Olivier N Francis | Université du Luxembourg (original) (raw)

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Papers by Olivier N Francis

Geodesy and Geodynamics, Nov 1, 2021

Previous studies show that the calculated loading effects from global ocean tide models do not ma... more Previous studies show that the calculated loading effects from global ocean tide models do not match actual measurements of gravity attraction and loading effects in Southeast Asia. In this paper, taking advantage of a unique network of gravity tidal stations all over the Chinese mainland, we compare the observed and modeled tidal loading effects on the basis of the most recent global ocean tide models. The results show that the average efficiencies of the ocean tidal loading correction for O 1 , K 1 , M 2 are 77%, 73% and 59%, respectively. The loading correction efficiencies using recent ocean tidal models are better than the 40 years old Schwiderskis model at coastal stations, but relative worse at stations far from ocean.

Zenodo (CERN European Organization for Nuclear Research), Nov 18, 2009

Zenodo (CERN European Organization for Nuclear Research), Sep 20, 2006

Frontiers in Earth Science, Jan 17, 2019

Journal Of Geophysical Research: Solid Earth, Jun 1, 2019

Considering the unstable inversion of ill-conditioned intermediate matrix required in each integr... more Considering the unstable inversion of ill-conditioned intermediate matrix required in each integral arc in the short-arc approach presented in Chen et al. (2015, https://doi.org/10.1002/ 2014JB011470), an optimized short-arc method via stabilizing the inversion is proposed. To account for frequency-dependent noise in observations, a noise whitening technique is implemented in the optimized short-arc approach. Our study shows that the optimized short-arc method is able to stabilize the inversion and eventually prolong the arc length to 6 hr. In addition, the noise whitening method is able to mitigate the impacts of low-frequency noise in observations. Using the optimized short-arc approach, a refined time series of Gravity Recovery and Climate Experiment (GRACE) monthly models called Tongji-Grace2018 has been developed. The analyses allow us to derive the following conclusions: (a) During the analyses over the river basins (i.e., Amazon, Mississippi, Irrawaddy, and Taz) and Greenland, the correlation coefficients of mass changes between Tongji-Grace2018 and others (i.e., CSR RL06, GFZ RL06, and JPL RL06 Mascon) are all over 92% and the corresponding amplitudes are comparable; (b) the signals of Tongji-Grace2018 agree well with those of CSR RL06, GFZ RL06, ITSG-Grace2018, and JPL RL06 Mascon, while Tongji-Grace2018 and ITSG-Grace2018 are less noisy than CSR RL06 and GFZ RL06; (c) clearer global mass change trend and less striping noise over oceans can be observed in Tongji-Grace2018 even only using decorrelation filtering; and (d) for the tests over Sahara, over 36% and 19% of noise reductions are achieved by Tongji-Grace2018 relative to CSR RL06 in the cases of using decorrelation filtering and combined filtering, respectively.

Geophysical Research Letters, Aug 28, 2016

Journal Of Geophysical Research: Solid Earth, Nov 1, 2019

Spring relative gravimeters are considered too unstable to provide useful information on long-ter... more Spring relative gravimeters are considered too unstable to provide useful information on long-term gravity variations. In this paper, we prove that the new generation of spring gravimeter gPhoneX can reach long-term stability at the μGal level (10 nm s −2) when the verticality of the gravimeter is maintained, if the instrumental drift can be correctly estimated. We conducted two comparisons with different gPhoneXs in different observatories and environmental conditions. In the "Walferdange Underground Laboratory for Geodynamics" in Luxembourg, we compared time series from the gPhoneX (with and without tilt control), with data from a superconducting gravimeter. We found an agreement at the μGal level when the tilt control is switched on. We validated this result by repeating the experiment at the "Geodesy in Karstic Environment" observatory in the south of France. The fit between the superconducting gravimeter and the gPhoneX hourly values gives similar results at all frequencies over 276 days of measurements. The linear correlation coefficient between the gPhoneX and superconducting gravimeter reaches 0.99, with a misfit of 6.0 nm s −2. We demonstrated that tilt-controlled gPhoneXs are suitable for long-term gravity monitoring.

Zenodo (CERN European Organization for Nuclear Research), Jul 22, 2023

介绍了2007年11月在欧洲卢森堡Walferdange举办的第二次绝对重力比对观测的概况及结果．有20台绝对重力仪参加了比对观测，在ECGS共15个测点上的比测结果标准偏差小于2．1×10^-... more 介绍了2007年11月在欧洲卢森堡Walferdange举办的第二次绝对重力比对观测的概况及结果．有20台绝对重力仪参加了比对观测，在ECGS共15个测点上的比测结果标准偏差小于2．1×10^-8m·s^-2。文章简述了数据处理方法以及结果分析，中国地震局地震研究所引进的FG5／232绝对重力仪参加了本次比对，为在中国大陆建立绝对重力基准提供了重要经验．

Zenodo (CERN European Organization for Nuclear Research), Feb 27, 2012

Zenodo (CERN European Organization for Nuclear Research), May 11, 2021

Zenodo (CERN European Organization for Nuclear Research), Jan 21, 2023

Zenodo (CERN European Organization for Nuclear Research), Sep 25, 2006

This report contains the results of absolute gravity measurements carried in The Netherlands duri... more This report contains the results of absolute gravity measurements carried in The Netherlands during June and July 2006. Acceleration due to gravity has been determined using an absolute gravimeter at the five following stations: Epen, Kootwiijk, Wageningen, Westerbork and Zundert. The absolute gravimeter FG5#216 (manufactured by MicrogLaCoste Inc.), from the European Center for Geodynamics and Seismology, was operated by Olivier Francis. Rene Reudink from TU Delft provided assistance and logistic support during the field campaign. In this campaign, we reoccupied 4 stations. One new station has been established in Wageningen to complement the Dutch absolute gravity network. The drop-to-drop scatters of the observations in Westerbork, Kootwijk and Zundert can be considered as medium range values. The station in Epen is still by far the best one. On the other hand, the microseismic noise level is very high at the new station in Wageningen: the drop-to-drop scatter is about 80 microgal to be compared with the 5 microgal in the station of Epen. Overall, the mean set standard deviations for all the station are excellent with the highest value around 2 microgal and close to 1 microgal for most of the stations. Data processing Raw data from the absolute gravimeters consist of vectors of time and position of the falling object during the drops. To obtain the gravity value, a linear equation representing the equation of motion is fit to the raw data including the gravity gradient which as been measured with relative meters. The data processing follows the protocol adopted during absolute gravimeters comparisons at the BIPM in Sèvres (Francis and van Dam, 2003). Geophysical corrections are applied to the raw gravity data: Earth tides using modelled tidal parameters, atmospheric pressure using a constant admittance, and the polar motion effect using pole positions from IERS. The g-soft version 6.0 software from MicrogLaCoste Inc. was used for the processing. All the text outputs as well as some figures are compiled in the annexes of this report for future reference. Vertical Gravity Gradient The vertical gravity gradient was measured with a relative spring gravimeter Scintrex CG5 in Kootwijk aand Wageningen. This gradient is needed to linearize the equation of motion but also to transfer the measured absolute gravity value from the reference height around 1.3 m to the floor. The final values of the linear vertical gravity gradient are given in Table 1.

Zenodo (CERN European Organization for Nuclear Research), Jun 15, 2015

Zenodo (CERN European Organization for Nuclear Research), Jul 31, 2018