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Expérimentation d’un gravimètre mobile léger et novateur pour la mesure du champ de gravité en fond de mer

Clément Roussel 1
1 GeF-L2G - Équipe de Géodésie et de Géomatique (Le Mans)
GeF - Laboratoire Géomatique et foncier
Abstract : One of the major challenges of modern gravimetry consists in determining mathematical models and digital maps of the Earth’s gravity field, the reliability of which is identical whatever the spatial scale considered in terrestrial, coastal, marine and submarine domains. Today, the harmonics of high degree corresponding to the short wavelengths of the gravity field are still affected by great uncertainties due to the diversity and the differences in precision and resolution of the gravimetric techniques making it possible to reach them. The main obstacle to improve the resolution and accuracy of models is that gravimetry and gradiometry mobile devices, the only instruments that allow homogeneous precision and spatial resolution acquisitions, are still bulky and energy-intensive, which prohibits their installation on terrestrial, aerial, surface and submarine drones. The interest of this type of carrier is to make acquisitions very close to the sources which considerably increases the restitution of the local variations of the gravity. The development of a new type of gravimetric sensor with small size and lower energy consumption appears therefore essential to answer the problematic posed by the measurement of the short wavelenghts of the gravity field.As part of its research in gravimetry, the Laboratoire Géomatique et Foncier (Cnam/GeF EA 4630), in collaboration with the Laboratory for Research Geodesy (LAREG) of the National Institute for Geographic and Forest Information (IGN), the Oceanic Domains Laboratory (LDO, UMR, CNRS 6538, UBO), the French Research Institute for the Exploitation of the Sea (IFREMER) and the Marine Hydrographic and Oceanographic Service (SHOM), develops an innovative instrument which allows the dynamic measurement of the Earth’s gravity field in the subsea domain.The system, called GraviMob (Gravimetry Mobile System), does not require a stabilized platform and is rigidly attached to the carrier vehicule, in this case an Autonomous Underwater Vehicule (AUV). The heart of the system consists of triads of accelerometers, allowing a vector measurement of the gravity. A Kalman filter, integrating the position and orientation data of the carrier vehicle, performs the estimation of the gravity field in a frame adapted to its interpretation. This instrumental prototype has been tested in the Mediterranean Sea during the year 2016. The comparison of the gravimetric signal obtained near the seabed with the surface gravimetric profiles, previously acquired by the SHOM, indicates a repeatability of the general trend of the gravimetric signal to within 5 mGal.This manuscript deals successively with the establishment of the observation equation of the GraviMob system, the calibration and orientation of the accelerometers, the gravity field estimation strategy by a Kalman filter, integrating an evolution model of the gravity field components and an observation model taking the measurement noise into account, the processing and analysis of the measurements acquired during its experimentation in the Mediterranean Sea, then the comparison of the gravimetric signal obtained with the reference data.
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Clément Roussel. Expérimentation d’un gravimètre mobile léger et novateur pour la mesure du champ de gravité en fond de mer. Géophysique [physics.geo-ph]. Conservatoire national des arts et metiers - CNAM, 2017. Français. ⟨NNT : 2017CNAM1099⟩. ⟨tel-01635884⟩

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