Fast computation of general forward gravitation problems

International audienceWe consider the well-known problem of the forward computation of the gradient of the gravity 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-cores clusters for days can now be carried-out on a desk computer in minutes. The computation of the topographic correction over France and the global topographic correction at the altitude of the satellite GOCE are proposed as numerical illustrations of the method

Joint analysis of GOCE gravity gradients data of gravitational potential and of gravity with seismological and geodynamic observations to infer mantle properties

International audienceJoint analysis of the seismic velocities and geoid, gravity and gravity gradients are used to constrain the viscosity profile within the mantle as well as the lateral density variations. Recent ESA's Gravity field and steady-state Ocean Circulation Explorer measurements of the second-order derivatives of the Earth's gravity potential give new possibilities to determine these mantle properties. Using a simple mantle model and seismic tomography results, we investigate how the gravitational potential, the three components of the gravity vector and the gravity gradients can bring information on the radial viscosity profile and on the mantle mass anomalies. We start with lateral density variations in the Earth's mantle based either on slab history or deduced from seismic tomography. The main uncertainties are: for the latter case, the relationship between seismic velocity and density-the so-called density/velocity scaling factor-and for the former case, the variation with depth of the density contrast between the cold slabs and the surrounding mantle. We perform a Monte Carlo search for the viscosity and the density/velocity scaling factor profiles within the mantle, which allows to fit the observed geoid, gravity and gradients of gravity. We compute the posterior probability distribution of the unknown parameters, and find that the gravity gradients improve the estimate of the scaling factor within the upper mantle, because of their sensitivity to the masses within the upper mantle, whereas the geoid and the gravity better constrain the scaling factor in the lower mantle. In the upper mantle, it is less than 0.02 in the upper part and about 0.08-0.14 in the lower part, and it is significantly larger for depths greater than 1200 km (about 0.32-0.34). In any case, the density/velocity scaling factor between 670 and 1150 km depth is not well constrained. We show that the viscosity of the upper part of the mantle is strongly correlated with the viscosity of the lower part of the mantle and that the viscosity profile is characterized by a decrease in the lower part of the upper mantle (about 10 20-2 × 10 20 Pa s) and by an increase (about 10 23-2 × 10 23 Pa s) at the top of the lower mantle (between 670 and 1150 km)

Evidence for postglacial signatures in gravity gradients: A clue in lower mantle viscosity

International audienceThe Earth's surface was depressed under the weight of ice during the last glaciations. Glacial Isostatic Adjustment (GIA) induces the slow recession of the trough that is left after deglaciation and is responsible for a contemporary uplift rate of more than 1 cm/yr around Hudson Bay. The present-day residual depression, an indicator of still-ongoing GIA, is difficult to identify in the observed topography, which is predominantly sensitive to crustal heterogeneities. According to the most widespread GIA models, which feature a viscosity of 2- 3 ×1021 Pa s on top of the lower mantle, the trough is approximately 100 m deep and cannot explain the observed gravity anomalies across North America. These large anomalies are therefore usually attributed to subcontinental density heterogeneities in the tectosphere or to slab downwelling in the deep mantle. Here, we use observed gravity gradients (GG) to show that the uncompensated GIA trough is four times larger than expected and that it is the main source of the North American static gravity signal. We search for the contribution to these GGs from mantle mass anomalies, which are deduced from seismic tomography and are mechanically coupled to the global mantle flow. This contribution is found to be small over Laurentia, and at least 82% of the GGs are caused by GIA. Such a contribution from GIA in these GG observations implies a viscosity that is greater than 1022 Pa s in the lower mantle. Our conclusions are a plea for GIA models with a highly viscous lower mantle, which confirm inferences from mantle dynamic models. Any change in GIA modelling has important paleoclimatological and environmental implications, encouraging scientists to re-evaluate the past ice history at a global scale. These implications, in turn, affect the contribution of bedrock uplift to the contemporaneous mass balance over Antarctica and Greenland and thus the present-day ice-melting rate as deduced from the GRACE space mission. Additionally, studies of the thermo-chemical structure of the lithosphere/crust under North America that exploit gravity or geodetic data should be corrected for a GIA model, which is not the case today

Absolute Quantum Gravimeter as a promising field sensor for volcano monitoring

International audienceLa Soufrière, an active volcano in Guadeloupe (French West Indies) monitored by the Volcanological and Seismological Observatory of Guadeloupe (OVSG), requires a comprehensive understanding of mass transfers, including water movements. To address this, a gravity repetition network was established in the 1980s following the volcano's last major eruption in 1976. In 2011, initial absolute measurements were conducted using a Micro-g Lacoste portable absolute gravity meter A10#14. As part of the EQUIPEX RESIF program, aimed at meeting the scientific community's seismic and gravimetric instrument needs in France, the first absolute quantum field gravimeter (AQG-B01) was acquired. This advanced instrument, designed for diverse applications, including volcano gravity monitoring, utilizes atom interferometry with lasers to measure gravity by manipulating 'atomic waves' with a cloud of free-falling cold atoms at a cycling rate of 2 Hz. In March 2023, a fieldwork was undertaken with the AQG-B01 as an initial step toward modern gravity monitoring of La Soufrière. The specific goals included reoccupying stations within the microgravity network and identifying new sites for expanding the network, selecting an appropriate location for a permanent station near the summit based on a Lacoste & Romberg D meter, testing the AQG-B01 under challenging tropical conditions (humidity up to 85%, mean temperature of 24 degree C), assessing the use of an external power supply for the AQG, and evaluating the ease of installation and accuracy of measurements with the AQG, as specified by the manufacturer. This work primarily focuses on the latter three objectives

