References of "Francis, Olivier 50001803"
     in
Bookmark and Share    
Full Text
Peer Reviewed
See detailCorrection to: Performance assessment of the relative gravimeter Scintrex CG-6
Francis, Olivier UL

in Journal of Geodesy (2022), 96(1), 1-1

Detailed reference viewed: 28 (0 UL)
Full Text
Peer Reviewed
See detailPerformance assessment of the relative gravimeter Scintrex CG-6
Francis, Olivier UL

in Journal of Geodesy (2021), 95:116

The new-generation relative gravimeter Scintrex CG6 is put on the test bench: Its performance is compared to its predecessor, the Scintrex-CG5. A CG5 Scintrex and a CG6 Scintrex were both submitted to the ... [more ▼]

The new-generation relative gravimeter Scintrex CG6 is put on the test bench: Its performance is compared to its predecessor, the Scintrex-CG5. A CG5 Scintrex and a CG6 Scintrex were both submitted to the same rigorous tests, which have been developed in recent years to highlight some defects of the CG5. The results show that the CG6 always performs better than the CG5. For instance, the instrumental drift is 5 times smaller for the CG6 than for the CG5. In the tidal bands, the noise level of the CG6 is 3 times lower than of the one of the CG5. We confirmed the tilt susceptibility of the CG5 and found that the CG6 is barely affected by long duration tilts before taking measurements. Unlike the CG5, the CG6 measurements are not influenced by external temperature variations. Overall, the CG6 provides more precise and stable gravity measurements compared to the CG5. [less ▲]

Detailed reference viewed: 33 (0 UL)
Full Text
See detailThe absolute Gravity Network of Haiti - Status Report 2021
Francis, Olivier UL; Sauveur, Renaldo UL; Beker, Neptune et al

Report (2021)

Detailed reference viewed: 75 (17 UL)
Full Text
Peer Reviewed
See detailEvaluation of global ocean tide models based on tidal gravity observations in China
Tan, Hongbo; Francis, Olivier UL; Wu, Guiju et al

in Geodesy and Geodynamics (2021), 12(6), 451-458

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 ... [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 O1, K1, M2 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. [less ▲]

Detailed reference viewed: 28 (0 UL)
Full Text
Peer Reviewed
See detailSNR-Based GNSS-R for Coastal Sea-Level Altimetry
Tabibi, Sajad UL; Sauveur, Renaldo UL; Guerrier, Kelly et al

in Geosciences (2021), 11(9 391),

Geodetic Global Navigation Satellite System reflectometry (GNSS-R) uses ground-based signals of opportunity to retrieve sea levels at an intermediate spatial scale. Geodetic GNSS-R is based on the ... [more ▼]

Geodetic Global Navigation Satellite System reflectometry (GNSS-R) uses ground-based signals of opportunity to retrieve sea levels at an intermediate spatial scale. Geodetic GNSS-R is based on the simultaneous reception of Line-of-Sight (LoS) and its coherent GNSS sea surface reflection (non-LOS) signals. The scope of this paper is to present geodetic GNSS-R applied to sea level altimetry. Signal-to-Noise Ratio (SNR) measurements from a Commercial Off-The-Shelf (COTS) geodetic-quality GNSS station at the Haiti Coast Guard Base in Port-au-Prince is used to retrieve sea levels in the International Terrestrial Reference Frame 2014 (ITRF2014). The GNSS-R sea levels are compared with those of the OTT Radar Level Sensor (RLS) installed vertically below the GNSS antenna. The Root-Mean-Square Error (RMSE) between the geodetic GNSS-R sea levels and OTT RLS records is 3.43 cm, with a correlation of 0.96. In addition, the complex differences between the OTT RLS records and 15-min GNSS-R sea levels using Global Positioning System (GPS) and Globalnaya Navigazionnaya Sputnikovaya Sistema (or Global Navigation Satellite System; GLONASS) signals for all the eight major tidal constituents are in mm-level agreement. Therefore, geodetic GNSS-R can be used as a complementary approach to the conventional method for sea level studies in a stable terrestrial reference frame. [less ▲]

