References of "van Dam, Tonie 50003245"
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See detailHydrogeological investigations at the Membach station, Belgium and application to correct long periodic gravity variations
Van Camp, M.; Vanclooster, M.; Crommen, O. et al

in Journal of Geophysical Research (2006), 111

A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the ... [more ▼]

A comprehensive hydrogeological investigation regarding the influence of variations in local and regional water mass on superconducting gravity measurements is presented for observations taken near the geodynamic station of Membach, Belgium. Applying a regional water storage model, the gravity contribution due to the elastic deformation of the Earth was derived. In addition, the Newtonian gravity effect induced by the local water mass variations was calculated, using soil moisture observations taken at the ground surface (about 48 m above the gravimeters). The computation of the gravimetric effect is based on a digital elevation model with spatially discretized rectangular prisms. The obtained results are compared with the observations of a superconducting gravimeter (SG). We find that the seasonal variations can be reasonably well predicted with the regional water storage model and the local Newtonian effects. Shorter-period effects depend on the local changes in hydrology. This result shows the sensitivity of SG observations to very local water storage changes. [less ▲]

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See detailSeasonal effect on vertical positioning by Satellite Laser Ranging and GPS on Absolute Gravity at the OCA geodetic station, Grasse, France
Nicolas, Joëlle; Nocquet, J.-M.; Van Camp, M. et al

in Geophysical Journal International (2006), 167(3), 1127-1137

We present a comparison of the vertical displacement monitored by independent techniques at the geodetic observatory of Grasse (France). Both Satellite Laser Ranging and Global Positioning System (GPS ... [more ▼]

We present a comparison of the vertical displacement monitored by independent techniques at the geodetic observatory of Grasse (France). Both Satellite Laser Ranging and Global Positioning System (GPS) vertical position time-series over the period 1998–2003 show a prominent annual signal with a magnitude of 5–6 mm and reaching a maximum every year in July. Results from 14 absolute gravity measurements are also discussed. We investigate the possible origin of the observed signal by comparing it with predictions from various local and regional contributions. GPS results from a local network indicate that the periodic annual elastic deformation of the ∼1270 m high karstic plateau due to local water storage loading does not exceed 1–2 mm. In contrast, a combination of global model prediction for atmospheric and hydrological loading explains more than 70 per cent of the annual and semi-annual observed signals. [less ▲]

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See detailA comparison of annual vertical crustal displacements from GPS and GRACE
van Dam, Tonie UL; Wahr, J.; Lavallée, David

in Journal of Geophysical Research (2006), 111

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity ... [more ▼]

We compare approximately 3 years of GPS height residuals (with respect to the International Terrestrial Reference Frame) with predictions of vertical surface displacements derived from the Gravity Recovery and Climate Experiment (GRACE) gravity fields for stations in Europe. An annual signal fit to the residual monthly heights, corrected for atmospheric pressure and barotropic ocean loading effects, should primarily represent surface displacements due to long-wavelength variations in water storage. A comparison of the annual height signal from GPS and GRACE over Europe indicates that at most sites, the annual signals do not agree in amplitude or phase. We find that unlike the annual signal predicted from GRACE, the annual signal in the GPS heights is not coherent over the region, displaying significant variability from site to site. Confidence in the GRACE data and the unlikely possibility of large-amplitude small- scale features in the load field not captured by the GRACE data leads us to conclude that some of the discrepancy between the GPS and GRACE observations is due to technique errors in the GPS data processing. This is evidenced by the fact that the disagreement between GPS and GRACE is largest at coastal sites, where mismodeling of the semidiurnal ocean tidal loading signal can result in spurious annual signals. [less ▲]

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See detailInternational Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2003
Francis, Olivier UL; van Dam, Tonie UL

Book published by Centre Européen de Géodynamique et de Séismologie (2006)

Detailed reference viewed: 64 (8 UL)
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See detailEffects of atmospheric pressure loading and seven-parameter transformations on estimates of geocenter motion and station heights from space geodetic observations
Tregoning, P.; van Dam, Tonie UL

in Journal of Geophysical Research (2005), 110

Variations in fluid loads such as the oceans and the atmosphere deform the surface of the Earth. The accuracy of station coordinates, in particular, heights, that can be estimated depends on how well one ... [more ▼]