Ultra-sensitive electrostatic planar acceleration gradiometer for airborne geophysical surveys

International audienceWe propose a new concept of gravity gradiometer, GREMLIT, for the determination of the spatial derivatives of gravitational acceleration during airborne surveys. The core of this instrument is the acceleration gradiometer composed of four ultra-sensitive electrostatic planar accelerometers, inheriting from technologies developed for the GRACE and GOCE satellite gravity missions. Data from these missions have greatly improved our knowledge of the Earth’s gravity field and its time variations. However, resolving wavelengths of a few 10 km or less, beyond the reach of the satellite resolution, is of utmost importance to study a number of crustal geophysical processes and geological structures. We first present the benefits for a new gravity gradiometer, then we describe the planar acceleration gradiometer, which put together with three orthogonal gyroscopes, constitutes the gravity gradiometer GREMLIT. The acceleration gradiometer enables measurement at one point of the horizontal spatial derivatives of the acceleration horizontal components. We explain the measurement principle and describe the computation of the gravity gradients along with the necessary ancillary measurements. From a detailed error budget analysis of the accelerometers, an expected spectral sensitivity below \text{1E/}\sqrt{\text{Hz}} is found in the [10-3, 0.2] Hz measurement bandwidth. To maintain such performance in flight, we finally discuss the adaptation of the acceleration gradiometer to the turbulent airborne environment. To limit the saturation of the accelerometers, we propose to cancel the common-mode output of the acceleration gradiometer by integrating the instrument on a double-gimbal platform controlled by the common-mode. We demonstrate on a real case study that with such a solution, it is technically possible to prevent the saturation of the accelerometers at least 95% of the time and it is not damaging to the airborne survey

From space to lithosphere: inversion of the GOCE gravity gradients. Supply to the Earth’s interior study

International audienceThe emergence of high resolution satellite measurements of the gravitational field (GOCEmission) offers promising perspectives for the study of the Earth’s interior. These new datacall for the development of innovant analysis and interpretation methods. Here we combine a forward prism computation with a Bayesian resolution approach to invert for these gravity gradient data configuration. We apply and test our new method on satellite data configuration, i.e. 225 km height with a global and homogeneous geographic distribution. We first quantify the resolution of our method according to both data and parameterization characteristics. It appears that for reasonable density contrast values (0.1 g.cm−3) crustal structures have to be wider than ∼28 km to be detectable in the GOCE signal. Deeper bodies are distinguishable for greater size (35 km size at 50 km depth, ∼80 km at 300 km depth). We invert the six tensor components, among which five are independent. By carefully testing each of them and their different combinations, we enlighten at rade off between the recovery of data and the sensitivity to inversion parameters. We particularly discussed this characteristic in terms of geometry of the synthetic model tested (structures orientation, 3-D geometry, etc.). In terms of RMS value, each component is always better explained if inverted solely, but the result is strongly affected by the inversion parameterization (smoothing, variances, etc.). On the contrary, the simultaneous inversion of several components displays a significant improvement for the global tensor recovery, more dependent on data than on density variance or on smoothness control. Comparing gravity and gradient inversions, we highlight the superiority of the GG data to better reproduce the structures especially in terms of vertical location. We succesfully test our method on a realistic case of a complex subduction case for both gradient and gravity data. While the imaging of small crustal structures requires terrestrial gravity dataset, the longest wave length of the slab is well recovered with both data sets. The precision and homogeneous coverage of GOCE data however, counterbalance the heterogeneous and often quite non existance coverage of terrestrial gravity data. This is particularly true in large areas which requires a coherent assemblage of heterogeneous data sets, or in highr elief, vegetally covered and offshore zone

Direct Measurement of Daily Evapotranspiration from a Deciduous Forest using a Superconducting gravimeter

We have a direct measurement of evapotranspiration at the 50 ha scal

Résif-Gmob et l'infrastructure gravimétrique nationale

International audienceRésif-Gmob, parc national d’instruments gravimétriques mobiles coordonné par le CNRS-Insu etcomposante instrumentale de Résif, offre aux équipes scientifiques françaises un ensemble de moyens, de mesures et de services pourdes études du champ de pesanteur. Ce parc instrumental comprend divers capteurs complémentaires permettant de mesurer avec une précision variant de quelques 10 -8 à 10 -11 g, les variations spatiales ou temporelles de la pesanteur

Résif-Gmob et l'infrastructure gravimétrique nationale

International audienceRésif-Gmob, parc national d’instruments gravimétriques mobiles coordonné par le CNRS-Insu etcomposante instrumentale de Résif, offre aux équipes scientifiques françaises un ensemble de moyens, de mesures et de services pourdes études du champ de pesanteur. Ce parc instrumental comprend divers capteurs complémentaires permettant de mesurer avec une précision variant de quelques 10 -8 à 10 -11 g, les variations spatiales ou temporelles de la pesanteur