References of "Francis, Olivier 50001803"
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See detailTongji-Grace02s and Tongji-Grace02k: high-precision static GRACE-only global Earth's gravity field models derived by refined data processing strategies
Qiujie, Chen; Yunzhong, Shen; Francis, Olivier UL et al

in Journal of Geophysical Research: Solid Earth (2018), 0(ja),

Abstract In order to derive high-precision static GRACE-only gravity field solutions, the following strategies were implemented in this study: (1) a refined accelerometer calibration model that treats ... [more ▼]

Abstract In order to derive high-precision static GRACE-only gravity field solutions, the following strategies were implemented in this study: (1) a refined accelerometer calibration model that treats monthly accelerometer scales as a 3-order polynomial and daily accelerometer biases as a 5-order polynomial was developed to calibrate accelerometer measurements; (2) the errors of the acceleration and attitude data were estimated together with the geopotential coefficients and accelerometer parameters on the basis of the weighted least-squares adjustments; (3) a nearly complete observation series of GRACE mission was used to decrease the condition number of normal equation; and (4) the GRACE data collected in lower orbit altitude were also included to decrease the condition number. Our results show that: (1) the refined accelerometer calibration model with much less parameters performs as well as previous methods (i.e. solving daily scales and hourly biases or estimating biases along with bias rates every two hours). However, it provides a system of more stable normal equation and less high-frequency noise in gravity field solutions; (2) high-frequency noise in the gravity field solution is reduced by modelling the errors of the acceleration and attitude data; (3) the geopotential coefficients at all degrees is greatly enhanced by using longer GRACE time series (especially the data by the end of 2010); and (4) due to lower orbit altitude, the GRACE data collected since 2014 lead to a significant improvement of the gravity field solution as the satellites are more sensitive to higher-frequency signal. Using the refined strategies, an unconstrained static solution (named Tongji-Grace02s) up to degree and order 180 was derived. For further suppressing the high-frequency noise, a regularization strategy based on the Kaula rule is applied to the degrees and orders beyond 80, leading to a regularized model Tongji-Grace02k. To validate the quality of the derived models, both Tongji-Grace02s and Tongji-Grace02k were compared to the latest GRACE-only models (i.e. GGM05S, ITU\_GRACE16, ITSG-Grace2014s and ITSG-Grace2014k) and validated using independent data (i.e. GNSS/Levelling data and DTU13 oceanic gravity data). Compared to other models, much less spatial noise in terms of global gravity anomalies with respect to the state-of-the-art model EIGEN6C4 and far higher accuracy at high degrees are achieved by Tongji-Grace02s. The same conclusions can be drawn for Tongji-Grace02k when the same analyses were applied to the regularized solutions ITSG-Grace2014k and Tongji-Grace02k. Validations with independent data confirm that Tongji-Grace02s has the least noise among the unconstrained GRACE-only models and Tongji-Grace02k is the one with the best accuracy among the regularized GRACE-only solutions. For the tests up to degree and order 180 using GNSS/Levelling data, the improvements of Tongji-Grace02s with respect to ITSG-Grace2014s reach 13\% over the Canada and 23\% in the Mexico. Even better, no less than 58\% of improvement is achieved by both Tongji-Grace02s relative to ITSG-Grace2014s and Tongji-Grace02k with respect to ITSG-Grace2014k in the validation based on DTU13 data. [less ▲]

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See detailGeophysical Investigation of the Pb–Zn Deposit of Lontzen–Poppelsberg, Belgium
Evrard, Maxime; Dumont, Gaël; Hermans, Thomas et al

in Minerals (2018), 8(6 233),

The drillhole information from the Lontzen–Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface ... [more ▼]

