. Toosi University of Technology, 470 Mirdamad Ave. West, Box 15875-4416, 19697 Tehran, Iran.
    Generation of vertical-horizontal and horizontal-horizontal gravity gradients using stochastically modified integral estimators2011In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 48, no 8, p. 1341-1358Article in journal (Refereed)
    Abstract [sv]

    The Earth's gravity field modelling is an ill-posed problem having a sensitive solution to the error of data. Satellite gravity gradiometry (SGG) is a space technique to measure the second-order derivatives of geopotential for modelling this field, but the measurements should be validated prior to use. The existing terrestrial gravity anomalies and Earth gravity models can be used for this purpose. In this paper, the second-order vertical–horizontal (VH) and horizontal–horizontal (HH) derivatives of the extended Stokes formula in the local north-oriented frame are modified using biased, unbiased and optimum types of least-squares modification. These modified integral estimators are used to generate the VH and HH gradients at 250 km level for validation purpose of the SGG data. It is shown that, unlike the integral estimator for generating the second-order radial derivative of geopotential, the system of equations from which the modification parameters are obtained is unstable for all types of modification, with large cap size and high degree, and regularization is strongly required for solving the system. Numerical studies in Fennoscandia show that the SGG data can be estimated with an accuracy of 1 mE using an integral estimator modified by a biased type least-squares modification. In this case an integration cap size of 2.5° and a degree of modification of 100 for integrating 30′ × 30′ gravity anomalies are required.

  • 142.
    Eshagh, Mehdi
    et al.
    Department of Geodesy, K.N.Toosi University of Technology, Tehran, Iran.
    Romeshkani, Mohsen
    Department of Geodesy, K.N.Toosi University of Technology, Tehran, Iran.
    Quality assessment of terrestrial gravity anomalies from GOCE gradiometric data and Earth's gravity models using variance component estimation2013In: Studia Geophysica et Geodaetica, ISSN 0039-3169, E-ISSN 1573-1626, Vol. 57, no 1, p. 67-83Article in journal (Refereed)
    Abstract [en]

     The satellite gravity gradiometry (SGG) data of the recent European satellite mission, the Gravity field and steady-state Ocean Circulation Explorer (GOCE), can be used as an external source for quality description of terrestrial gravity anomalies and the Earth's gravity models (EGMs). In this study integral estimators are provided and modified in a least-squares sense to regenerate the SGG data of GOCE from terrestrial gravity anomalies and an existing EGM. Based on the differences between the generated and real GOCE SGG data, condition adjustment models are constructed and variance component estimation (VCE) is applied for balancing the a priori errors of data with these differences. Here, a 1-month orbit of GOCE is considered over Iran and the condition adjustment models and VCE process are used to calibrate the errors of the GOCE data, terrestrial gravity anomalies of the area and the EGM. Numerical studies over Iran show that the a priori errors of the GOCE data and the EGM were properly presented. Also the average error of the terrestrial gravity anomalies, with a resolution of 0.5° × 0.5°, after condition adjustment and VCE process using Tzz, Tx, Tyz and −Txx −Tyy is about 30 mGal.

  • 143.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Natural Sciences and Electrical and Surveying Engineering.
    Sprlak, Michal
    University of West Bohemia, NTIS - New Technologies for the Information Society, Faculty of Applied Sciences, Plzeň, Czech Republic.
    On the integral inversion of satellite-to-satellite velocity differences for local gravity field recovery: A theoretical study2016In: Celestial mechanics & dynamical astronomy, ISSN 0923-2958, E-ISSN 1572-9478, Vol. 124, no 2, p. 124-144Article in journal (Refereed)
    Abstract [en]

