| 1. |
- Alizadeh Khameneh, Mohammad Amin, et al.
(författare)
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Optimization of deformation monitoring networks using finite element strain analysis
- 2018
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Ingår i: Journal of Applied Geodesy. - : Walter de Gruyter GmbH. - 1862-9016 .- 1862-9024. ; 2:2, s. 187-197
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Tidskriftsartikel (refereegranskat)abstract
- An optimal design of a geodetic network can fulfill the requested precision and reliability of the network, and decrease the expenses of its execution by removing unnecessary observations. The role of an optimal design is highlighted in deformation monitoring network due to the repeatability of these networks. The core design problem is how to define precision and reliability criteria. This paper proposes a solution, where the precision criterion is defined based on the precision of deformation parameters, i. e. precision of strain and differential rotations. A strain analysis can be performed to obtain some information about the possible deformation of a deformable object. In this study, we split an area into a number of three-dimensional finite elements with the help of the Delaunay triangulation and performed the strain analysis on each element. According to the obtained precision of deformation parameters in each element, the precision criterion of displacement detection at each network point is then determined. The developed criterion is implemented to optimize the observations from the Global Positioning System (GPS) in Skåne monitoring network in Sweden. The network was established in 1989 and straddled the Tornquist zone, which is one of the most active faults in southern Sweden. The numerical results show that 17 out of all 21 possible GPS baseline observations are sufficient to detect minimum 3 mm displacement at each network point. © 2018 Walter de Gruyter GmbH, Berlin/Boston.
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| 2. |
- Alizadeh Khameneh, Mohammad Amin, et al.
(författare)
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The Effect of Instrumental Precision on Optimisation of Displacement Monitoring Networks
- 2016
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Ingår i: Acta Geodaetica et Geophysica Hungarica. - : Springer Science and Business Media LLC. - 1217-8977 .- 1587-1037 .- 2213-5812 .- 2213-5820. ; 51:4, s. 761-772
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Tidskriftsartikel (refereegranskat)abstract
- In order to detect the geo-hazards, different deformation monitoring networks are usually established. It is of importance to design an optimal monitoring network to fulfil the requested precision and reliability of the network. Generally, the same observation plan is considered during different time intervals (epochs of observation). Here, we investigate the case that instrumental improvements in sense of precision are used in two successive epochs. As a case study, we perform the optimisation procedure on a GPS monitoring network around the Lilla Edet village in the southwest of Sweden. The network was designed for studying possible displacements caused by landslides. The numerical results show that the optimisation procedure yields an observation plan with significantly fewer baselines in the latter epoch, which leads to saving time and cost in the project. The precision improvement in the second epoch is tested in several steps for the Lilla Edet network. For instance, assuming two times better observation precision in the second epoch decreases the number of baselines from 215 in the first epoch to 143 in the second one.
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| 3. |
- Bagherbandi, Mohammad, et al.
(författare)
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Recovery of Moho's undulations based on the Vening Meinesz-Moritz theory from satellite gravity gradiometry data : A simulation study
- 2012
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Ingår i: Advances in Space Research. - : Elsevier BV. - 0273-1177 .- 1879-1948. ; 49:6, s. 1097-1111
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Tidskriftsartikel (refereegranskat)abstract
- In the gravimetric approach to determine the Moho depth an isostatic hypothesis can be used. The Vening Meinesz-Moritz isostatic hypothesis is the recent theory for such a purpose. Here, this theory is further developed so that the satellite gravity gradiometry (SGG) data are used for recovering the Moho depth through a nonlinear integral inversion procedure. The kernels of its forward and inverse problems show that the inversion should be done in a larger area by 5 than the desired one to reduce the effect of the spatial truncation error of the integral formula. Our numerical study shows that the effect of this error on the recovered Moho depths can reach 6 km in Persia and it is very significant. The iterative Tikhonov regularization in a combination with either generalized cross validation or quasi-optimal criterion of estimating the regularization parameter seems to be suitable and the solution is semi-convergent up to the third iteration. Also the Moho depth recovered from the simulated SGG data will be more or less the same as that obtained from the terrestrial gravimetric data with a root mean square error of 2 km and they are statistically consistent.
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| 4. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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A case study on displacement analysis of Vasa warship
- 2018
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Ingår i: Journal of Geodetic Science. - : DE GRUYTER POLAND SP ZOO. - 2081-9919 .- 2081-9943. ; 8:1, s. 43-54
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Tidskriftsartikel (refereegranskat)abstract
- Monitoring deformation of man-made structures is very important to prevent them from a risk of collapse and save lives. Such a process is also used for monitoring change in historical objects, which are deforming continuously with time. An example of this is the Vasa warship, which was under water for about 300 years. The ship was raised from the bottom of the sea and is kept in the Vasa museum in Stockholm. A geodetic network with points on the museum building and the ship's body has been established and measured for 12 years for monitoring the ship's deformation. The coordinate time series of each point on the ship and their uncertainties have been estimated epoch-wisely. In this paper, our goal is to statistically analyse the ship's hull movements. By fitting a quadratic polynomial to the coordinate time series of each point of the hull, its acceleration and velocity are estimated. In addition, their significance is tested by comparing them with their respective estimated errors after the fitting. Our numerical investigations show that the backside of the ship, having highest elevation and slope, has moved vertically faster than the other places by a velocity and an acceleration of about 2 mm/year and 0.1 mm/year(2), respectively and this part of the ship is the weakest with a higher risk of collapse. The central parts of the ship are more stable as the ship hull is almost vertical and closer to the floor. Generally, the hull is moving towards its port and downwards.
