References of "van Dam, Tonie 50003245"
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See detailComment on "Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium" by A. Demoulin and A. Collignon
Camelbeeck, Thierry; Van Camp, Michel; Jongmans, Denis et al

in Journal of Geophysical Research (2002), 107(B11), 2281-2281

Comment on ‘‘Nature of the recent vertical ground movements inferred from high-precision leveling data in an intraplate setting: NE Ardenne, Belgium’’ by A. Demoulin and A. Collignon

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See detailThe New IERS Special Bureau for Loading (SBL)
van Dam, Tonie UL; Plag, Hans-Peter; Blewitt, Geoffrey et al

in International VLBI Service for Geodesy and Astrometry: General Meeting Proceedings (2002)

Currently, the establishment of the International Earth Rotation Service (IERS) Special Bureau for Loading (SBL) is in progress as part of the IERS Global Geophysical Fluids Center (GGFC). The main ... [more ▼]

Currently, the establishment of the International Earth Rotation Service (IERS) Special Bureau for Loading (SBL) is in progress as part of the IERS Global Geophysical Fluids Center (GGFC). The main purpose of the SBL is to provide reliable, consistent model predictions of loading signals that have been thoroughly tested and validated. The products will describe at least the surface deformation, gravity signal and geo-center variations due to the various surface loading processes in reference frames relevant for direct comparison with existing geodetic observing techniques. To achieve these goals, major scientific advances are required with respect to the Earth model, the theory and algorithms used to model deformations of the Earth as well as improvements in the observational data related to surface loading. [less ▲]

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See detailEvaluation of the precision of using absolute gravimeters to calibrate superconducting gravimeters
Francis, Olivier UL; van Dam, Tonie UL

in Metrologia (2002), 39(5), 485-488

We present an experiment in which four different FG5 absolute gravimeters (AG) were operated simultaneously alongside a superconducting relative gravimeter (SG). We demonstrate that 0.1 % precision can be ... [more ▼]

We present an experiment in which four different FG5 absolute gravimeters (AG) were operated simultaneously alongside a superconducting relative gravimeter (SG). We demonstrate that 0.1 % precision can be achieved on the calibration factor of the SG by comparison with AG measurements, independently of the FG5 instrument used for the calibration and of the offsets among the FG5 absolute values. This experiment demonstrates the robustness of using any FG5 absolute gravimeter to calibrate any SG. This result is of value to geoscientists analysing data from (a) globally distributed SGs which most probably have been calibrated using different instruments; or (b) any individual SG calibrated with different FG5 absolute gravimeters. [less ▲]

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See detailGravity Changes due to Continental Water Storage
van Dam, Tonie UL; Wahr, John M.; Milly, P. Chris D. et al

in Journal of the Geodetic Society of Japan (2001), 47(1), 249-254

Five years of global continental water storage variations are used to predict the effects of long-wavelength, long-period variability in water storage on gravity observations. At the sites of existing ... [more ▼]

Five years of global continental water storage variations are used to predict the effects of long-wavelength, long-period variability in water storage on gravity observations. At the sites of existing superconducting gravimeters, the modeled gravity changes have root-mean-square (RMS) values of as much as 7 mu Gals, with ranges of up to 22 mu Gals. Variations much larger than these values can be found over large regions the globe. We find that the gravity effects are predominantly annual in character. We also find that the modeled responses to water loading exhibit long-period variations that could be mistaken for secular tectonic trends when observed over a time span of a few years. [less ▲]

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See detailResults from the Fifth Internationl Comparison of Absolute Gravimeters, ICAG97
Robertsson, L.; Francis, Olivier UL; van Dam, Tonie UL et al

in Metrologia (2001), 38

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters ... [more ▼]

The fifth in the series of International Comparisons of Absolute Gravimeters (ICAG) was held at the Bureau International des Poids et Measures (BIPM) in November 1997. Fifteen absolute gravimeters participated in the comparison. The mean gravity value obtained at station A (0.9 m) at the BIPM was found to be 980 925 707.8 µGal with a standard uncertainty of 2.8 µGal. This is consistent with the results obtained in previous comparisons at this site. Conclusions based on the analysis of the present results and proposals for future activities are presented. [less ▲]

