Surface atmospheric pressure excitation of the translational mode of the inner core

Using hourly atmospheric surface pressure field from ECMWF (European Centre for Medium-Range Weather Forecasts) and from NCEP (National Centers for Environmental Prediction) Climate Forecast System Reanalysis (CFSR) models, we show that atmospheric pressure fluctuations excite the translational oscillation of the inner core, the so-called Slichter mode, to the sub-nanogal level at the Earth surface. The computation is performed using a normal-mode formalism for a spherical, self-gravitating anelastic PREM-like Earth model. We determine the statistical response in the form of power spectral densities of the degree-one spherical harmonic components of the observed pressure field. Both hypotheses of inverted and non-inverted barometer for the ocean response to pressure forcing are considered. Based on previously computed noise levels, we show that the surface excitation amplitude is below the limit of detection of the superconducting gravimeters, making the Slichter mode detection a challenging instrumental task for the near future

Earth's Interior from VLBI and Superconducting Gravimeter Data

International audienceWe analyze VLBI delays back to 1984 from the permanent geodetic network, and ten superconducting gravimeter records from the Global Geodynamic Project spanning more than 7 years. From the former data, we deduce nutation o sets, and from the latter we get gravimetric factors. Comparison of these observed quantities against theoretical expressions of the core and mantle admittance to the tidal potential allows us to estimate Love numbers and resonant frequencies and quality factors of the core. We point out strengths and deficiencies of each technique in their ability to retrieve the Earth's interior parameters

Application of a non-linear damped harmonic analysis method to the normal modes of the Earth

International audienceThe search for weak oscillations in noisy records requires the development of analysis tools more sophisticated than the Fourier spectrum analysis. We present in this paper a new non-linear damped harmonic analysis (NLDHA) method that enables us to detect any damped harmonic signal hidden in noise and to simultaneously estimate the amplitude, frequency, phase and damping factor of the harmonic signal. We also compare this newly developed NLDHA method with a conventional auto-regressive method called ESPRIT. The application of the NLDHA method to the seismic normal modes 0S2 and 0S3 excited after the 2004 Sumatra-Andaman earthquake highlights its ability to precisely recover the frequencies and quality factors of the singlets that constitute these two seismic modes. The application of the NLDHA in the sub-seismic frequency band reveals the presence of some unmodelled waves but no detection of the translational oscillation of the inner core has been claimed

A Search for the Free Inner Core Nutation in VLBI Data

International audienceWe investigate the time-frequency spectrum of VLBI nutation series, including operational series released by IVS analysis centers. We focus on the interannual prograde band, where the signal as- sociated with the free inner core nutation (FICN) should be searched. We do not conclude on the detection of the FICN. However, we point out some signi cant but unexplained structures within the FICN frequency band. We also investigate whether their origin is geophysical or they are artifacts due to VLBI analysis methods or reference frame effects

Monitoring of groundwater redistribution in a karst aquifer using a superconducting gravimeter

Geodetic tools monitor the earth’s deformation and gravity field. They are presently sensitive enough to record subtle changes triggered by hydrological processes, thus providing complementary data to standard hydrological measurements. Among these tools, superconducting gravimeter (SG) have proven useful to unravel groundwater redistribution, which significantly alter the gravity field. In the frame of the EquipEx MIGA (Matter wave-laser based Interferometer Gravitation Antenna) project, one SG (iOSG-24) was set up in July 2015 in the Low-noise Underground Laboratory (LSBB) at Rustrel, France, in a gallery located 500 m beneath the surface. In this work, we analyse the underground iOSG-24 gravity time series together with hydro-meteorological data and basic gravity modelling. We find that the gravimeter recorded the redistribution of water in the ground and that most of this redistribution occurs in the unsaturated zone located above the gravimeter. Nevertheless, residuals between our model and the gravity data suggest the occurrence of large lateral fluxes and rapid runoff not considered in our model. We discuss how the setting of a second SG, planned in July 2018, at the surface of the LSBB could help unravelling such hydrological processes

Editorial note for the geodesy and geodynamics journal special issue contemporary research in geodynamics and earth tides-Selection from the 19th international symposium on geodynamics and earth tides, 2021, Wuhan, China

Modern geodetic technologies such as high-precision ground gravity measurement, satellite gravity measurement, global navigation satellite system, remote sensing, etc. provide rich observation data for monitoring various geodynamic processes on the of the global earth and on its surface surface and inside. The 19th International Symposium on Geodynamics and Earth Tides brought together scientific researchers from 26 countries around the world, shared the application of various measurements in different geoscience issues, covering Earth tidal deformation, oceanic and atmospheric loading effects, earthquake cycle, hydrology, Earth rotation changes, etc., and provided a very valuable exchange platform for global peers. Keywords: Earth tides, Gravity, Earthquake, Earth rotation, Hydrolog

Variations temporelles de la gravité en relation avec la dynamique interne de la Terre - Apport des gravimètres supraconducteurs