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| 5. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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A strategy to calibrate errors of Earth gravity models
- 2014
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Ingår i: Journal of Applied Geophysics. - : Elsevier BV. - 0926-9851 .- 1879-1859. ; 103:April, s. 215-220
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Eshagh, Mehdi, 1977-
(författare)
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A theoretical discussion on Vening Meinesz-Moritz inverse problem of isostasy
- 2016
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 207:3, s. 1420-1431
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Tidskriftsartikel (refereegranskat)abstract
- The Moho surface can be determined according to isostatic theories and the recent Vening Meinesz-Moritz (VMM) theory of isostasy has been successful for this purpose. In this paper, we will study this method from a theoretical prospective and try to find its connection to the Airy-Heiskanen (AH) and Vening Meinesz original theories. We develop Jeffrey’s inverse solution to isostasy according to the recent developments of the VMM method and compare both methods in similar situations. We will show that they are generalisations of the AH model in a global and continuous domain. In the VMM spherical harmonic solution for Moho depth, the mean Moho depth contributes only to the zero-degree term of the series, whilst in Jeffrey’s solution it contributes to all frequencies. We improve the VMM spherical harmonic series further so that the mean Moho can contribute to all frequencies of the solution. This modification makes the VMM global solution superior to the Jeffrey one, but in a global scale, the difference between both solutions is less than 3 km. Both solutions are asymptotically-convergent and we present two methods to obtain smooth solutions for Moho from them.
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| 7. |
- Eshagh, Mehdi, 1977-
(författare)
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A theoretical study on terrestrial gravimetric data refinement by earth gravity models
- 2014
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Ingår i: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 62:1, s. 158-171
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Tidskriftsartikel (refereegranskat)abstract
- The idea of this paper is to present estimators for combining terrestrial gravity data with Earth gravity models (EGMs) and produce a high-quality source of the Earth's gravity field data through all wavelengths. To do so, integral and point-wise estimators are mathematically developed, based on the spectral combination theory, in such a way that they combine terrestrial data with one and/or two Earth gravity models. The integral estimators are developed so that they become biased or unbiased to a priori information. For testing the quality of the estimators, their global mean square errors (MSEs) are generated using an Earth gravity model08 model and one of the recent products of the gravity field and steady-state ocean circulation explorer (GOCE) mission. Numerical results show that the integral estimators have smaller global root mean square errors (RMSEs) than the point-wise ones but they are not efficient practically. The integral estimator of the biased type is the most suited due to its smallest global root mean square error comparing to the rest of the estimators. Due largely to the omission errors of Earth gravity models the point-wise estimators are not sensitive to the Earth gravity model commission error; therefore, the use of high-degree Earth gravity models is very influential for reduction of their root mean square errors. Also it is shown that the use of the ocean circulation explorer Earth gravity model does not significantly reduce the root mean square errors of the presented estimators in the presence of Earth gravity model08. All estimators are applied in the region of Fennoscandia and a cap size of 2° for numerical integration and a maximum degree of 2500 for generation of band-limited kernels are found suitable for the integral estimators.
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| 8. |
- Eshagh, Mehdi, Professor, 1977-
(författare)
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An optimal design of GNSS interference localisation wireless security network based on time-difference of arrivals for the Arlanda international airport
- 2022
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Ingår i: Journal of Geodetic Science. - : De Gruyter Open. - 2081-9919 .- 2081-9943. ; 12:1, s. 154-164
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Tidskriftsartikel (refereegranskat)abstract
- Today, most of the aircrafts are navigated by global navigation satellite systems (GNSSs). Landing is a dangerous phase of a flight especially when an airport runway is not clearly seen from the aircrafts. In such cases, GNSSs are useful for a safe landing under the circumstances that healthy signals, free of any interference, reach to GNSSs receiver antennas mounted on the aircrafts. This shows the importance of establishing GNSS interference localisation security networks around airports. Designing a good configuration for the points with GNSS antennas at for receiving interference signals is important for a successful localisation of the interference device. Here, the time-difference of the arrivals of an interference signal to such points or anchor nodes (ANs), are used as observables, and a security network with four ANs is optimally designed along the runways of the Arlanda airport to reduce the dilution of precision (DOP) of the network. Our study showed that by such an optimisation, the maximum DOP value can reduce by 50% meaning a significant increase in the probability of a successful interference device localisation.
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| 9. |
- Eshagh, Mehdi, Professor, 1977-
(författare)
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Analysis of the gravity field, direct and inverse problems
- 2022
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Ingår i: Journal of Geodetic Science. - : De Gruyter Open. - 2081-9919 .- 2081-9943. ; 12:1, s. 244-245
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Recension (refereegranskat)abstract
- This new book deals with, as its titles says, the analysis and process of gravity field of the Earth. The book is mathematically oriented, and the authors have explained how Mathematics is used for local analysis of the gravity field. The basic and required background knowledge, needed for understanding the book, are given in early chapters as well as appendices. They are used for explaining gravity field analysis from measurement surveys, corrections, and processes to direct and inverse problems in exploration Geophysics with interesting exercises, numerical and real examples.
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| 10. |
- Eshagh, Mehdi, Professor, 1977-, et al.
(författare)
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Comparison of gravimetric and mantle flow solutions for sub-lithopsheric stress modeling and their combination
- 2018
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Ingår i: Geophysical Journal International. - : Oxford University Press (OUP). - 0956-540X .- 1365-246X. ; 213:2, s. 1013-1028
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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