The drillhole information from the Lontzen–Poppelsberg site has demonstrated three orebodies and has allowed the estimation of the extension of the lodes, their dip, and the location at the ground surface. The localisation of the lodes makes them excellent targets for further exploration with geophysics. This deposit is classified as a Mississippi Valley Type (MVT) deposit. It consists mainly of Pb–Zn–Fe sulphides that display contrasting values in resistivity, chargeability, density, and magnetic susceptibility, with regards to the sedimentary host rocks. The dipole–dipole direct current (DC) resistivity and induce polarization (IP) profiles have been collected and inverted to successfully delineate the Pb–Zn mineralization and the geological structures. Short-spacing EM34 electromagnetic conductivity data were collected mainly on the top of Poppelsberg East lode and have revealed a conductive body matching with the geologically modelled mineralization. Gravity profiles have been carried out perpendicularly to the lode orientation; they show a strong structural anomaly. High resolution ground magnetic data were collected over the study area, but they showed no anomaly over the ore deposits. The geophysical inversion results are complementary to the model based on drill information, and allow us to refine the delineation of the mineralization. The identification of the geophysical signatures of this deposit permits targeting new possible mineralization in the area. [less ▲]

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See detailRecording Belgium's Gravitational History
Van Camp, Michel; Francis, Olivier UL; Lecocq, Thomas

in EOS : Transactions, American Geophysical Union (2017)

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See detailThe gravity of geophysics
Van Camp, Michel; de Viron, Olivier; Watlet, Arnaud et al

in EOS : Transactions, American Geophysical Union (2017), 98

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See detailUsing GPS and absolute gravity observations to separate the effects of present-day and Pleistocene ice-mass changes in South East Greenland
van Dam, Tonie UL; Francis, Olivier UL; Wahr, J. et al

in Earth and Planetary Science Letters (2017), 459

Measurements of vertical crustal uplift from bedrock sites around the edge of the Greenland ice sheet (GrIS) can be used to constrain present day mass loss. Interpreting any observed crustal displacement ... [more ▼]

Measurements of vertical crustal uplift from bedrock sites around the edge of the Greenland ice sheet (GrIS) can be used to constrain present day mass loss. Interpreting any observed crustal displacement around the GrIS in terms of present day changes in ice is complicated, however, by the glacial isostatic adjustment (GIA) signal. With GPS observations alone, it is impossible to separate the uplift driven by present day mass changes from that due to ice mass changes in the past. Wahr et al. (1995) demonstrated that viscoelastic surface displacements were related to the viscoelastic gravity changes through a proportionality constant that is nearly independent of the choice of Earth viscosity or ice history model. Thus, by making measurements of both gravity and surface motion at a bedrock site, the viscoelastic effects could be removed from the observations and we would be able to constrain present day ice mass changes. Alternatively, we could use the same observations of surface displacements and gravity to determine the GIA signal. In this paper, we extend the theory of Wahr et al. (1995) by introducing a constant, Z, that represents the ratio between the elastic changes in gravity and elastic uplift at a particular site due to present day mass changes. Further, we combine 20 yrs of GPS observations of uplift with eight absolute gravity observations over the same period to determine the GIA signal near Kulusuk, a site on the southeastern side of the GrIS, to experimentally demonstrate the theory. We estimate that the GIA signal in the region is 4.49 ± 1.44 mm/yr and is inconsistent with most previously reported model predictions that demonstrate that the GIA signal here is negative. However, as there is very little in situ data to constrain the GIA rate in this part of Greenland, the Earth model or the ice history reconstructions could be inaccurate (Khan et al., 2016). Improving the estimate of GIA in this region of Greenland will allow us to better determine the present day changes in ice mass in the region, e.g. from GRACE. [less ▲]

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See detailRegional comparison of absolute gravimeters, EURAMET.M.G-K2 key comparison
Pálinkáš, V.; Francis, Olivier UL; Val'ko, M. et al

in Metrologia (2017), 54(1A), 07012

In the framework of the regional EURAMET.M.G-K2 comparison of absolute gravimeters, 17 gravimeters were compared in November 2015. Four gravimeters were from different NMIs and DIs, they were used to link ... [more ▼]

