| 1. |
- Amin, Hadi, et al.
(författare)
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A global vertical datum defined by the conventional geoid potential and the Earth ellipsoid parameters
- 2020
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Konferensbidrag (populärvet., debatt m.m.)abstract
- According to the classical Gauss–Listing definition, the geoid is the equipotential surface of the Earth’s gravity field that in a least-squares sense best fits the undisturbed mean sea level. This equipotential surface, except for its zero-degree harmonic, can be characterized using the Earth’s Global Gravity Models (GGM). Although nowadays, the satellite altimetry technique provides the absolute geoid height over oceans that can be used to calibrate the unknown zero-degree harmonic of the gravimetric geoid models, this technique cannot be utilized to estimate the geometric parameters of the Mean Earth Ellipsoid (MEE). In this study, we perform joint estimation of W0, which defines the zero datum of vertical coordinates, and the MEE parameters relying on a new approach and on the newest gravity field, mean sea surface, and mean dynamic topography models. As our approach utilizes both satellite altimetry observations and a GGM model, we consider different aspects of the input data to evaluate the sensitivity of our estimations to the input data. Unlike previous studies, our results show that it is not sufficient to use only the satellite componentof a quasi-stationary GGM to estimate W0. In addition, our results confirm a high sensitivity of the applied approach to the altimetry-based geoid heights, i.e. mean sea surface and mean dynamic topography models. Moreover, as W0 should be considered a quasi-stationary parameter, we quantify the effect of time-dependent Earth’s gravity field changes as well as the time-dependent sea-level changes on the estimation of W0. Our computations resulted in the geoid potential W0 = 62636848.102 ± 0.004 m2s-2 and the semi-major and –minor axes of the MEE,a = 6378137.678 ± 0.0003 m and b = 6356752.964 ± 0.0005 m, which are 0.678 and 0.650 m larger than those axes of the GRS80 reference ellipsoid, respectively. Moreover, a new estimation for the geocentric gravitational constant was obtained as GM = (398600460.55 ± 0.03) × 106 m3s-2.
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| 2. |
- Amin, Hadi, et al.
(författare)
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Evaluation of the Closure of Global Mean Sea Level Rise Budget over January 2005 to August 2016
- 2019
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Sea level changes over time because of water mass exchange among the oceans and continents, ice sheets, and atmosphere. It fluctuates also due to variations of seawater salinity and temperature known as the steric contributor. GRACE-based Stokes coefficients provide a valuable source of information, about the water mass exchange as the main contributor to the Earth’s gravity field changes, within decadal scales. Moreover, measuring seawater temperature and salinity at different layers of ocean depth, Argo floats help to model the steric component of Global Mean Sea Level. In this study, we evaluate the Global Mean Sea Level (GMSL) budget closure using satellite altimetry, GRACE, and Argo products. Hereof, considering the most recent released GRACE monthly products (RL06), we examine an iterative remove-restore method to minimize the effect of artifact leaked large signal from ice sheets and land hydrology. In addition, the effect of errors and biases in geophysical model corrections, such as GIA, on the GMSL budget closure is estimated. Moreover, we quantify the influence of spatial and decorrelation filtering of GRACE data on the GMSL budget closure. In terms of the monthly fluctuations of sea level, our results confirm that closing the GMSL budget is highly dependent on the choice of the spatial averaging filter. In addition, comparing the trends and variations for both the global mean sea level time series and those estimated for mass and steric components, we find that spatial averaging functions play a significant role in the sea level budget closure.