Detailed reference viewed: 57 (12 UL)
Full Text
Peer Reviewed
See detailGravity monitoring of underground flash flood events to study their impact on groundwater recharge and the distribution of karst voids
Watlet, A.; Van Camp, M.; Francis, Olivier UL et al

in Water Resources Research (2020), 56(n/a), 2019026673

Abstract Flash flood events are expected to become increasingly common with the global increases in weather extremes. They are a significant natural hazard that affects karst landscapes, which host large ... [more ▼]

Abstract Flash flood events are expected to become increasingly common with the global increases in weather extremes. They are a significant natural hazard that affects karst landscapes, which host large resources of drinking water worldwide. The role played by underground flood events in the karst aquifer recharge is complex due to the heterogeneity of the basement which remains poorly understood. We present the analysis of 20 in-cave flash flood events affecting the Rochefort karst system (Belgium) using continuous gravity measurements at one single station, and water level sensors installed in caves. Underground flood events typically produce a peak in the gravity signal, due to an increase in the associated mass change. After the flood, the gravity values drop but remain slightly increased compared to before the flood event. Via forward gravity modeling, we demonstrate that this remaining anomaly can be reasonably explained by the infiltration of local rainfall within the karst system rather than by allogenic recharge of the aquifer. Flash floods are mainly restricted to connected voids. This allows us to utilize them as proxies to investigate the distribution of cavities in the karst system. Forward modeling of the gravitational attraction induced by the mapped caves being flooded yields a gravity signal much smaller than the observed one. We conclude that at least 50 more cavities than those previously mapped are required to match the measured anomalies. This presents opportunities for implementing similar approaches in other diverse porous media, using gravity monitoring of hydrological processes (e.g. infiltration fronts, hydrothermalism or tide effects in coastal aquifers) as proxies to characterize underground properties. [less ▲]

Detailed reference viewed: 138 (6 UL)
Full Text
Peer Reviewed
See detailCan GNSS-R Detect Abrupt Water Level Changes?
Tabibi, Sajad UL; Francis, Olivier UL

in Remote Sensing (2020), 12(21 3614),

Global navigation satellite system reflectometry (GNSS-R) uses signals of opportunity in a bi-static configuration of L-band microwave radar to retrieve environmental variables such as water level. The ... [more ▼]

Global navigation satellite system reflectometry (GNSS-R) uses signals of opportunity in a bi-static configuration of L-band microwave radar to retrieve environmental variables such as water level. The line-of-sight signal and its coherent surface reflection signal are not separate observables in geodetic GNSS-R. The temporally constructive and destructive oscillations in the recorded signal-to-noise ratio (SNR) observations can be used to retrieve water-surface levels at intermediate spatial scales that are proportional to the height of the GNSS antenna above the water surface. In this contribution, SNR observations are used to retrieve water levels at the Vianden Pumped Storage Plant (VPSP) in Luxembourg, where the water-surface level abruptly changes up to 17 m every 4-8 h to generate a peak current when the energy demand increases. The GNSS-R water level retrievals are corrected for the vertical velocity and acceleration of the water surface. The vertical velocity and acceleration corrections are important corrections that mitigate systematic errors in the estimated water level, especially for VPSP with such large water-surface changes. The root mean square error (RMSE) between the 10-min multi-GNSS water level time series and water level gauge records is 7.0 cm for a one-year period, with a 0.999 correlation coefficient. Our results demonstrate that GNSS-R can be used as a new complementary approach to study hurricanes or storm surges that cause abnormal rises of water levels. [less ▲]

Detailed reference viewed: 123 (18 UL)
Full Text
See detailImpact of Atmospheric Circulation on Flooding Occurrence and Type in Luxembourg (Central Western Europe)
Meyer, Judith; Douinot, Audrey; Zehe, Erwin et al

Scientific Conference (2020)

Detailed reference viewed: 22 (0 UL)
Full Text
See detailMeasuring gravity changes for decades
Van Camp, Michel; de Viron, Olivier; Meurers, Bruno et al

Scientific Conference (2020)

Detailed reference viewed: 23 (1 UL)
Full Text
Peer Reviewed
See detailThe results of CCM.G-K2.2017 key comparison
WU, Shuqing; FENG, Jinyang; LI, Chunjian et al

in Metrologia (2020), 57(1A), 07002--07002

The CCM.G-K2.2017 comparison was organised for the purpose of determination of the degree of equivalence of the national standards for free-fall acceleration measurement. The comparison was held in the ... [more ▼]