In the framework of the regional EURAMET.M.G-K2 comparison of absolute gravimeters, 17 gravimeters were compared in November 2015. Four gravimeters were from different NMIs and DIs, they were used to link the regional comparison to the CCM.G.K2 by means of linking converter. Combined least-squares adjustments with weighted constraint was used to determine KCRV. Several pilot solutions are presented and compared with the official solution to demonstrate influences of different approaches (e.g. definition of weights and the constraint) on results of the adjustment. In case of the official solution, all the gravimeters are in equivalence with declared uncertainties. == Main text To reach the main text of this paper, click on Final Report [http://www.bipm.org/utils/common/pdf/final_reports/M/G-K2/EURAMET.M.G-K2.pdf] . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ [http://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 ▲]

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See detailRegional comparison of absolute gravimeters SIM.M.G-K1 key comparison
Newell, D. B.; Westrum, D. Van; Francis, Olivier UL et al

in Metrologia (2017), 54(1A), 07019

Twelve absolute gravimeters were compared during the regional Key Comparison SIM.M.G-K1 of absolute gravimeters. The four gravimeters were from different NMIs and DIs. The comparison was linked to the CCM ... [more ▼]

Twelve absolute gravimeters were compared during the regional Key Comparison SIM.M.G-K1 of absolute gravimeters. The four gravimeters were from different NMIs and DIs. The comparison was linked to the CCM.G-K2 through EURAMET.M.G-K2 via the DI gravimeter FG5X-216. Overall, the results and uncertainties indicate an excellent agreement among the gravimeters, with a standard deviation of the gravimeters' DoEs better than 1.3 μGal. In the case of the official solution, all the gravimeters are in equivalence well within the declared uncertainties. == Main text To reach the main text of this paper, click on Final Report [http://www.bipm.org/utils/common/pdf/final_reports/M/G-K1/SIM.M.G-K1.pdf] . Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/ [http://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 ▲]

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See detailRetrieving hydrological connectivity from empirical causality in karst systems
Delforge, D.; Vanclooster, M.; Van Camp, M. et al

Scientific Conference (2017)

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See detailGeophysics From Terrestrial Time-Variable Gravity Measurements
Van Camp, Michel; de Viron, Olivier; Watlet, Arnaud et al

in Reviews of Geophysics (2017), 55

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the ... [more ▼]

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. [less ▲]

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See detailSingle station Monitoring of Volcanoes Using Seismic ambient noise
De Plaen, Raphaël UL; Lecocq, Thomas; Caudron, Corentin et al

Poster (2016, December 16)

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See detailStudy of space weather impact on Antarctica ionosphere from GNNS data
Bergeot, Nicolas; Chevalier, J.-M.; Bruyninx, Carine et al

Poster (2016, April 29)

The impact of solar activity on the ionosphere at polar latitudes is not well known compare to low and mid-latitudes due to lack of experimental observations, especially over Antarctica. Consequently, one ... [more ▼]

The impact of solar activity on the ionosphere at polar latitudes is not well known compare to low and mid-latitudes due to lack of experimental observations, especially over Antarctica. Consequently, one of the present challenges of the Space Weather community is to better characterize (1) the climatological behavior of the polar ionosphere in response to variations of the solar activity and (2) the different response of the ionosphere at high latitudes during extreme solar events and geomagnetic storms. For that, the combination of GNSS measurements (e.g. GPS, GLONASS and Galileo) on two separate frequencies allows determining the ionospheric delay between a ground receiver and a satellite. This delay is function of the integrated number of electrons encountered in the ionosphere along the signal ray path, called the Total Electron Content (TEC). It is thus possible to study the behavior of ionospheric TEC at different time and spatial scales from the observations of a network of permanent GNSS stations. In the frame of GIANT-LISSA and IceCon projects we installed since 2009 five GNSS stations around the Princess Elisabeth station. We used these stations additionally to other stations from the IGS global network to estimate the ionospheric TEC at different locations over Antarctica. This study presents this regional data set during different solar activity levels and discusses the different climatological behaviors identified in the ionosphere at these high latitudes. Finally, we will show few examples of typical TEC disturbances observed during extreme solar events. [less ▲]