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| 3. |
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| 4. |
- Bagherbandi, Mohammad, Professor
(författare)
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Challenges and Solutions for Establishing Precise Geodetic Control Networks: Introducing an Innovative Method
- 2024
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Human-made infrastructure, such as dams, bridges, tunnels, and high towers, requires highly precise geodetic control networks and continuous monitoring to detect potential failure risks and plan civil engineering maintenance works. In classical 2D geodetic networks, reducing slope distances to horizontal ones is an important task for engineers. The common practice for this reduction involves using vertical angles and applying trigonometric rules. However, using vertical angles introduces systematic errors, primarily due to air refraction, deflections of the vertical (DOV), and the geometric effects of the reference surface, whether it is a sphere or an ellipsoid. Therefore, employing vertical angles in establishing geodetic control networks in 2D is challenging due to these systematic errors. To mitigate the refraction and DOV effects, reciprocal observations of vertical angles can be considered, especially if the elevation differences are small. In this study, we quantify these effects and propose an innovative solution to eliminate these systematic errors in small-scale geodetic networks. Specifically, we propose a new technique that does not rely on vertical angles for the reduction of distances, which is called the network-aided method. Thus, the geometric, physical, and refraction effects cancel out in this method. The results of this study hold significant importance for surveying guidelines. The main advantage of the proposed method is less fieldwork and, hence cost reduction since there is no need for different OFF-construction (reference) and ON-construction (monitoring) networks. Consequently, the number of network points will be less than in traditional networks. There is no need for reciprocal observations since vertical angles are not utilized, while the precision remains equal or even superior (in terms of quality factors i.e., higher redundancy numbers and smaller error ellipses).
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| 5. |
- Bagherbandi, Mohammad, Professor, et al.
(författare)
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How isostasy explains continental rifting in East Africa?
- 2020
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Konferensbidrag (populärvet., debatt m.m.)abstract
- The principle of isostasy plays an important role to understand the relation between different geodynamic processes. Although, it is difficult to find an exact method that delivers a complete image of the Earth structure. However, gravimetric methods are alternative to provide images of the interior of the Earth. The Earth’s crust parameters, i.e. crustal depth and crust-mantle density contrast, can reveal adequate information about the solid Earth system such as volcanic activity, earthquake and continental rifting. Hence, in this study, a combine Moho model using seismic and gravity data is determined to investigate the relationship between the isostatic state of the lithosphere and seismic activities in East Africa. Our results show that isostatic equilibrium and compensation states are closely correlated to the seismicity patterns in the study area. For example, several studies suggest that African superplume causes the rift valley, and consequently differences in crustal and mantle densities occur. This paper presents a method to determine the crustal thickness and crust-mantle density contrast and consequently one can observe low-density contrast (about 200 kg/m3 ) and thin crust (about 30 km) near the triple junction plate tectonics in East Africa (Afar Triangle), which confirms the state of overcompensation in the rift valley areas. Furthermore, the density structure of the lithosphere shows a large correlation with the earthquake activity, sub-crustal stress and volcanic distribution across East Africa.
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| 6. |
- Bagherbandi, Mohammad, Professor, et al.
(författare)
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Studying permafrost using GRACE and in situ data in the northern high-latitudes regions
- 2019
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Konferensbidrag (populärvet., debatt m.m.)abstract
- There is an exceptional opportunity of achieving simultaneous and complementary data from a multitude of geoscience and environmental near-earth orbiting artificial satellites to study phenomena related to the climate change e.g. sea level change, ice melting, soil moisture variation, temperature changes, and earth surface deformations. In this study, we focus on permafrost thawing and its associated gravity change, and organic material changes using GRACE data and other satellite- and ground-based observations. The estimation of permafrost changes requires combining information from various sources, particularly using the gravity field change, surface temperature change, and GIA. The most significant factor for careful monitoring of the permafrost thawing is the fact that this process could be responsible for releasing an additional enormous amount of greenhouse gases emitted to the atmosphere, most importantly to mention Carbone dioxide and Methane that are currently stored in the frozen ground. The results of a preliminary numerical analysis reveal a possible existence of a high correlation between the secular trends of greenhouse gases, temperature and equivalent water thickness in the selected regions. Furthermore, according to our estimates based on processing the GRACE data, the groundwater storage attributed to the due to permafrost thawing increased at the annual rates of 3.4, 3.8, 4.4 and 4.0 cm, in Siberia, northern Alaska, and Canada. Despite a rather preliminary character of our results, these findings indicate that the methodology developed and applied in this study should be improved by incorporating the in situ permafrost measurements.
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| 7. |
- Bagherbandi, Mohammad, Professor, et al.