The CCM.G-K2.2017 comparison was organised for the purpose of determination of the degree of equivalence of the national standards for free-fall acceleration measurement. The comparison was held in the Changping Campus of National Institute of Metrology China (NIM), from October to November in 2017. This is the first time that such a comparison is organized outside of the Europe continent and establishes a new global comparison sites in China. This comparison is also the largest ever organized with the participation of 13 instruments. We give the list of the participants who actually performed measurements during the comparison, the data (raw absolute gravity measurements and their uncertainties) submitted by the participants as well as the results of the vertical gravity gradient at the comparison sites. The measurement strategy is briefly discussed and the data elaboration is presented. Finally, the results of the data adjustment are presented including the degrees of equivalence (DoE) of the absolute gravimeters and the key comparison reference values (KCRVs). Overall, the measurements of KC instruments are all consistent given the declared uncertainties. Main text To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/. The final report has been peer-reviewed and approved for publication by the CCM, according to the provisions of the CIPM Mutual Recognition Arrangement (CIPM MRA). [less ▲]

Detailed reference viewed: 252 (4 UL)
Full Text
See detailLong-term monitoring with spring-based gravimeters: tilt-control benefits and application to the Rochefort Cave Laboratory (Belgium)
Fores, Benjamin; Watlet, Arnaud; Van Camp, Michel et al

Scientific Conference (2020)

Detailed reference viewed: 20 (0 UL)
Full Text
Peer Reviewed
See detailTidal analysis of GNSS reflectometry applied for coastal sea level sensing in Antarctica and Greenland
Tabibi, Sajad UL; Geremia-Nievinski, Felipe; Francis, Olivier UL et al

in Remote Sensing of Environment (2020), 248

We retrieve sea levels in polar regions via GNSS reflectometry (GNSS-R), using signal-to-noise ratio (SNR) observations from eight POLENET GNSS stations. Although geodetic-quality antennas are designed to ... [more ▼]

We retrieve sea levels in polar regions via GNSS reflectometry (GNSS-R), using signal-to-noise ratio (SNR) observations from eight POLENET GNSS stations. Although geodetic-quality antennas are designed to boost the direct reception from GNSS satellites and to suppress indirect reflections from natural surfaces, the latter can still be used to estimate the sea level in a stable terrestrial reference frame. Here, typical GNSS-R retrieval methodology is improved in two ways, 1) constraining phase-shifts to yield more precise reflector heights and 2) employing an extended dynamic filter to account for the second-order height rate of change (vertical acceleration). We validate retrievals over a 4-year period at Palmer Station (Antarctica), where there is a co-located tide gauge (TG). Because ice contaminates the long-period tidal constituents, we focus on the main tidal species (daily and subdaily), by employing a deseasonalization filter. The difference between sub-hourly GNSS-R retrievals of the ocean surface and TG records has a root-mean-square error (RMSE) of 15.4 cm and a correlation of 0.903, while the tidal prediction has a RMSE of 1.9 cm and a correlation of 0.998. There is excellent millimetric agreement between the two sensors for most eight major tidal constituents, with the exception of luni-solar diurnal (K1), principal solar (S2), and luni-solar semidiurnal (K2) components, which are biased in GNSS-R due to the leakage of the GPS orbital period. We also compare the GNSS-R tidal constituents from seven additional POLENET sites, without co-located TG, to global and local ocean tide models. We find that the root-sum-square-error (RSSE) of eight major constituents varies between 26.0 cm and 56.9 cm for different models. Given that the agreement in tidal constituents between the TG and GNSS-R was better at Palmer Station, we conclude that assimilating the GNSS-R retrievals into tidal models would improve their accuracy in Antarctica and Greenland, provided that care is exercised to avoid the orbital period overtones and also sea ice. [less ▲]

Detailed reference viewed: 170 (27 UL)
Full Text
See detailHydrogeological effects on terrestrial gravity measurements
Van Camp, Michel; de Viron, Olivier; Dassargues, Alain et al

Scientific Conference (2019, December 10)

For the 20 last years, terrestrial and satellite gravity measurements have reached such a precision that they allow for identification of the signatures from water storage fluctuations. In particular ... [more ▼]