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See detailCrustal displacements due to continental water loading
van Dam, Tonie UL; Wahr, J.; Milly, P. C. D. et al

in Geophysical Research Letters (2001), 28(4), 651-654

The effects of long-wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root-mean-square (RMS ... [more ▼]

The effects of long-wavelength (>100 km), seasonal variability in continental water storage on vertical crustal motions are assessed. The modeled vertical displacements (ΔrM) have root-mean-square (RMS) values for 1994–1998 as large as 8 mm, with ranges up to 30 mm, and are predominantly annual in character. Regional strains are on the order of 20 nanostrain for tilt and 5 nanostrain for horizontal deformation. We compare ΔrM with observed Global Positioning System (GPS) heights (ΔrO) (which include adjustments to remove estimated effects of atmospheric pressure and annual tidal and non-tidal ocean loading) for 147 globally distributed sites. When the ΔrO time series are adjusted by ΔrM, their variances are reduced, on average, by an amount equal to the variance of the ΔrM. Of the ΔrO time series exhibiting a strong annual signal, more than half are found to have an annual harmonic that is in phase and of comparable amplitude with the annual harmonic in the ΔrM. The ΔrM time series exhibit long-period variations that could be mistaken for secular tectonic trends or postglacial rebound when observed over a time span of a few years. [less ▲]

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See detailAbout Time Variations of Gravity
Melchior, P.; van Dam, Tonie UL; Francis, Olivier UL et al

in Computational Seismology and Geodynamics (2001)

Advances in gravity instrumentation have allowed for the determination of the absolute acceleration of gravity to a precision of 3–5 μGal and observations of tidally driven changes in gravity on the order ... [more ▼]

Advances in gravity instrumentation have allowed for the determination of the absolute acceleration of gravity to a precision of 3–5 μGal and observations of tidally driven changes in gravity on the order of nanogals. With observations of gravity and changes in gravity at these levels of precision we are able to investigate problems such as the resonance of the Earth's liquid inner core, to discriminate between the various ocean tidal models, understand the effects of atmospheric pressure loading on gravity observations, and perhaps to measure ice mass changes in Greenland. In this paper, we report on some of our results using absolute and superconducting gravimeter data. We describe a project to establish a site for international comparisons of absolute gravimeters in Luxembourg. [less ▲]

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See detailGPS measurements of vertical crustal motion in Greenland
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(D24), 33755-33759

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component ... [more ▼]

We have analyzed 5 years of continuous Global Positioning System (GPS) measurements taken at Kellyville, just off the western margin of the ice sheet in southern Greenland. A fit to the vertical component gives a negative secular uplift rate of −5.8±1.0 mm/yr. A negative rate (i.e., a subsidence) is consistent with archeological and historical evidence that the surrounding region has been subsiding over the last 3 kyr. However, it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice prior to 4 ka years ago, which predict that Kellyville should be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting −7.8±3.6 mm/yr discrepancy is too large to be the result of loading from present-day changes in nearby ice. We show, instead, that it is consistent with independent suggestions that the western ice sheet margin in this region of Greenland may have advanced by ≈50 km during the past 3–4 kyr. [less ▲]

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See detailGeodetic measurements in Greenland and their implications
Wahr, John; van Dam, Tonie UL; Larson, Kristine et al

in Journal of Geophysical Research (2001), 106(B8), 16567-16581

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two ... [more ▼]

