| 1. |
- Adamczyk, Anna, et al.
(författare)
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Application of first-arrival tomography to characterize a quick clay landslide site in Southwest Sweden
- 2013
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Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 61:5, s. 1057-1073
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Tidskriftsartikel (refereegranskat)abstract
- First-arrival traveltime tomography was applied to high-resolution seismic data acquired over a known quick-clay landslide scar near the Gota River in southwest Sweden in order to reveal the geometry and physical properties of clay-related normally consolidated sediments. Investigated area proved to be a challenging environment for tomographic imaging because of large P-wave velocity variations, ranging from 500 to 6000 m/s, and relatively steeply-dipping bedrock. Despite these challenges, P-wave velocity models were obtained down to ca. 150 m for two key 2D seismic profiles (each about 500-m long) intersecting over the landslide scar. The models portrait the sandwich-like structure of marine clays and coarse-grained consolidated sediments, but the estimated resolution (20 m) is too small to distinguish thin layers within this structure. Modelled velocity structures match well the results of reflection seismic processing and resistivity tomography available along the same profiles.
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| 2. |
- Eshagh, Mehdi
(författare)
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Alternative Expressions for Gravity Gradients in Local North-Oriented Frame and Tensor Spherical Harmonics
- 2010
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Ingår i: ACTA GEOPHYS. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 58:2, s. 215-243
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Tidskriftsartikel (refereegranskat)abstract
- The traditional expressions for gravity gradients in local north-oriented frame and tensor spherical harmonics have complicated forms involved with first- and second-order derivatives of spherical harmonics and also singular terms. In this paper we present alternative expressions for these quantities, which are simpler and contain no singular terms. The presented formulas are useful for those disciplines of geosciences which are involved with potential theory, tensor spherical harmonics and second-order derivatives of spherical harmonic series in the local northoriented frame. A simple numerical test on the solution of the gradiometric boundary value problems presents the correctness of these new expressions and ability of the solutions to continue the gravity gradients from satellite level down to sea level using spherical harmonics.
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| 3. |
- Eshagh, Mehdi
(författare)
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Comparison of two approaches for considering laterally varying density in topographic effect on satellite gravity gradiometric data
- 2010
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Ingår i: ACTA GEOPHYS. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 58:4, s. 661-686
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Tidskriftsartikel (refereegranskat)abstract
- The satellite gravity gradiometric data are influenced by laterally varying density in topographic masses, while in most of studies a constant density for the masses was considered. This assumption causes an error in estimating the topographic effect. This paper theoretically and numerically investigates the methods of Sjoberg as well as Novak and Grafarend to consider the laterally varying density for topographic masses in formulation of topographic potential in spherical harmonics.
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| 4. |
- Eshagh, Mehdi
(författare)
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On integral approach to regional gravity field modelling from satellite gradiometric data
- 2011
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Ingår i: ACTA GEOPHYS. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 59:1, s. 29-54
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Tidskriftsartikel (refereegranskat)abstract
- Solution of the gradiometric boundary value problems leads to three integral formulas. If we are satisfied with obtaining a smooth solution for the Earth's gravity field, we can use the formulas in regional gravity field modelling. In such a case, satellite gradiometric data are integrated on a sphere at satellite level and continued downward to the disturbing potential (geoid) at sea level simultaneously. This paper investigates the gravity field modelling from a full tensor of gravity at satellite level. It studies the truncation bias of the integrals as well as the filtering of noise of data. Numerical studies show that by integrating T (zz) with 1 mE noise and in a cap size of 7A degrees, the geoid can be recovered with an error of 12 cm after the filtering process. Similarly, the errors of the recovered geoids from T (xz,yz) and T (xx-yy, 2xy) are 13 and 21 cm, respectively.
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| 5. |
- Eshagh, Mehdi, 1977-, et al.
(författare)
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On regularized time varying gravity field models based on GRACE data and their comparisons with hydrological models
- 2013
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Ingår i: Acta Geophysica. - : Springer. - 1895-6572 .- 1895-7455. ; 61:1, s. 1-17
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Tidskriftsartikel (refereegranskat)abstract
- 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
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| 6. |
- Eshagh, Mehdi, et al.
(författare)
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Smoothing impact of isostatic crustal thickness models on local integral inversion of satellite gravity gradiometry data
- 2011
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Ingår i: ACTA GEOPHYSICA. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 59:5, s. 891-906
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Tidskriftsartikel (refereegranskat)abstract
- 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. Here we 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 on Vening 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.
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| 7. |
- Gido, Nureldin A. A., et al.
