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An analysis of gravitational gradients in rotated frames and their relation to oriented mass sources / Isabelle Panet in Journal of geophysical research : Solid Earth, vol 123 n° 12 (December 2018)  

Public [article]  inJournal of geophysical research : Solid Earth > vol 123 n° 12 (December 2018) . - pp 11062 -11090 Titre : An analysis of gravitational gradients in rotated frames and their relation to oriented mass sources Type de document : Article/Communication Auteurs : Isabelle Panet , Auteur Année de publication : 2018 Article en page(s) : pp 11062 -11090 Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique [Termes descripteurs IGN] analyse de sensibilité [Termes descripteurs IGN] champ de pesanteur terrestre [Termes descripteurs IGN] gradient de gravitation [Termes descripteurs IGN] levé gravimétrique [Termes descripteurs IGN] masse de la Terre [Termes descripteurs IGN] modèle de géopotentiel [Termes descripteurs IGN] repère de référence Résumé : (auteur) Many mass sources within the Earth and its fluid envelopes show elongated geometries, aligning with the orientations of plate boundaries and plate motions, coastlines, rivers, and drainage basins for instance. To enhance their identification and separation in global or regional gravity observations and models, a dedicated method based on gravitational gradients analysis is presented here. This approach provides a detailed description of the geographic pattern of the gravity variations, which are accurately mapped thanks to the regular spatial coverage of high‐accuracy satellite data and arise from lateral density changes within the planet. First, gravity gradients are defined at different spatial scales in spherical frames, which are rotated along the radial axis according to the orientation of the source. The sensitivity of these gradients to the mass distribution inside a spherical Earth is described and analytical expressions relating the source to the observable are introduced. Then, the gravity gradients responses at different spatial scales to flat, elementary mass sources located at the surface and at increasing depth are studied. Specifically, the paper investigates how a source width and orientation can be determined, for localized and oscillatory mass anomalies with different width‐to‐length aspect ratios. This theoretical case study aims at providing a basis for the analysis of more complex mass structures, when applying the presented method to static or time‐varying satellite gravity field models. It may help deciphering the nature of the gravity sources by the detection of meaningful geometries and orientations in the gravity field. Numéro de notice : A2018-655 Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1029/2018JB016717 date de publication en ligne : 05/12/2018 En ligne : https://doi.org/10.1029/2018JB016717 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=93520 [article]

Topographic gravitational potential up to second-order derivatives: an examination of approximation errors caused by rock-equivalent topography (RET) / Michael Kuhns in Journal of geodesy, vol 90 n° 9 (September 2016)  

Public [article]  inJournal of geodesy > vol 90 n° 9 (September 2016) . - pp 883 – 902 Titre : Topographic gravitational potential up to second-order derivatives: an examination of approximation errors caused by rock-equivalent topography (RET) Type de document : Article/Communication Auteurs : Michael Kuhns, Auteur ; Christian Hirt, Auteur Année de publication : 2016 Article en page(s) : pp 883 – 902 Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique [Termes descripteurs IGN] erreur d'approximation [Termes descripteurs IGN] glace [Termes descripteurs IGN] masse d'eau [Termes descripteurs IGN] masse de la Terre [Termes descripteurs IGN] modèle de densité [Termes descripteurs IGN] potentiel de pesanteur terrestre [Termes descripteurs IGN] roche [Termes descripteurs IGN] rock-equivalent topography Résumé : (auteur) In gravity forward modelling, the concept of Rock-Equivalent Topography (RET) is often used to simplify the computation of gravity implied by rock, water, ice and other topographic masses. In the RET concept, topographic masses are compressed (approximated) into equivalent rock, allowing the use of a single constant mass–density value. Many studies acknowledge the approximate character of the RET, but few have attempted yet to quantify and analyse the approximation errors in detail for various gravity field functionals and heights of computation points. Here, we provide an in-depth examination of approximation errors associated with the RET compression for the topographic gravitational potential and its first- and second-order derivatives. Using the Earth2014 layered topography suite we apply Newtonian integration in the spatial domain in the variants (a) rigorous forward modelling of all mass bodies, (b) approximative modelling using RET. The differences among both variants, which reflect the RET approximation error, are formed and studied for an ensemble of 10 different gravity field functionals at three levels of altitude (on and 3 km above the Earth’s surface and at 250 km satellite height). The approximation errors are found to be largest at the Earth’s surface over RET compression areas (oceans, ice shields) and to increase for the first- and second-order derivatives. Relative errors, computed here as ratio between the range of differences between both variants relative to the range in signal, are at the level of 0.06–0.08 % for the potential, ∼3–7 % for the first-order derivatives at the Earth’s surface (∼0.1 % at satellite altitude). For the second-order derivatives, relative errors are below 1 % at satellite altitude, at the 10–20 % level at 3 km and reach maximum values as large as ∼20 to 110 % near the surface. As such, the RET approximation errors may be acceptable for functionals computed far away from the Earth’s surface or studies focussing on the topographic potential only. However, for derivatives of the functionals computed near the Earth’s surface, the use of RET introduces very spurious errors, in some cases as large as the signal, rendering it useless for smoothing or reducing of field observation, thus rigorous mass modelling should be used for both spatial and spectral domain methods. Numéro de notice : A2016-657 Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern En ligne : http://dx.doi.org/10.1007/s00190-016-0917-6 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=81859 [article]