We describe results from an ongoing experiment in Greenland, in which we are using absolute gravity and continuous Global Positioning System (GPS) measurements to study vertical crustal motion at two locations along the edge of the ice sheet: Kellyville, located about one third of the way up the western ice margin, and Kulusuk, located along the eastern ice margin at about the same latitude as Kellyville. The GPS measurements suggest average crustal uplift rates of -5.8±1.0 mm/yr at Kellyville and -2.1±1.5 mm/yr at Kulusuk. There have not yet been enough absolute gravity occupations to permit useful secular gravity solutions at either location. The negative uplift rate at Kellyville is consistent with independent archeological and historical evidence that the southwestern edge of the continent has been subsiding over the last 3000 years, but it is inconsistent with estimates of the Earth's continuing viscoelastic response to melting ice during the early Holocene, which predict that Kellyville is likely to be uplifting, rather than subsiding, by 2.0±3.5 mm/yr. The resulting -7.8±3.6 mm/yr discrepancy between the observed and predicted uplift rates is too large to be caused by loading from present-day changes in nearby ice. However, it is consistent with independent suggestions that the western ice sheet margin in this region may have advanced by ≈50 km during the past 3000-4000 years. If this advance did occur and if the crustal subsidence it induces is not removed from altimeter measurements of Greenland ice sheet elevations, then the altimeter solutions could underestimate the true snow/ice thickness change by 5-10 mm/yr along portions of the western margin of the ice sheet. [less ▲]

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See detailGlobal Positioning System and Gravity Used to Study Greenland Ice
van Dam, Tonie UL; Larson, Kristine; Wahr, John et al

in Earth in Space (2001), 13(5), 1-16

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See detailAn International Reference Station for Inter-comparison of Absolute Gravimeters (ISIAG) in Walferdange, Luxembourg: The GRAVILUX Project
D'OREYE, N.; van Dam, Tonie UL; Francis, Olivier UL

in Bulletin d'Information du Bureau Gravimétrique International (2000), (86), 27-36

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See detailMeasuring postglacial rebound with GPS and absolute gravity
Larson, Kristine M.; van Dam, Tonie UL

in Geophysical Research Letters (2000), 27(23), 3925-3928

We compare vertical rates of deformation derived from continuous Global Positioning System (GPS) observations and episodic measurements of absolute gravity. We concentrate on 4 sites in a region of North ... [more ▼]

We compare vertical rates of deformation derived from continuous Global Positioning System (GPS) observations and episodic measurements of absolute gravity. We concentrate on 4 sites in a region of North America experiencing postglacial rebound. The rates of uplift from gravity and GPS agree within one standard deviation for all sites. The GPS vertical deformation rates are signi cantly more precise than the gravity rates, primarily because of the denser temporal spacing provided by continuous GPS tracking. We conclude that continuous GPS observations are more cost e cient and provide more precise estimates of vertical deformation rates than campaign style gravity observations where systematic errors are di cult to quantify. [less ▲]

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See detailUsing GPS and Gravity to Infer Ice Mass Changes in Greenland
van Dam, Tonie UL; Larson, Kristine; Wahr, John et al

in EOS (2000), 81(37), 421-427

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See detailNetwork of Superconducting Gravimeters Benefits a Number of Disciplines
Crossley, D.; Hinderer, J.; Casula, G. et al

in EOS (1999), 80(11), 121-126

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See detailSeasonal Motion in the Annapolis, Maryland GPS Monument
Schenewerk, M.; van Dam, Tonie UL; Nerem, Steven R.

in GPS Solutions (1999), 2(3), 41-49

The permanent GPS tracking site at Annapolis, MD shows a 7-mm seasonal signal primarily in its horizontal position. It is suggested that thermal expansion of the pier on which the antenna rests is the ... [more ▼]

The permanent GPS tracking site at Annapolis, MD shows a 7-mm seasonal signal primarily in its horizontal position. It is suggested that thermal expansion of the pier on which the antenna rests is the source of this motion. A simple numerical model of the pier reproduces the observed motion of the GPS antenna, lending credence to this hypothesis. Although adding an additional level of complexity, this motion is predictable and the site retains it s value for high precision monitoring. Although the arrangement of this GPS site it somewhat uncommon, these results emphasize the importance of the underlying antenna monumentation when measuring crustal motions. [less ▲]