Variations in fluid loads such as the oceans and the atmosphere deform the surface of the Earth. The accuracy of station coordinates, in particular, heights, that can be estimated depends on how well one can separate these surface deformations from the associated translational motion between the center of mass of the solid Earth and the total Earth (CM). We applied simulated atmospheric pressure loading effects to the coordinates of sites in the CM frame to explore to what level of accuracy both geocenter motion and accurate station coordinates can actually be recovered from geodetic analyses. We found that standard seven-parameter transformations (three rotations, three translations, scale) generally recover about 80% of the geocenter motion; however, the inclusion of a scale factor permits the aliasing of surface loading deformation, introducing scale errors of up to 0.3 ppb and daily height errors as large as 4 mm. This limits the geophysical studies that can be performed accurately using the results of geodetic analyses where the magnitudes of the signals are small (e.g., tectonic movement of tide gauges, uplift rates for interpreting glacial isostatic adjustment). The quality of the geodetic results is extremely sensitive to the number and distribution of sites used to estimate the transformations and becomes worse when regional (rather than global) sets of sites are used. If the scale factor parameter is omitted, then the amount of aliasing of surface loading effects is reduced considerably and more accurate site velocities and geocenter motion estimates are achieved. [less ▲]

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See detailEffect of gravitational consistency and mass conservation on seasonal surface mass loading models
Clarke, Peter J.; Lavallée, David A.; Blewitt, Geoffrey et al

in Geophysical Research Letters (2005), 32(L08306), 1-5

Increasingly, models of surface mass loads are used either to correct geodetic time coordinates by removing seasonal and other ‘‘noise’’, or for comparison with other geodetic parameters. However, models ... [more ▼]

Increasingly, models of surface mass loads are used either to correct geodetic time coordinates by removing seasonal and other ‘‘noise’’, or for comparison with other geodetic parameters. However, models of surface loading obtained by simply combining the mass redistribution due to individual phenomena will not in general be self- consistent, in that (i) the implied global water budget will not be mass-conserving, and (ii) the modelled sea level will not be an equipotential surface of Earth’s total gravity field. We force closure of the global water budget by allowing the ‘‘passive’’ ocean to change in mass. This medium-term passive ocean response will not be a uniform change in non- steric ocean surface height, but must necessarily be spatially variable to keep the ‘‘passive’’ ocean surface on an equipotential. Using existing load models, we demonstrate the effects of our consistency theory. Geocenter motion is amplified significantly, by up to 43%. [less ▲]

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See detailForcing of Polar Motion in the Chandler Frequency Band: A Contribution to Understanding Interannual Climate Variations
Plag, H.-P.; Chao, B. F.; Gross, R. S. et al

in Cahiers du Centre Européen de Géodynamique et de Séismologie (2005), 24

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See detailResults of the international comparison of absolute gravimeters in Walferdange (Luxembourg) of November 2003
Francis, Olivier UL; van Dam, Tonie UL; Amalvict, M. et al

in Jekeli, C.; Bastos, L.; Fernandes, J. (Eds.) Gravity, Geoid and Space Missions - GGSM 2004 (2005)

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See detailAtmospheric loading corrections applied to GPS data at the observation level
Tregoning, T.; van Dam, Tonie UL

in Geophysical Research Letters (2005), 32(L22310), 1-4

Space-geodetic techniques can detect elastic deformation of the Earth caused by atmospheric pressure loading (ATML). However, it has not yet been demonstrated whether these surface displacements should be ... [more ▼]

Space-geodetic techniques can detect elastic deformation of the Earth caused by atmospheric pressure loading (ATML). However, it has not yet been demonstrated whether these surface displacements should be accounted for at the time of reduction of the observations or by applying time-averaged values to the coordinates after the analysis of the observations. An analysis of the power spectral density of the ATML predicted vertical deformation shows that, aside from the diurnal and semi-diurnal periods, there is very little power in the sub-daily frequencies. The present tidal ATML models improve the analysis at sites near the equator but seem to degrade the height estimates elsewhere. The majority of the non-tidal deformation can be modelled by applying a daily-averaged correction to daily estimates of coordinates but a greater improvement in height RMS is found if non-tidal ATML is applied at the observation level. [less ▲]