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See detailSingle station monitoring of volcanoes using seismic ambient noise
De Plaen, Raphaël UL; Lecocq, Thomas; Caudron, Corentin et al

in Geophysical Research Letters (2016)

Seismic ambient noise cross correlation is increasingly used to monitor volcanic activity. However, this method is usually limited to volcanoes equipped with large and dense networks of broadband stations ... [more ▼]

Seismic ambient noise cross correlation is increasingly used to monitor volcanic activity. However, this method is usually limited to volcanoes equipped with large and dense networks of broadband stations. The single station approach may provide a powerful and reliable alternative to the classical “cross-stations” approach when measuring variation of seismic velocities. We implemented it on the Piton de la Fournaise in Reunion Island, a very active volcano with a remarkable multi-disciplinary continuous monitoring. Over the past decade, this volcano was increasingly studied using the traditional cross-correlation technique and therefore represents a unique laboratory to validate our approach. Our results, tested on stations located up to 3.5 km from the eruptive site, performed as well as the classical approach to detect the volcanic eruption in the 1-2 Hz frequency band. This opens new perspectives to successfully forecast volcanic activity at volcanoes equipped with a single 3-component seismometer. [less ▲]

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See detailTemporal variation of tidal parameters in superconducting gravimeter time-series
Meurers, Bruno; Van Camp, Michel; Francis, Olivier UL et al

in Geophysical Journal International (2016), 205(1), 284-300

Analysing independent 1-yr data sets of 10 European superconducting gravimeters (SG) reveals statistically significant temporal variations of M2 tidal parameters. Both common short-term (<2 yr) and long ... [more ▼]

Analysing independent 1-yr data sets of 10 European superconducting gravimeters (SG) reveals statistically significant temporal variations of M2 tidal parameters. Both common short-term (<2 yr) and long-term (>2 yr) features are identified in all SG time-series but one. The averaged variations of the amplitude factor are about 0.2‰. The path of load vector variations equivalent to the temporal changes of tidal parameters suggests the presence of an 8.85 yr modulation (lunar perigee). The tidal waves having the potential to modulate M2 with this period belong to the 3rd degree constituents. Their amplitude factors turn out to be much closer to body tide model predictions than that of the main 2nd degree M2, which indicates ocean loading for 3rd degree waves to be less prominent than for 2nd degree waves within the M2 group. These two different responses to the loading suggest that the observed modulation is more due to insufficient frequency resolution of limited time-series rather than to time variable loading. Presently, SG gravity time-series are still too short to prove if time variable loading processes are involved too as in case of the annual M2 modulation known to appear for analysis intervals of less than 1 yr. Whatever the variations are caused by, they provide the upper accuracy limit for earth model validation and permit estimating the temporal stability of SG scale factors and assessing the quality of gravity time-series. [less ▲]

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See detailMultiscale hydrogeological and hydrogeophysical approach to monitor vadose zone hydrodynamics of a karst system
Watlet, Arnaud; Poulain, Amaël; Francis, Olivier UL et al

Poster (2016)

The vadose zone of karst systems plays an important role on the water dynamics. In particular, temporary perched aquifers can appear in the subsurface due to changes of weather conditions, reduced ... [more ▼]