(författare)
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Towards a world vertical datum defined by the geoid potential and Earth’s ellipsoidal parameters
- 2018
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Sustainable development and digitalization need reliable data. Geospatial data becomes a more and more important tool in society for many kinds of research of immediate use, but also for future planning and enterprise. Harmonization of geodata is very important for data producers and organizations, e.g. for mapping agencies. Establishing a uniform horizontal/vertical reference system is a basic prerequisite for combining data from different sources, and for allowing cross-border presentations and analyzes. If we do not use the same reference for positioning, it is not certain that one can compose reliable geodata from different organizations.The overall aim of this study is to provide a theoretical and practical solution to unifying height systems in order to overcome systematic datum inconsistencies in height data and digital terrain models. The study deals with a variety of issues in physical geodesy such as Earth’s gravity field, sea level rise, sea surface topography and GNSS data. The advent of satellite altimetry in the 1970s provided a tool for the realization of a global vertical datum as being the equipotential surface of the Earth’s gravity field that minimizes the sea-surface topography (SST) all over the oceans in a least-squares sense. This leads to a direct determination of the geoid potential (W0) from satellite altimetry and an Earth Gravitational Model (EGM). In contrast, here we will first determine the Mean Earth Ellipsoid parameters and from these follows W0. This means that once the size of the axes of the globally best-fitting ellipsoid is determined, W0 follows. A major problem with this method is that satellite altimetry is only successful over the oceans, but the method requires global data. This problem is solved by employing satellite altimetry and the EGM in a practical combination.
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| 8. |
- Gido, Nureldin A. A., et al.
(författare)
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Satellite monitoring of mass changes and ground subsidence in Sudan’s oil fields using GRACE and Sentinel-1 data
- 2020
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Konferensbidrag (populärvet., debatt m.m.)abstract
- Monitoring environmental hazards, due to natural and anthropogenic causes, is one of the important issues, which requires proper data, models, and cross-validation of the results. The geodetic satellite missions, e.g. the Gravity Recovery and Climate Experiment (GRACE) and Sentinel-1, are very useful in this aspect. GRACE missions are dedicated to model the temporal variations of the Earth’s gravity field and mass transportation in the Earth’s surface, whereas Sentinel-1 collects Synthetic Aperture Radar (SAR) data which enables us to measure the ground movements accurately. Extraction of large volumes of water and oil decreases the reservoir pressure, form compaction and consequently land subsidence occurs which can be analyzed by both GRACE and Sentinel-1 data. In this paper, large-scale groundwater storage (GWS) changes are studied using the GRACE monthly gravity field models together with different hydrological models over the major oil reservoirs in Sudan, i.e. Heglig, Bamboo, Neem, Diffra and Unity-area oil fields. Then we correlate the results with the available oil wells production data for the period of 2003-2012. In addition, using the only freely available Sentinel-1 data, collected between November 2015 and April 2019, the ground surface deformation associated with this oil and water depletion is studied. Due to the lack of terrestrial geodetic monitoring data in Sudan, the use of GRACE and Sentinel-1 satellite data is very valuable to monitor water and oil storage changes and their associated land subsidence over our region of interest. Our results show that there is a significant correlation between the GRACE-based GWS change and extracted oil and water volumes. The trend of GWS changes due to water and oil depletion ranged from -18.5 to -6.2mm/year using the CSR GRACE monthly solutions and the best tested hydrological model in this study. Moreover, our Sentinel-1 SAR data analysis using Persistent Scatterer Interferometry (PSI) method shows high rate of subsidence i.e. -24.5, -23.8, -14.2 and -6 mm/year over Heglig, Neem, Diffra and Unity-area oil fields respectively. The results of this study can help us to control the integrity and safety of operations and infrastructure in that region, as well as to study the groundwater/oil storage behavior.
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| 9. |
- Shafiei, Mehdi, et al.