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See detailModeling environmental loading effects, Invited, Proceedings EGGS, Ed. H.-P. Plag and S. Zerbini
van Dam, Tonie UL; Wahr, John

in Physics and Chemistry of the Earth (1998), 23

Temporal variations in the geographic distribution of atmospheric, hydrologic and oceanic mass load and deform the surface of the Earth. In many instances, the deformation is large enough to be detected ... [more ▼]

Temporal variations in the geographic distribution of atmospheric, hydrologic and oceanic mass load and deform the surface of the Earth. In many instances, the deformation is large enough to be detected with space based geodetic techniques as well as with terrestrial gravity observations. For example, atmospheric loading induced crustal deformations on the order of 20 mm are possible at high latitudes with accompanying changes in gravity of 6 μGals. Non-tidal ocean loading effects can typically cause 5 mm (2 mm root-mean-square, RMS) in vertical positioning at coastal geodetic sites with displacements of up to 10 mm possible. Deformation associated gravity changes are usually on the order of 2-3 μGals, however peak-to-peak changes of 5 μGals are also predicted. The effects of regional ground water variations on geodetic measurements are less well known. Model results indicate that annual changes in gravity and vertical positioning can be as large as 2 μGals and 5 mm for sites where there is significant annual snowfall. We present a review of work done to date to address these issues. [less ▲]

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See detailA detailed analysis of tropospheric effects on geodetic observations at TMGO
Schenewerk, M.; van Dam, Tonie UL; Sasagawa, G. et al

in Physics and Chemistry of the Earth (1998), 23(1), 103-106

Improvements in geodetic tools are making previously subtle effects significant. Two examples affecting GPS observations are atmospheric loading of the Earth's crust and the tropospheric delays ... [more ▼]

Improvements in geodetic tools are making previously subtle effects significant. Two examples affecting GPS observations are atmospheric loading of the Earth's crust and the tropospheric delays, specifically the wet component. Each measurement, tropospheric delays and site coordinates, requires unambiguous determination of the other to achieve the highest accuracy. Table Mountain Geophysical Observatory (TMGO) is a unique site where a long history of observations from two complementary techniques, GPS and superconducting gravimetry, have been accrued. In particular, the superconducting gravity measurements provide a unique baseline for evaluating GPS vertical estimates over a variety of time frames. Positional estimates for TMGO using these techniques will be compared. Tropospheric effects will be identified and discussed. The ability for GPS to make subdaily, daily, and long term vertical estimates will be evaluated. [less ▲]

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See detailChesapeake Bay subsidence monitored as wetlands loss continues
Nerem, R. S.; van Dam, Tonie UL; Schenewerk, M.

in EOS (1998), 79

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See detailTwo years of continuous measurements of tidal and nontidal variations of gravity in Boulder, Colorado
van Dam, Tonie UL; Francis, Olivier UL

in Geophysical Research Letters (1998), 25(3), 393-396

We report here on the results of an analysis of 2 years of data from NOAA’s superconducting gravimeter located at the Table Mountain Gravity Observatory in Boulder, Colorado. Observed tidal parameters ... [more ▼]

We report here on the results of an analysis of 2 years of data from NOAA’s superconducting gravimeter located at the Table Mountain Gravity Observatory in Boulder, Colorado. Observed tidal parameters, corrected for ocean loading effects, are compared with theoretical tidal parameters predicted for a non-hydrostatic inelastic Earth model and demonstrate excellent agreement. Tidal residuals, corrected for polar motion and a linear instrument drift are highly correlated with gravity changes measured by two absolute gravimeters over the same time period. The admittance to local pressure i s found to be -0.356 mGal/mbar. However, this admittance factor is found to be seasonally and frequency dependent. Correlations between rainfall events and gravity changes are observed. Attempts to model these gravity changes as exponential functions of time were unsuccessful. [less ▲]

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See detailSea-level rise studied in Chesapeake Bay as wetlands loss continues
Nerem, R. S.; Schenewerk, M.; van Dam, Tonie UL

in Earth in Space (1998), 10(12), 156-157

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