Fast computation of general forward gravitation problems / Fabien Casenave in Journal of geodesy, vol 90 n° 7 (July 2016)  

Public [article]  inJournal of geodesy > vol 90 n° 7 (July 2016) . - pp 655 – 675 Titre : Fast computation of general forward gravitation problems Type de document : Article/Communication Auteurs : Fabien Casenave, Auteur ; Laurent Métivier , Auteur ; Gwendoline Pajot-Métivier , Auteur ; Isabelle Panet , Auteur Année de publication : 2016 Article en page(s) : pp 655 – 675 Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique [Termes descripteurs IGN] algorithme [Termes descripteurs IGN] France (géographie physique) [Termes descripteurs IGN] masse de la Terre [Termes descripteurs IGN] potentiel de pesanteur terrestre [Termes descripteurs IGN] vitesse Résumé : (auteur) We consider the well-known problem of the forward computation of the gradient of the gravitational potential generated by a mass density distribution of general 3D geometry. Many methods have been developed for given geometries, and the computation time often appears as a limiting practical issue for considering large or complex problems. In this work, we develop a fast method to carry out this computation, where a tetrahedral mesh is used to model the mass density distribution. Depending on the close- or long-range nature of the involved interactions, the algorithm automatically switches between analytic integration formulae and numerical quadratic formulae, and relies on the Fast Multipole Method to drastically increase the computation speed of the long-range interactions. The parameters of the algorithm are empirically chosen for the computations to be the fastest possible while guarantying a given relative accuracy of the result. Computations that would load many-core clusters for days can now be carried out on a desk computer in minutes. The computation of the contribution of topographical masses to the Earth’s gravitational field at the altitude of the GOCE satellite and over France are proposed as numerical illustrations of the method. Numéro de notice : A2016-427 Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern DOI : 10.1007/s00190-016-0900-2 date de publication en ligne : 08/04/2016 En ligne : http://dx.doi.org/ 10.1007/s00190-016-0900-2 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=81321 [article]

Contribution of mass density heterogeneities to the quasigeoid-to-geoid separation / Robert Tenzer in Journal of geodesy, vol 90 n° 1 (January 2016)  