(författare)
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Studying permafrost by integrating satellite and in situ data in the northern high-latitude regions
- 2019
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Ingår i: Acta Geophysica. - : Springer. - 1895-6572 .- 1895-7455. ; 67:2, s. 721-734
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Tidskriftsartikel (refereegranskat)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. These satellite missions provide the information about the various phenomena, such as 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 (in terms of the groundwater storage), and organic material changes using the gravity recovery and climate experiment (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 glacial isostatic adjustment. The most significant factor for a 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 carbon dioxide (CO2) 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 (CO2), temperature and equivalent water thickness (in permafrost active layer) in the selected regions. Furthermore, according to our estimates based on processing the GRACE data, the groundwater storage attributed due to permafrost thawing increased at the annual rates of 3.4, 3.8, 4.4 and 4.0 cm, respectively, in Siberia, North Alaska and Canada (Yukon and Hudson Bay). Despite a rather preliminary character of our results, these findings indicate that the methodology developed and applied in this study should be further improved by incorporating the in situ permafrost measurements.
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| 8. |
- Grusson, Youen
(författare)
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Assessment of potential soil erosion and sediment yield in the semi-arid Nfis basin (High Atlas, Morocco) using the SWAT model
- 2019
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Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 67, s. 263-272
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Tidskriftsartikel (refereegranskat)abstract
- Arid and semi-arid regions face multiple problems in terms of water management, particularly where surface water is a primary resource. On the northern flank of the High Atlas Mountains (Morocco), deforestation has resulted in soil erosion and siltation of reservoirs. Better characterization of sediment yield is important for reducing the impact of siltation and prolonging the lifespan of dams. The Soil and Water Assessment Tool was used for modeling the Nfis basin in the southern Tensift watershed, leading to a better understanding of the rate of siltation behind Lalla Takerkoust dam. Runoff and sediment yield simulations were evaluated using graphical and statistical methods. The SWAT model performed well in estimating sediment load during the calibration period from 1990 to 2015 (Nash-Sutcliffe efficiency=0.5-0.62, R-2=0.5-0.61). The model enabled the determination of soil loss within each hydrological response unit in the watershed. The overall rate was approximately 123tHa(-1) for an average annual rainfall of 315mmyr(-1). This high yield has to be taken into account for effective water-resources management in the Nfis basin.
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| 9. |
- Kiamehr, Ramin, et al.
(författare)
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Interpretation of general geophysical patterns in Iran based on GRACE gradient component analysis
- 2008
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Ingår i: Acta Geophysica. - : Springer Science and Business Media LLC. - 1895-6572 .- 1895-7455. ; 56:2, s. 440-454
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Tidskriftsartikel (refereegranskat)abstract
- Only with satellites it is possible to cover the entire Earth densely with gravity field related measurements of uniform quality within a short period of time. However, due to the altitude of the satellite orbits, the signals of individual local masses are strongly damped. Based on the approach of Petrovskaya and Vershkov we determine the gravity gradient tensor directly from the spherical harmonic coefficients of the recent EIGEN-GL04C combined model of the GRACE satellite mission. Satellite gradiometry can be used as a complementary tool to gravity and geoid information in interpreting the general geophysical and geodynamical features of the Earth. Due to the high altitude of the satellite, the effects of the topography and the internal masses of the Earth are strongly damped. However, the gradiometer data, which are nothing else than the second order spatial derivatives of the gravity potential, efficiently counteract signal attenuation at the low and medium frequencies. In this article we review the procedure for estimating the gravity gradient components directly from spherical harmonics coefficients. Then we apply this method as a case study for the interpretation of possible geophysical or geodynamical patterns in Iran. We found strong correlations between the cross-components of the gravity gradient tensor and the components of the deflection of vertical, and we show that this result agrees with theory. Also, strong correlations of the gravity anomaly, geoid model and a digital elevation model were found with the diagonal elements of the gradient tensor.
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| 10. |
- Kiamehr, Ramin, et al.
(författare)
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Interpretation of the general geophysical patterns of Iran based on the gradient components analysis of the GRACE
- 2008
-
Ingår i: Acta Geophysica. - : Springer. - 1895-6572 .- 1895-7455. ; 56:2, s. 440-454
-
Tidskriftsartikel (refereegranskat)abstract
- Only with satellites it is possible to cover the entire Earth densely with gravityfield related measurements of uniform quality within a short period of time. However,due to the altitude of the satellite orbits, the signals of individual local massesare strongly damped. Based on the approach of Petrovskaya and Vershkov we determinethe gravity gradient tensor directly from the spherical harmonic coefficientsof the recent EIGEN-GL04C combined model of the GRACE satellite mission. Satellitegradiometry can be used as a complementary tool to gravity and geoid informationin interpreting the general geophysical and geodynamical features of theEarth. Due to the high altitude of the satellite, the effects of the topography and theinternal masses of the Earth are strongly damped. However, the gradiometer data,which are nothing else than the second order spatial derivatives of the gravity potential,efficiently counteract signal attenuation at the low and medium frequencies.In this article we review the procedure for estimating the gravity gradientcomponents directly from spherical harmonics coefficients. Then we apply thismethod as a case study for the interpretation of possible geophysical or geodynamicalpatterns in Iran. We found strong correlations between the cross-components ofthe gravity gradient tensor and the components of the deflection of vertical, and weshow that this result agrees with theory. Also, strong correlations of the gravityanomaly, geoid model and a digital elevation model were found with the diagonalelements of the gradient tensor.
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