    The gravity field can be recovered locally from the satellite-to-satellite velocity differences (VDs) between twin-satellites moving in the same orbit. To do so, three different integral formulae are derived in this paper to recover geoid height, radial component of gravity anomaly and gravity disturbance at sea level. Their kernel functions contain the product of two Legendre polynomials with different arguments. Such kernels are relatively complicated and it may be impossible to find their closed-forms. However, we could find the one related to recovering the geoid height from the VD data. The use of spectral forms of the kernels is possible and one does not have to generate them to very high degrees. The kernel functions are well-behaving meaning that they reduce the contribution of far-zone data and for example a cap margin of 7∘ is enough for recovering gravity anomalies. This means that the inversion area should be larger by 7∘ from all directions than the desired area to reduce the effect of spatial truncation error of the integral formula. Numerical studies using simulated data over Fennoscandia showed that when the distance between the twin-satellites is small, higher frequencies of the anomalies can be recovered from the VD data. In the ideal case of having short distance between the satellites flying at 250 km level, recovering radial component of gravity anomaly with an accuracy of 7 mGal is possible over Fennoscandia, if the VD data is contaminated only with the spatial truncation error, which is an ideal assumption. However, the problem is that the power of VD signal is very low when the satellites are close and it is very difficult to recognise the signal amongst the noise of the VD data. We also show that for a successful determination of gravity anomalies at sea level from an altitude of 250 km mean VDs with better accuracy than 0.01 mm/s are required. When coloured noise at this level is used for the VDs at 250 km with separation of 300 km, the accuracy of recovery will be about 11 mGal over Fennoscandia. In the case of using the real velocities of the satellites, the main problems are downward/upward continuation of the VDs on the mean orbital sphere and taking the azimuthal integration of them.

  • 144.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Mathematics, Computer and Surveying Engineering.
    Steinberger, Bernhard
    Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, Telegrafenberg, Potsdam, Germany & Centre for Earth Evolution and Dynamics (CEED), University of Oslo, Postboks 1028 Blindern, Oslo, Norway.
    Tenzer, Robert
    Hong Kong Polytechnic University, Department of Land Surveying and Geo-Informatics, 11 Yuk Chai Rd, Hung Hom, Hong Kong.
    Tassara, Andrés
    Universidad de Concepción, Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Victor Lamas 1290, Concepción, Chile.
    Comparison of gravimetric and mantle flow solutions for sub-lithopsheric stress modeling and their combination2018In: Geophysical Journal International, ISSN 0956-540X, E-ISSN 1365-246X, Vol. 213, no 2, p. 1013-1028Article in journal (Refereed)
    Abstract [en]

    Based on Hager and O’Connell’s solution to mantle flow equations, the stresses induced by mantle convection are determined using the density and viscosity structure in addition to topographic data and a plate velocity model. The solution to mantle flow equations requires the knowledge of mantle properties that are typically retrieved from seismic information. Large parts of the world are, however, not yet covered sufficiently by seismic surveys. An alternative method of modeling the stress field was introduced by Runcorn. He formulated a direct relation between the stress field and gravity data, while adopting several assumptions, particularly disregarding the toroidal mantle flow component and mantle viscosity variations. A possible way to overcome theoretical deficiencies of Runcorn’s theory as well as some practical limitations of applying Hager and O’Connell’s theory (in the absence of seismic data) is to combine these two methods. In this study, we apply a least-squares analysis to combine these two methods based on the gravity data inversion constraint on mantle flow equations. In particular, we use vertical gravity gradients from the Gravity field and steady state Ocean Circulation Explorer that are corrected for the gravitational contribution of crustal density heterogeneities prior to applying a localized gravity-gradient inversion. This gravitational contribution is estimated based on combining the Vening Meinesz-Moritz and flexural isostatic theories. Moreover, we treat the non-isostatic effect implicitly by applying a band-limited kernel of the integral equation during the inversion. In numerical studies of modeling, the stress field within the South American continental lithosphere we compare the results obtained after applying Runcorn and Hager and O’Connell’s methods as well as their combination. The results show that, according to Hager and O’Connell’s (mantle flow) solution, the maximum stress intensity is inferred under the northern Andes. Additional large stress anomalies are detected along the central and southern Andes, while stresses under most of old, stable cratonic formations aremuch less pronounced or absent. A prevailing stress-vector orientation realistically resembles a convergent mantle flow and downward currents under continental basins that separate Andean Orogeny from the Amazonian Shield and adjacent cratons. Runcorn’s (gravimetric) solution, on the other hand, reflects a tectonic response of the lithosphere to mantle flow, with the maximum stress intensity detected along the subduction zone between the Nazca and Altiplano plates and along the convergent tectonic margin between the Altiplano and South American plates. The results also reveal a very close agreement between the results obtained from the combined and Hager and O’Connell’s solutions. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.

