On the calibration of a superconducting gravimeter using absolute gravity measurements

International audienceA 24 hr continuous parallel registration between an absolute free-fall gravimeter and a relative cryogenic gravimeter is analysed. Different adjustment procedures (L,, L2 norms) are applied to the sets of absolute and relative readings in order to estimate the value of the calibration factor of the superconducting meter, as well as its uncertainty. In addition, a sensitivity test is performed to investigate the influence of some parameters (like the laser frequency and its short-term drift) upon this factor. The precision in the calibration factor is found to be better than 1 per cent, but systematic effects related to the short time interval may add another one and half per cent uncertainty. From preliminary results, it appears that this calibration experiment leads to a close agreement between the values of the gravimetric factor for the reference tidal wave O1 observed with the superconducting meter and the theoretical value (Dehant-Wahr body tide + ocean loading)

A simple method to retrieve the complex eigenfrequency of the Earth's nearly diurnal-free wobble; application to the Strasbourg superconducting gravimeter data

International audienceWe have analysed more than four years of data from the Strasbourg superconducting gravimeter to retrieve the period and damping of the nearly diurnal-free wobble (NDFW). The removal of noise spikes is found to be crucial for an accurate determination of tidal-wave amplitudes and phases. A new simple algorithm is derived which allows an analytical solution for the NDFW pertod and damping using the complex gravimetric factors of three resonant diurnal waves. The results show a huge reduction of the confidence intervals when compared with a previous investigation from a Lacoste Romberg spring meter operated at the same station. Our results are in close agreement with values obtained from two other European superconducting gravimeters. The results are also compared with respect to values inferred from very long baseline interferometry (VLBI) measurements

Fluid core dynamics and degree-one deformations: Slichter mode and geocenter motions

International audienceMagnetic pressures as well as viscous tractions exist at the Core–Mantle-Boundary (CMB) and Inner Core Boundary (ICB) induced by the fluid motions. These internal pressure and tangential tractions involve degree-one deformations. Degree-one deformation has particular characteristics related to geodesy as well as to mechanics: it is related to the center of mass of the Earth and is strongly dependent on the choice of the origin of the reference frame. If the reference frame is centered to the center of mass of the Earth (as usual in elasto-gravitational studies), there is no possible degree-one geopotential perturbations in the Free Space, and consequently, the total degree-one force acting on the Earth has to be equal to zero. It imposes a relation on the degree-one coefficients of the pressures and tractions acting, respectively, at the boundary of the fluid outer core. In this study, we analytically compute the degree-one elasto-gravitational deformations for a simple Earth model (constituted of three homogeneous layers: a solid deformable inner core, a liquid outer core and an elastic mantle), and we present two geophysical applications: (1) first, at short time-scale, we investigate the Slichter mode and its possible excitation by dynamic pressure at the core boundary. (2) Second, at decadal time-scale, we compute the gravity perturbations and the geocenter motion induced by degree-one geostrophic pressures acting at both CMB and ICB

Investigations of Tides from the Antiquity to Laplace

International audienceTidal phenomena along the coasts were known since the prehistoric era, but a long journey of investigations through the centuries was necessary from the Greco-Roman Antiquity to the modern era to unravel in a quasi-definitive way many secrets of the ebb and flow. These investigations occupied the great scholars from Aristotle to Galileo, Newton, Euler, d'Alembert, Laplace, and the list could go on. We will review the historical steps which contributed to an increasing understanding of the tides

Analytical Solutions of Love Numbers for a Hydrostatic Ellipsoidal Incompressible Homogeneous Earth

International audienceTidal forces acting on the Earth cause deformations and mass redistribution inside the planet involving surface motions and variation in the gravity field, which may be observed in geodetic experiments. Because for space geodesy it is now necessary to achieve the mm level in tidal displacements, we take into account the hydrostatic flattening of the Earth in the computation of the elasto-gravitational deformations. Analytical solutions are derived for the semi-diurnal tides on a slightly elliptical homogeneous incompressible elastic model. That simple analytical Earth's model is not a realistic representation of any real planet, but it is useful to understand the physics of the problem and also to check numerical procedures. We rediscover and discuss the Love's solutions and obtain new analytical solutions for the tangential displacement. We extend these analytical results to some geodetic responses of the Earth to tidal forces such as the perturbation of the surface gravity field, the tilt and the deviation of the vertical with reference to the Earth's axis

The effects of the atmospheric pressure on nutations

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