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See detailSuperconducting gravimeters in seismology
Francis, Olivier UL; van Dam, Tonie UL

in Newsletter of the European-Mediterranean Seismological Centre (2004), 21

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See detailObserving and assessing non-tidal ocean loading using ocean, continuous GPS and gravity data in the Adriatic area
Zerbini, S.; Matonti, F.; Raicich, R. et al

in Geophysical Research Letters (2004), 31(L23609), 1-5

The effect of nontidal ocean loading (NTOL) is observed in the height series of four permanent GPS stations in the northern Adriatic. A validation of the ECCO model is performed by comparing model ... [more ▼]

The effect of nontidal ocean loading (NTOL) is observed in the height series of four permanent GPS stations in the northern Adriatic. A validation of the ECCO model is performed by comparing model estimates of sea-level anomalies from tide-gauges with TOPEX/ POSEIDON data, and ECCO model estimates of bottom pressure with those derived from temperature and salinity observations. The amplitudes of theECCO sea-level anomaly are found to be 1.4 times smaller than observations; bottom pressure is 2 times smaller. Using a Green’s functions approach to determine elastic deformations, the ECCO ocean bottom pressure is used to estimate surface displacements at the GPS sites. Model results were compared with the height series and with the observed NTOL effect. The height series and the predicted NTOL are highly correlated at all four stations. The analysis performed on superconducting gravimeter data at the Medicina station also shows high correlation. [less ▲]

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See detailProceedings of the Workshop: The Sate of GPS Vertical Positioning Precision: Separation of Earth Processes by Space Geodesy
van Dam, Tonie UL; Francis, Olivier UL

Book published by Centre Européen de Géodynamique et de Séismologie (2004)

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See detailIndication of the uplift of the Ardenne in long-term gravity variations in Membach (Belgium)
Francis, Olivier UL; Van Camp, Michel; van Dam, Tonie UL et al

in Geophysical Journal International (2004), 158(1), 346-352

We report on the results of 7 yr of collocated gravity observations made with an FG5 abso- lute (AG) gravimeter and a GWR C-Series superconducting gravimeter (SG) located at the Membach Geophysical ... [more ▼]

We report on the results of 7 yr of collocated gravity observations made with an FG5 abso- lute (AG) gravimeter and a GWR C-Series superconducting gravimeter (SG) located at the Membach Geophysical Station in eastern Belgium. The SG gravity residuals track changes in gravity periodically observed by the AG, at the microgal level. Further, in the SG resid- ual signal we distinguish a quasi-seasonal term that can be mostly explained by variations in local water storage effects. In the AG time-series we observe a small trend in the gravity of −0.6 ± 0.1 μGal yr−1 perhaps indicating that the Membach Station is being displaced up- wards by about 3.0 mm yr−1. An uplift of the region is confirmed by Global Positioning System (GPS) measurements performed 3 km away. We are able to explain the features in the gravity time-series in terms of water storage variability, post-glacial rebound and tectonic activity. [less ▲]

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See detailComparison between modeled and Observed Gravity Tidal Parameters at Ny-Alesund, Svalbard
Sato, Tadahiro; Tamura, Y; Matsumoto, K et al

in Francis, Olivier; van Dam, Tonie (Eds.) Proceedings of the Workshop: IMG-2002 Instrumentation and Metrology in Gravimetry (2003)

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See detailProceedings of the Workshop: IMGC-2002 Instrumentation and Metrology in Gravimetry
Francis, Olivier UL; van Dam, Tonie UL

Book published by Centre Européen de Géodynamique et de Séismologie (2003)

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See detailVertical crustal motion observed in the BIFROST project
Scherneck, Hans-Georg; Johansson, Jan M.; Koivula, Hannu et al

in Journal of Geodynamics (2003), 35(4-5), 425-441

This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations ... [more ▼]