The Earth's dynamics and interior structure are badly constrained, particularly in the deep interior, because the surface induced signals are of weak amplitude and low frequency and reach the detection level of seismometers. Gravimetry is then a privileged tool to investigate the Earth's interior at these long periods. Time varying gravity is permanently recorded at the surface by a worldwide network of cryogenic relative gravimeters. We study the noise levels of all these gravimetric stations over a wide frequency range, from the long period seismic normal modes to the tides. We show that superconducting gravimeters can uniquely contribute to the study of the low frequency Earth's vibrations, which sample the Earth from the surface to the core. We highlight (1) the splitting due to rotation and ellipticity of the 0S2 normal mode into five singlets, (2) the first observation of the 2S1 seismic mode, after the 2001, June 23rd Peru earthquake with a magnitude 8.4.We search for the normal mode of translation of the inner core, which has never been clearly observed. Its detection is essential as it would constrain the density jump at the inner core boundary, probably also the viscosity and the outer core stratification. For that purpose, we develop some stacking, automatic detection of signals methods and a wavelet representation.La dynamique et la structure interne de la Terre sont mal contraintes en profondeur, car les signaux induits en surface, de faible amplitude et basse fréquence, atteignent le seuil de détection des sismomètres. La gravimétrie devient un outil privilégié d'investigation de la Terre profonde aux longues périodes. Les variations temporelles de la gravité sont enregistrées en permanence par des gravimètres relatifs cryogéniques. Nous étudions les niveaux de bruit de ces stations sur un large spectre de fréquences allant des modes propres sismiques longue période jusqu'aux marées. Nous montrons que les gravimètres supraconducteurs apportent une contribution unique à l'étude des modes de vibration les plus graves qui échantillonnent la Terre de la surface jusqu'au noyau. Nous mettons en avant (1) l'éclatement dû à la rotation et à l'ellipticité du mode propre 0S2 en cinq singlets, (2) la première observation du mode sismique 2S1, après le séisme du Pérou du 23 juin 2001 de magnitude 8.4.Nous nous consacrons à la recherche du mode propre de translation de la graine 1S1 qui n'a jamais été observé sans ambiguïté. Sa détection est primordiale car elle permettrait de contraindre le saut de densité à l'interface graine - noyau liquide, ainsi que la viscosité à cette interface et la stratification du noyau fluide. Pour cela nous développons des méthodes de sommation, de détection automatique des signaux et une famille d'ondelettes

Observations Gravimétriques et Modélisation de la Dynamique Globale de la Terre

The observation and analysis of Earth’s normal modes bring some information on the deep Earth’s interior and dynamics. We can cite the Earth’s rotational and seismic modes. In the search of the signature of some elusive signals associated with the Earth’s inner core oscillations (the « Slichter modes ») that have never been detected, we have theoretically computed their expected amplitudes for various geophysical processes : the surface atmospheric masses, the earthquakes and the pressure flows in the fluid outer core. We have also briefly looked at the possible effect of the kinetics of phase changes on the Slichter mode. For that, we have used a simple model of dendritic growth at the ICB and shown that the phase changes do not disturb the Slichter motion. Besides, we have reconciled the gravimetric and geodetic VLBI nutation observations of the Earth’s response to the lunisolar tides by inverting the resonance parameters associated with the free core nutation. The free inner-core nutation has not yet been detected, either in VLBI nutation data, or in surface gravimetric records. Finally, we consider the interactions of gravitational waves with the Earth through their coupling with seismic modes.L’observation et l’analyse des modes propres de la Terre apportent des contraintes fondamentales sur la structure et sur la dynamique interne de notre planète. On peut ainsi citer les modes de rotation et les modes sismiques. Dans le cadre de la recherche de petits signaux venant de la Terre profonde et induits par les modes oscillatoires de la graine (les « modes de Slichter ») qui n’ont jamais été détectés, nous avons modélisé leur excitation par plusieurs sources possibles : les masses atmosphériques, les séismes et des variations de pressions fluides dans le noyau liquide. Nous avons également abordé le problème de la cinématique des changements de phase des cristaux de fer qui existent à ces conditions élevées de température et de pression. Ce problème de transition de phase durant un déplacement de l’interface noyau liquide - graine solide pourrait rendre ces modes d’oscillation de la graine inexistants. Une première estimation reposant sur un modèle simple de croissance dendritique à l’ICB a montré que les changements de phase ne perturbent pas les modes de Slichter. En outre, nous avons réconcilié les observables gravimétriques et géodésiques VLBI des marées luni-solaires et leurs effets en termes de réponse de la Terre en déformation (marées gravimétriques) et en rotation (nutations astronomiques), en inversant les paramètres de la résonance associée à la nutation propre du noyau. La nutation propre de la graine n’a cependant pas encore été détectée, ni dans les données de nutation VLBI, ni dans les données gravimétriques de surface. Finalement, nous nous intéressons à l’interaction des ondes gravitationnelles avec la Terre, en particulier via leur couplage avec les modes propres sismiques