  • 145.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Tenzer, Robert
    University of West Bohemia Plzen,New Technologies for the Information Society (NTIS), Czech Republic.
    Lithospheric stress tensor from gravity and lithospheric structure models2017In: Pure and Applied Geophysics, ISSN 0033-4553, E-ISSN 1420-9136, Vol. 174, no 7, p. 2677-2688Article in journal (Refereed)
    Abstract [en]

    n this study we investigate the lithospheric stresses computed from the gravity and lithospheric structure models. The functional relation between the lithospheric stress tensor and the gravity field parameters is formulated based on solving the boundary-value problem of elasticity in order to determine the propagation of stresses inside the lithosphere, while assuming the horizontal shear stress components (computed at the base of the lithosphere) as lower boundary values for solving this problem. We further suppress the signature of global mantle flow in the stress spectrum by subtracting the long-wavelength harmonics (below the degree of 13). This numerical scheme is applied to compute the normal and shear stress tensor components globally at the Moho interface. The results reveal that most of the lithospheric stresses are accumulated along active convergent tectonic margins of oceanic subductions and along continent-to-continent tectonic plate collisions. These results indicate that, aside from a frictional drag caused by mantle convection, the largest stresses within the lithosphere are induced by subduction slab pull forces on the side of subducted lithosphere, which are coupled by slightly less pronounced stresses (on the side of overriding lithospheric plate) possibly attributed to trench suction. Our results also show the presence of (intra-plate) lithospheric loading stresses along Hawaii islands. The signature of ridge push (along divergent tectonic margins) and basal shear traction resistive forces is not clearly manifested at the investigated stress spectrum (between the degrees from 13 to 180).

  • 146.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Divison of Natural Sciences, Surveying and Mechanical Engineering.
    Tenzer, Robert
    Wuhan University, China.
    Sub-crustal stress determined using gravity and crust structure models2015In: Computational Geosciences, ISSN 1420-0597, E-ISSN 1573-1499, p. 115-125Article in journal (Refereed)
    Abstract [en]

    The sub-crustal stress induced by mantle convection has been traditionally computed using the Runcorn formulae of solving the Navier-Stokes problem. The main disadvantage of this method is a limited spectral resolution (up to degree 25 of spherical harmonics) due to a divergence of the spherical harmonic expression. To improve the spectral resolution, we propose a new method of computing the horizontal components of the sub-crustal stress based on utilising the stress function with a numerical differentiation. According to the proposed method, the stress function is functionally related to the gravity and crust structure models expressed in terms of spherical harmonics, instead of directly relating the stress components with partial derivatives of these spherical harmonics. The stress components are then computed from the stress function by applying a numerical differentiation. This modification increases the degree-dependent convergence domain of the asymptotically convergent series and consequently allows computing the stress components to a higher spectral resolution, which is compatible with currently available global crustal models. We further utilise the solution to the Vening Meinesz-Moritz inverse problem of isostasy in definition of the stress function. This definition facilitates a variable crustal thickness instead of assuming only a constant value adopted in the Runcorn formulae. The crustal thickness and sub-crustal stress are then determined directly from gravity data and a crustal structure model. We apply this numerical approach to compute the sub-crustal stress globally. Regional results are also presented and discussed over study areas of oceanic subduction zones, convergent continent-to-continent collision zones and hotspots. We demonstrate that the largest (in magnitude) sub-crustal stress occurs mainly along seismically active convergent tectonic plate boundaries.

  • 147.
    Eshagh, Mehdi
    et al.
    University West, Department of Engineering Science, Division of Computer, Electrical and Surveying Engineering.
    Zoghi, Sedigheh
    Division of Geodesy and Satellite Positioning, Royal Institute of Technology (KTH), Stockholm, Sweden.
    Local error calibration of EGM08 geoid using GNSS/levelling data2016In: Journal of Applied Geophysics, ISSN 0926-9851, E-ISSN 1879-1859, Vol. 130, no July, p. 209-217Article in journal (Refereed)
    Abstract [en]

    The geoid error, computed from EGM08, is unrealistically large due to the continuation of the spherical harmonic coefficient errors down to the surface of the reference ellipsoid. In this study, we try to calibrate such an error by the differences between the EGM08 and GNSS/levelling geoids over Fennoscandia. We use the variance component estimation procedure through combined adjustments of the geoid and GNSS/levelling heights using corrector surfaces of 4-, 5- and 7-parameter. We also develop a simple iterative method to calibrate the geoid error from the a posteriori variance factor and the errors of GNSS/levelling geoid. Our numerical investigations show that performing the separate adjustment and variance component estimation for each country with a two-component stochastic model is more successful than performing it in the whole area with a five-component model. The number of GNSS/levelling data over Sweden and Norway are much larger than those in Denmark and Finland. This causes that the corrector surfaces are fitted better in these countries and consequently the estimated errors for the geoid become larger than what they should be in the others. Based on a 7-parameter corrector surface model, the average error of the EGM08 geoid becomes 12, 17, 51 and 34 mm, in Sweden, Denmark, Norway and Finland, respectively. If the two-component stochastic model is used in a combined adjustment over Fennoscandia this average error will be 48 mm.

