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Characterisation of a new mobile absolute quantum gravimeter : application in groundwater storage monitoring

Abstract : Quantum gravimeters provide the possibility of continuous, high-frequency absolute gravity monitoring while remaining user-friendly and transportable. This thesis assessed high precision performance measures of the first commercial absolute quantum gravimeters AQG#A01 and AQG#B01 developed by Muquans. This was carried out in comparison with high precision absolute and relative gravimeters and additional geophysical and environmental data, in controlled conditions and experiments in view of future deployment in field conditions.Both AQG devices allow stable measurements of g of several weeks. Significant drifts in time have not been observed. The two instruments have been transported and re-installed several times between sites and had been successfully applied in different conditions. The sensitivity of the AQG#A01 is better than 10 nm/s² after 24 h, which the AQG#B01 achieves after only one hour in a calm environment. For noisier environments, the sensitivity after one hour of the AQG#B01 is 20 to 30 nm/s². The repeatability of the AQG#B01 is reported as better than 50 nm/s². Changes of instrument tilt and external temperature (20 - 30 °C) and combination of both did not influence the measurement of gravitational attraction. These results were also confirmed during two weeks of acquisition in an urban garage during which the measurement of g remained unaffected by fast temperature changes.A rainfall event at the Larzac geodetic observatory caused a gravity increase of 100 nm/s² in December 2019, which was detected with the AQG#B01 in agreement with the superconducting relative gravimeter (GWR, iGrav#002) and corresponding Bouguer slab approximation. The potential gain in precision and time saved makes the AQG#B01 a promising instrument for e.g. large-scale gravity mapping. Such studies were formerly only feasible using a relative gravimeter that requires repeated acquisition loops and a reference absolute gravimeter for drift corrections. The AQG#B01 can be used without a reference instrument: It provides stable, repeatable measurements of absolute gravity while being transportable and user-friendly. Continuous monitoring at high precision allows for studies of high temporal resolution at different scales. The AQG#B01 would especially be suited for the monitoring of transient mass changes at durations (e.g. a few weeks) that are too short to justify the effort of installing a stationary, superconducting gravimeter. To reliably detect transient phenomena, a drift-free and repeatable determination of g is required for which e.g. spring relative gravimeters are not suitable. There are aspects that are still under investigations, such as the potential effect of the sensor head's orientation, the Coriolis effect, on the measurement of g and the assessment of the accuracy in view of differences between the AQG#B01 and the absolute gravimeter (Micro-g LaCoste, FG5#228) that is used as a reference.Time-lapse ground-based gravimetry is increasingly applied in subsurface hydrology to monitor water storage dynamics. The complementary spatial sensitivities of gravity and vertical gravity gradients (VGG) can be used to deduct the spatial characteristics of subsurface mass changes. VGG were estimated from one year of monthly relative gravity surveys on three different heights on three locations inside the Larzac observatory. The repeatability of VGG estimations was found to be better than 23 ± 9 E. The study suggests the influence of heterogeneous soil saturation patterns on VGG and the potential of differential VGG monitoring in resolving spatial mass distributions. Observed time-lapse, differential VGG changes provided additional constraints to the subsurface model. Combined VGG and gravity monitoring in hydrogeology is a promising new approach for hydrogeophysical subsurface imaging, which could find practical application in gravity monitoring during hydraulic aquifer testing.
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Submitted on : Wednesday, April 28, 2021 - 3:12:09 PM
Last modification on : Friday, November 4, 2022 - 3:43:05 AM
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  • HAL Id : tel-03211175, version 1

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Anne-Karin Cooke. Characterisation of a new mobile absolute quantum gravimeter : application in groundwater storage monitoring. Geophysics [physics.geo-ph]. Université Montpellier, 2020. English. ⟨NNT : 2020MONTG043⟩. ⟨tel-03211175⟩

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