(författare)
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A satellite-based gravimetric approach to GIA modelling
- 2018
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Konferensbidrag (populärvet., debatt m.m.)abstract
- In view of the GRACE Follow-On mission, we will study the capability of temporal gravity field from the GRACE data to detect the present gravity variation in the process of Glacial Isostatic Adjustment (GIA). Motivated to reducing the dependency of GIA to the knowledge of the ice load history modelling, also, to researching various GRACE-type signal analysis methodsA number of gravimetric GIA modelling methods are investigated in order to their compatibility with a total number of 515 GPS data points in North America and Fennoscandia. We investigate three mathematical methods, namely regression, principal component, and independent component analysis (ICA) in extracting the GIA signal from the GRACE monthly geoid height. To exploit the GRACE data to their maximum spatial resolution we will utilize some regularization techniques to recover the signal-to-noise ratio of the short wavelengths. One of the results of this investigation is the relative success of the ICA method. We will produce our final gravimetric model using the fast-ICA algorithm of Hyvärinen and Oja (2000). The gravimetric models will be shown to be in a complete agreement with the GPS data and the best GIA forward model for the areas near the centre of the uplifting regions. We will show that the gravimetric method provides a relatively high-resolution GIA model, while largest discrepancies occur in Alaska, and Svalbard collocated with the present ice mass changes.Finally, we assimilate the best gravimetric models and the GPS data into a GIA forward model, for North America and Fennoscandia, while the contribution of the forward model is set to minimum and compare it with the ICE-6g_C (Peltier et al. 2015) model. We found that for the whole area subjected to epeirogenic movement, their discrepancies reach to the extrema at -1.8 and +3.3, and -0.45 and +0.75 mm⁄a, respectively. Further improvement of the gravimetric model is achieved after reducing for the strong hydrological gravity signals.
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| 10. |
- Tenzer, Robert, et al.
(författare)
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Theoretical and practical aspects of defining the heights for planets and moons
- 2018
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Konferensbidrag (populärvet., debatt m.m.)abstract
- In planetary sciences, the geodetic (geometric) heights defined with respect to the reference surface (the sphere or the ellipsoid) or with respect to the center of the planet/moon are typically used for mapping topographic surface, compilation of global topographic models, detailed mapping of potential landing sites, and other space science and engineering purposes. Nevertheless, certain applications, such as studies of gravity-driven mass movements, require the physical heights to be defined with respect to the equipotential surface. Taking the analogy with terrestrial height systems, the realization of height systems for telluric planets and moons could be done by means of defining the orthometric and geoidal heights. In this case, however, the definition of the orthometric heights in principle differs. Whereas the terrestrial geoid is described as an equipotential surface that best approximates the mean sea level, such a definition for planets/moons is irrelevant in the absence of (liquid) global oceans. A more natural choice for planets and moons is to adopt the geoidal equipotential surface that closely approximates the geometric reference surface (the sphere or the ellipsoid). In this study, we address these aspects by proposing a more accurate approach for defining the orthometric heights for telluric planets and moons from available topographic and gravity models, while adopting the average crustal density in the absence of reliable crustal density models. In particular, we discuss a proper treatment of topographic masses in the context of gravimetric geoid determination. In numerical studies, we investigate differences between the geodetic and orthometric heights, represented by the geoidal heights, on Mercury, Venus, Mars, and Moon. Our results reveal that these differences are significant. The geoidal heights on Mercury vary from − 132 to 166 m. On Venus, the geoidal heights are between − 51 and 137 m with maxima on this planet at Atla Regio and Beta Regio. The largest geoid undulations between − 747 and 1685 m were found on Mars, with the extreme positive geoidal heights under Olympus Mons in Tharsis region. Large variations in the geoidal geometry are also confirmed on the Moon, with the geoidal heights ranging from − 298 to 461 m. For comparison, the terrestrial geoid undulations are mostly within ± 100 m. We also demonstrate that a commonly used method for computing the geoidal heights that disregards the differences between the gravity field outside and inside topographic masses yields relatively large errors. According to our estimates, these errors are − 0.3/+ 3.4 m for Mercury, 0.0/+ 13.3 m for Venus, − 1.4/+ 125.6 m for Mars, and − 5.6/+ 45.2 m for the Moon.
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