For the 20 last years, terrestrial and satellite gravity measurements have reached such a precision that they allow for identification of the signatures from water storage fluctuations. In particular, hydrogeological effects induce significant time-correlated signature in the gravity time series. Gravity response to rainfall is a complex function of the local geologic and climatic conditions, e.g., rock porosity, vegetation, evaporation, and runoff rates. The gravity signal combines contributions from many geophysical processes, source separation being a major challenge. At the local scale and short-term, the associated gravimetric signatures often exceed the tectonic and GIA effects, and monitoring gravity changes is a source of information on local groundwater mass balance, and contributes to model calibrations. Some aquifer main characteristics can then be inferred by combining continuous gravity, geophysical and hydrogeological measurements. In Membach, Belgium, a superconducting gravimeter has monitored gravity continuously for more than 24 years. This long time series, together with 300 repeated absolute gravity measurements and environmental monitoring, has provided valuable information on the instrumental, metrological, hydrogeological and geophysical points of view. This has allowed separating the signal sources and monitoring partial saturation dynamics in the unsaturated zone, convective precipitation and evapotranspiration at a scale of up to 1 km², for signals smaller than 1 nm/s², equivalent to 2.5 mm of water. Based on this experience, another superconducting gravimeter was installed in 2014 in the karst zone of Rochefort, Belgium. In a karst area, where the vadose zone is usually thicker than in other contexts, combining gravity measurements at the surface and inside accessible caves is a way to separate the contribution from the unsaturated zone lying between the two instruments, from the saturated zone underneath the cave, and the common mode effects from the atmosphere or other regional processes. Those experiments contribute to the assessment of the terrestrial hydrological cycle, which is a major challenge of the geosciences associated with key societal issues: availability of freshwater, mitigation of flood hazards, or measurement of evapotranspiration. [less ▲]

Detailed reference viewed: 84 (3 UL)
Full Text
See detailAbsolute Gravity and Uplift in the Yellowstone Caldera
van Dam, Tonie UL; Francis, Olivier UL

Scientific Conference (2019, December 10)

GPS time-series of uplift show that points in and around the caldera have gone through cycles of uplift, followed by subsidence since observations began about three decades ago. A dramatic increase in the ... [more ▼]

GPS time-series of uplift show that points in and around the caldera have gone through cycles of uplift, followed by subsidence since observations began about three decades ago. A dramatic increase in the uplift rate started in 2004 at the GPS station LKWY near Yellowstone Lake and Old Faithful, OFWY. Since 2010, the sites have subsided, began uplifting again in 2014 coincidentally after a M 4.8 earthquake near the Norris Geyser Basin, and then started subsiding again in 2016. The cause of the episodic uplift and subsidence and the spatial pattern of the surface displacement are not yet well understood. The 2003-2009 episode of rapid uplift is believed to result from deep source magma intrusion simultaneous with depressurization of the hydrothermal systems beneath the Norris Geyser Basin. But whether it is caused by the intrusion of magma from a distant reservoir, or by the expulsion and localized trapping of pressurized water and gas from rock that is already in-place, is not known. We have taken observations of absolute gravity at LKWY and OFWY almost annually since 2009. In this presentation, we compare gravity and uplift and provide some insight into the mechanism driving the uplift/subsidence cycles. [less ▲]

Detailed reference viewed: 113 (6 UL)
Full Text
Peer Reviewed
See detailAn optimized short-arc approach: methodology and application to develop refined time series of Tongji-Grace2018 GRACE monthly solutions
Chen, Qiujie; Shen, Yunzhong; Chen, Wu et al

in Journal of Geophysical Research. Solid Earth (2019), 124(6), 6010-6038

Abstract 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), an optimized short-arc method ... [more ▼]

Abstract 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), 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 the optimized short-arc method is able to stabilize the inversion and eventually prolong the arc length to 6 hours. 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 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. [less ▲]

Detailed reference viewed: 207 (16 UL)
See detailEvaluation of global ocean tide models based on tidal gravity observations in China
Tan, H.; Francis, Olivier UL; Shen, C. et al

Scientific Conference (2019)

Solid Earth is affected by tidal cycles triggered by the gravity attraction of the celestial bodies. However, about 70% the Earth is covered with seawater which is also affected by the tidal forces. In ... [more ▼]