Public [article]  inJournal of geodesy > vol 90 n° 1 (January 2016) . - pp 65-80 Titre : Contribution of mass density heterogeneities to the quasigeoid-to-geoid separation Type de document : Article/Communication Auteurs : Robert Tenzer, Auteur ; Christian Hirt, Auteur ; Pavel Novák, Auteur ; et al., Auteur Année de publication : 2016 Article en page(s) : pp 65-80 Note générale : bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Géodésie physique [Termes descripteurs IGN] anomalie de pesanteur [Termes descripteurs IGN] champ de pesanteur terrestre [Termes descripteurs IGN] harmonique sphérique [Termes descripteurs IGN] masse de la Terre [Termes descripteurs IGN] modèle de densité [Termes descripteurs IGN] quasi-géoïde [Termes descripteurs IGN] topographie Résumé : (auteur) The geoid-to-quasigeoid separation is often computed only approximately as a function of the simple planar Bouguer gravity anomaly and the height of the computation point while disregarding the contributions of terrain geometry and anomalous topographic density as well as the sub-geoid masses. In this study we demonstrate that these contributions are significant and, therefore, should be taken into consideration when investigating the relation between the normal and orthometric heights particularly in the mountainous, polar and geologically complex regions. These contributions are evaluated by applying the spectral expressions for gravimetric forward modelling and using the EIGEN-6C4 gravity model, the Earth2014 datasets of terrain, ice thickness and inland bathymetry and the CRUST1.0 sediment and (consolidated) crustal density data. Since the global crustal density models currently available (e.g. CRUST1.0) have a limited accuracy and resolution, the comparison of individual density contributions is—for consistency—realized with a limited spectral resolution up to a spherical harmonic degree 360 (or 180). The results reveal that the topographic contribution globally varies between −0.33 and 0.57 m, with maxima in Himalaya and Tibet. The contribution of ice considerably modifies the geoid-to-quasigeoid separation over large parts of Antarctica and Greenland, where it reaches ∼0.2 m. The contributions of sediments and bedrock are less pronounced, with the values typically varying only within a few centimetres. These results, however, have still possibly large uncertainties due to the lack of information on the actual sediment and bedrock density. The contribution of lakes is mostly negligible; its maxima over the Laurentian Great Lakes and the Baikal Lake reach only several millimetres. The contribution of the sub-geoid masses is significant. It is everywhere negative and reaches extreme values of −4.43 m. According to our estimates, the geoid-to-quasigeoid separation globally varies within −4.19 and 0.26 m while the corresponding values computed according to a classical definition are only negative and reach extreme values of −3.5 m. A comparison of these results reveals that inaccuracies caused by disregarding the terrain geometry and mass density heterogeneities distributed within the topography and below the geoid surface can reach ±2 m or more in the mountainous regions. Numéro de notice : A2016-019 Thématique : POSITIONNEMENT Nature : Article nature-HAL : ArtAvecCL-RevueIntern En ligne : http://link.springer.com/article/10.1007/s00190-015-0858-5/fulltext.html Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=79466 [article]

Exploring mass variations in the Earth system / Mike Sips in Cartography and Geographic Information Science, Vol 43 n° 1 (January 2016)  

Public [article]  inCartography and Geographic Information Science > Vol 43 n° 1 (January 2016) . - pp 3 - 15 Titre : Exploring mass variations in the Earth system Type de document : Article/Communication Auteurs : Mike Sips, Auteur ; Andrea Unger, Auteur ; Tobias Rawald, Auteur ; Ingo Sagsen, Auteur Année de publication : 2016 Article en page(s) : pp 3 - 15 Note générale : Bibliographie Langues : Anglais (eng) Descripteur : [Vedettes matières IGN] Applications de géodésie spatiale [Termes descripteurs IGN] analyse visuelle [Termes descripteurs IGN] champ de pesanteur terrestre [Termes descripteurs IGN] données GRACE [Termes descripteurs IGN] exploration de données [Termes descripteurs IGN] géophysique interne [Termes descripteurs IGN] masse de la Terre Résumé : (Auteur) Geophysical processes cause a redistribution of masses within system Earth called mass transport. This mass transport induces variations in the observed gravity field of the Earth. A common scientific approach to draw conclusions about geophysical processes is to determine the imprint of individual geophysical processes in observed gravity field variations. For this purpose, modelers follow a sequence of specialized steps. From this sequence, we identified in close collaboration with Earth system modelers at the German Research Center for GeoSciences (GFZ) steps that can significantly benefit from Visual Analytics: (a) finding an applicable release of observed gravity field variations that exhibit the geophysical process of interest (b) separating individual geophysical processes in observed gravity field variations and (c) confirming the reduction of observed gravity field variations to the geophysical process of interest. We identified important analytical requirements for our Visual Analytics approach based on a user- and task-centered design. By providing tailored support for the identified requirements, our Visual Analytics approach provides a valuable expansion of the modeler’s toolbox. Numéro de notice : A2016-109 Thématique : POSITIONNEMENT Nature : Article DOI : 10.1080/15230406.2015.1031702 En ligne : https://doi.org/10.1080/15230406.2015.1031702 Format de la ressource électronique : URL article Permalink : https://documentation.ensg.eu/index.php?lvl=notice_display&id=79927 [article]

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