The vadose zone of karst systems plays an important role on the water dynamics. In particular, temporary perched aquifers can appear in the subsurface due to changes of weather conditions, reduced evapotranspiration and the vertical gradients of porosity and permeability. Although many difficulties are usually encountered when studying karst environments due to their heterogeneities, cave systems offer an outstanding opportunity to investigate vadose zone from the inside. We present a multi-scale study covering two years of hydrogeological and geophysical monitoring of the Lomme Karst System (LKS) located in the Variscan fold-and-thrust belt (Belgium), a region (∼3000 ha) that shows many karstic networks within Devonian limestone units. Hydrogeological data cover the whole LKS and involve e.g. flows and levels monitoring or tracer tests performed in both vadose and saturated zones. Such data bring valuable information on the hydrological context of the studied area at the catchment scale. Combining those results with geophysical measurements allows validating and imaging them at a smaller scale, with more integrative techniques. Hydrogeophysical measurements are focused on only one cave system of the LKS, at the Rochefort site (∼40 ha), taking benefit of the Rochefort Cave Laboratory (RCL) infrastructures. In this study, a microgravimetric monitoring and an Electrical Resistivity Tomography (ERT) monitoring are involved. The microgravimetric monitoring consists in a superconducting gravimeter continuously measuring gravity changes at the surface of the RCL and an additional relative gravimeter installed in the underlying cave located 35 meters below the surface. While gravimeters are sensible to changes that occur in both the vadose zone and the saturated zone of the whole cave system, combining their recorded signals allows enhancing vadose zone’s gravity changes. Finally, the surface ERT monitoring provide valuable information at the (sub)-meter scale on the hydrological processes that occur in the vadose zone. Seasonal water variations and preferential flow path are observed. This helps separating the hydrological signature of the vadose zone from that of the saturated zone. [less ▲]

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See detailContinuous gravimetric monitoring as an integrative tool for exploring hydrological processes in the Lomme Karst System (Belgium)
Watlet, A.; Van Camp, M. J.; Poulain, A. et al

Scientific Conference (2016)

Karst systems are highly heterogeneous which makes their hydrology difficult to understand. Geophysical techniques offer non-invasive and integrative methods that help interpreting such systems as a whole ... [more ▼]

Karst systems are highly heterogeneous which makes their hydrology difficult to understand. Geophysical techniques offer non-invasive and integrative methods that help interpreting such systems as a whole. Among these techniques, gravimetry has been increasingly used in the last decade to characterize the hydrological behavior of complex systems, e.g. karst environments or volcanoes. We present a continuous microgravimetric monitoring of 3 years in the karstic area of Rochefort (Belgium), that shows multiple occurrences of caves and karstic features. The gravity record includes measurements of a GWR superconducting gravimeter, a Micro-g LaCoste gPhone and an absolute FG5 gravimeter. Together with meteorological measurements and a surface/in-cave hydrogeological monitoring, we were able to improve the knowledge of hydrological processes. On the one hand, the data allowed identifying seasonal groundwater content changes in the unsaturated zone of the karst area, most likely to be linked to temporary groundwater storage occurring in the most karstified layers closed to the surface. Combined with additional geological information, modelling of the gravity signal based on the vertical potential of the gravitational attraction was then particularly useful to estimate the seasonal recharge leading to the temporary subsurface groundwater storage. On the other hand, the gravity monitoring of flash floods occurring in deeper layers after intense rainfall events informed on the effective porosity gradient of the limestones. Modelling was then helpful to identify the hydrogeological role played by the cave galleries with respect to the hosting limestones during flash floods. These results are also compared with measurements of an in-cave gravimetric monitoring performed with a gPhone spring gravimeter. An Electrical Resistivity Tomography monitoring is also conducted at site and brings additional information useful to verify the interpretation made with the gravimetric monitoring [less ▲]

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See detailGroundwater Storage in a Karst Vadose Zone Evidenced Using Gravimetric and Surface-to-borehole ERT Monitoring Systems
Watlet, A.; Kaufmann, O.; Francis, Olivier UL et al

Poster (2015, September 06)

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See detailBilateral comparison between the FG5X-216 and FG5-242
Francis, Olivier UL; Ullrich, Christian

Report (2015)

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See detailRelative gravity measurements in Ilulissat in July 2013
Francis, Olivier UL

Report (2015)

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