This paper reports from investigations on the robustness of estimated rates of intraplate motion from the continuous GPS project BIFROST (Baseline Inferences from Fennoscandian Rebound Observations, Sealevel and Tectonics). We study loading effects due to ocean, atmosphere and hydrology and their impact on estimated rate parameters. We regularly find the admittance of a modelled perturbation at less than fifty percent of the full effect. We think that the finding relates to a difficult noise situation at all periods, and that a satisfying model for the dominating noise source has not been found yet. An additional reason for low admittance is found in the mapping process of the no-fiducial network solution into a conventional reference frame. [less ▲]

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See detailInvestigation of hydrological and atmospheric loading by space geodetic techniques
Schuh, H.; Eastermann, G.; Cretaux, J.-F. et al

in International Association of Geodesy Symposia (2003), 126

Observations of sea level can only be interpreted correctly if land motion in particular in terms of vertical deformation of coastal areas is taken into account. In the last decades space geodetic ... [more ▼]

Observations of sea level can only be interpreted correctly if land motion in particular in terms of vertical deformation of coastal areas is taken into account. In the last decades space geodetic techniques such as VLBI (Very Long Baseline Interferometry), SLR (Satellite Laser Ranging), the GPS (Global Positioning System), and Doris (Doppler Orbitography and Radio positioning Integrated by Satellite) have proved to be very powerful for determining displacements of points on the solid Earth. These can be modeled by using various geodynamical parameters, e.g. the Love and Shida numbers in the model of the solid Earth tides and site-dependent amplitudes and phases of the ocean loading models. Today, the small deformations associated with the response of the Earth to atmospheric and hydrological loading are of growing interest. These effects cause site-dependent vertical displacements with ranges up to ±30mm due to atmospheric pressure variations and due to mass redistribution in surface fluid envelopes, in particular in continental water reservoirs (soil moisture, snow, and groundwater). Several new global and regional models of soil moisture and snow depths are now available and can be validated by space geodetic techniques. This paper is intended to give a short overview about state-of-the-art of modeling loading effects. A short introduction to the Special Bureau for Loading within the Global Geophysical Fluid Center (GGFC) of the IERS will be given, too. Finally, it will be shown how the effects influence the results of high precision space geodetic measurements. The paper mainly concentrates on vertical crustal motions on seasonal and interannual time scales observed by VLBI and describes also some results obtained from Doris. [less ▲]

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See detailProcessing of the Absolute data of the ICAG01
Francis, Olivier UL; van Dam, Tonie UL

in Francis, Olivier; van Dam, Tonie (Eds.) Proceedings of the Workshop: IMG-2002 Instrumentation and Metrology in Gravimetry (2003)

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See detailGGFC Special Bureau for Loading: Current status and plans
van Dam, Tonie UL; Plag, H. P.; Francis, Olivier UL et al

in Richter, Bernd; Schwegmann, Wolfgang; Dick, Wolfgang R. (Eds.) Proceedings of the IERS Workshop on Combination Research and Global Geophysical Fluids (2003)

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See detailResults of the Sixth International Comparison of Absolute Gravimeters, ICAG-2001
Vitushkin, L.; Becker, M.; Jiang, Z. et al

in Metrologia (2002), 39(5), 407-427

The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sevres. Seventeen absolute gravimeters were used ... [more ▼]

The Sixth International Comparison of Absolute Gravimeters was held from 5 June to 28 August 2001 at the Bureau International des Poids et Mesures (BIPM), Sevres. Seventeen absolute gravimeters were used to make measurements at five sites of the BIPM gravity network. The vertical gravity gradients at the sites and the ties between them were also measured using seventeen relative gravimeters. For the first time the ties were also measured using absolute gravimeters. Various methods of processing the absolute and relative data were tested to calculate the results. The final results of ICAG-2001 are presented. The acceleration due to gravity at a height of 0.90 m is given as (980925701.2 ± 5.5) Gal and (980928018.8 ± 5.5) Gal for sites A and B, respectively, calculated using a combined adjustment of the absolute and relative data. [less ▲]

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