Solid Earth is affected by tidal cycles triggered by the gravity attraction of the celestial bodies. However, about 70% the Earth is covered with seawater which is also affected by the tidal forces. In the coastal areas, the ocean tidal loading (OTL) can reach up to 10% of the earth tide, 90% for tilt, and 25% for strain (Farrell, 1972). Since 2007, a high-precision continuous gravity observation network in China has been established with 78 stations. The long-term high-precision tidal data of the network can be used to validate, verifying and even improve the ocean tidal model (OTM). In this paper, tidal parameters of each station were extracted using the harmonic analysis method after a careful editing of the data. 8 OTMs were used for calculating the OTL. The results show that the Root-Mean-Square of the tidal residuals (M0) vary between 0.078-1.77 μgal, and the average errors as function of the distance from the sea for near(0-60km), middle(60-1000km) and far(>1000km) stations are 0.76, 0.30 and 0.21 μgal. The total final gravity residuals (Tx) of the 8 major constituents (M2, S2, N2, K2, K1, O1, P1, Q1) for the best OTM has amplitude ranging from 0.14 to 3.45 μgal. The average efficiency for O1 is 77.0%, while 73.1%, 59.6% and 62.6% for K1, M2 and Tx. FES2014b provides the best corrections for O1 at 12 stations, while SCHW provides the best for K1 ,M2 and Tx at 12 , 8 and 9 stations. For the 11 costal stations, there is not an obvious best OTM. The models of DTU10, EOT11a and TPXO8 look a litter better than FES2014b, HAMTIDE and SCHW. For the 17 middle distance stations, SCHW is the best OTM obviously. For the 7 far distance stations, FES2014b and SCHW model are the best models. But the correction efficiency is worse than the near and middle stations'. The outcome is mixed: none of the recent OTMs performs the best for all tidal waves at all stations. Surprisingly, the Schwiderski's model although is 40 years old with a coarse resolution of 1° x 1° is performing relative well with respect to the more recent OTM. Similar results are obtained in Southeast Asia (Francis and van Dam, 2014). It could be due to systematic errors in the surroundings seas affecting all the ocean tides models. It's difficult to detect, but invert the gravity attraction and loading effect to map the ocean tides in the vicinity of China would be one way. [less ▲]

Detailed reference viewed: 78 (2 UL)
Full Text
Peer Reviewed
See detailLong-term stability of tilt-controlled gPhoneX gravimeters
Fores, Benjamin UL; Klein, Gilbert UL; Le Moigne, Nicolas et al

in Journal of Geophysical Research. Solid Earth (2019), 124(11), 12264-12276

Abstract 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 ... [more ▼]

Abstract 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. [less ▲]

Detailed reference viewed: 219 (14 UL)
Full Text
Peer Reviewed
See detailTemporal Changes of Seismic Velocity Caused by Volcanic Activity at Mt. Etna Revealed by the Autocorrelation of Ambient Seismic Noise
De Plaen, Raphael S. M.; Cannata, Andrea; Cannavo', Flavio et al

in Frontiers in Earth Science (2019), 6

On active volcanoes, ambient noise-based seismic interferometry, able to detect very slight variations in seismic velocity associated with magma transport towards the surface, can be a very useful ... [more ▼]

On active volcanoes, ambient noise-based seismic interferometry, able to detect very slight variations in seismic velocity associated with magma transport towards the surface, can be a very useful monitoring tool. In this work, we performed the autocorrelation of ambient seismic noise recorded at Mt. Etna volcano, by three stations located close to the active summit craters, during April 2013 - October 2014. Such an interval was chosen because of the number and variety of eruptions. The method implemented to perform autocorrelation was the phase cross-correlation, which does not require normalization of the signals. The detected seismic velocity variations were very consistent for all three stations throughout the study period, mainly ranging between 0.3 and -0.2%, and were time-related to both sequences of paroxysmal eruptions and more effusive activities. In particular, we observed seismic velocity decreases accompanying paroxysmal eruptions, suggesting an intense pressurization within the plumbing system, which created an area of extensional strain with crack openings. It is worth noting that classical cross-station approach failed to detect seismic velocity changes related to volcano activity. In addition, seismic velocity variations over time were integrated with ground deformation data recorded by GPS stations and volcanic tremor centroid locations. Finally, we showed that, although the investigated frequency band (1-2 Hz) contains most of the volcanic tremor energy, our results did not indicate a particular contamination of seismic velocity variation measurements by variations of tremor sources. [less ▲]

Detailed reference viewed: 223 (11 UL)