  • 148.
    Feldt, Robert
    et al.
    Blekinge Institute of Technology.
    Lefteris, Angelis
    Aristotle University of Thessaloniki, Dept. of Informatics.
    Torkar, Richard
    Blekinge Institute of Technology.
    Samuelsson, Maria
    University West, Department of Economics and IT, Division of Computer Science and Informatics.
    Links between the personalities, views and attitudes of software engineers2010In: Information and Software Technology, ISSN 0950-5849, E-ISSN 1873-6025, Vol. 52, no 6, p. 611-624Article in journal (Refereed)
    Abstract [en]

    Context:Successful software development and management depends not only on the technologies, methods and processes employed but also on the judgments and decisions of the humans involved. These, in turn, are affected by the basic views and attitudes of the individual engineers.Objective:The objective of this paper is to establish if these views and attitudes can be linked to the personalities of software engineers.Methods:We summarize the literature on personality and software engineering and then describe an empirical study on 47 professional engineers in ten different Swedish software development companies. The study evaluated the personalities of these engineers via the IPIP 50-item five-factor personality test and prompted them on their attitudes towards and basic views on their professional activities.Results:We present extensive statistical analyses of their responses to show that there are multiple, significant associations between personality factors and software engineering attitudes. The tested individuals are more homogeneous in personality than a larger sample of individuals from the general population.Conclusion:Taken together, the methodology and personality test we propose and the associated statistical analyses can help find and quantify relations between complex factors in software engineering projects in both research and practice.

  • 149.
    Feldt, Robert
    et al.
    Blekinge Institute of Technology.
    Torkar, Richard
    Blekinge Institute of Technology.
    Angelis, Lefteris
    Aristotle University of Thessaloniki.
    Samuelsson, Maria
    University West, Department of Economics and IT, Division of Computer Science and Informatics.
    Towards individualized software engineering: empirical studies should collect psychometrics2008In: Proceedings of the 2008 international workshop on Cooperative and human aspects of software engineering: International Conference on Software Engineering, ACM , 2008, p. 49-52Conference paper (Other academic)
  • 150.
    Fernström, Nathalie
    et al.
    University West, Department of Economics and IT, Divison of Informatics.
    Holmeby, Emilia
    University West, Department of Economics and IT, Divison of Informatics.
    Ungdomars påverkan av sociala medier2016Independent thesis Basic level (degree of Bachelor), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    The Internet has helped people around the world gain new opportunities to communicate with each other. Global communication has also been streamlined, especially with the phenomenon of social media. Social media is a big part of many people's daily lives these days and it is a phenomenon that many people use every day. This has resulted in many young people spending a very large part of their waking hours to the use of social media. But there are also downsides and implications of using social media. It is not only the risks relating to what information young people choose to share, but also what information they consume and to which degree they consume information. Social media has become a venue for different types of social comparison. The aim of the present study is to investigate what impact using various social media has on young people's self-image and self-esteem. To achieve this purpose, we made a quantitative study by distributing a survey with relevant questions to young people between 15-19 years in Sweden. The selection was done by snowball sampling which means we distributed a link to the survey to secondary schools throughout Sweden and through a viral post on Facebook with a request to share the survey. The answers from the survey were analysed by comparing means, bivariate regression analysis and independent t-tests. The result was then related to previous research and Festinger's (1954) Social comparison theory as a theoretical framework. The survey measured the self-image and self-esteem of the respondents' using the Rosenberg Self-Esteem scale and the State Self-Esteem scale. Young people today has much wider social network than in the past thanks to social media. Therefore they have the opportunity to compare their appearance with and get affected by far more people than before. Young people are also concerned about how they appear on social media, which have been shown to influence young people's self-esteem and self-image greatly. The results in this study suggest that girls generally have a lower self-image and self-esteem than boys. Girls are most affected by the appearance related factors, even if these factors also affect men significantly. The boys are affected not only by the appearance related aspects, but also especially the questions regarding popularity, masculinity and power. They have also been shown to have a more risky behaviour on social media than girls. The conclusion of this study is that most respondents are affected, but to varying degrees, by comparison on social media.

1234567 101 - 150 of 495
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf