References of "van Dam, Tonie 50003245"
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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 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 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 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 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 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 detailUsing GPS and Gravity to Infer Ice Mass Changes in Greenland
van Dam, Tonie UL; Larson, Kristine; Wahr, John et al

in EOS : Transactions, American Geophysical Union (2000), 81(37), 421-427

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

in EOS : Transactions, American Geophysical Union (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 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 : Transactions, American Geophysical Union (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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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 detailPredictions of crustal deformation and of geoid and sea level variability caused by oceanic at atmospheric loading
van Dam, Tonie UL; Wahr, J.; Chao, Y. et al

in Geophysical Journal International (1997), 129(3), 507-517

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See detailAnalysis of data from the 1994 international comparison of absolute gravimeters with a single computational protocol
Sasagawa, G.; Klopping, F.; van Dam, Tonie UL

in Metrologia (1995), 32

Data from the 1994 International Comparison of Absolute Gravimeters were processed with the National Oceanic and Atmospheric Administration (NOAA) Geosciences Lab protocols and processing algorithms. The ... [more ▼]

Data from the 1994 International Comparison of Absolute Gravimeters were processed with the National Oceanic and Atmospheric Administration (NOAA) Geosciences Lab protocols and processing algorithms. The rootmean-square discrepancy between results obtained from individual groups and the NOAA processing is 2,3 μGal; the average discrepancy is 1,5 μGal. The sources of the discrepancies are probably (i) differences in the vertical gradients used for datum level transfers, (ii) inconsistencies between processing algorithms, and (iii) differences in data editing criteria. [less ▲]

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See detailDetection of atmospheric pressure loading using Very Long Baseline Interferometry measurements
van Dam, Tonie UL; Herring, T. A.

in Journal of Geophysical Research (1994), 99(B3), 4505-4518

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates ... [more ▼]

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates of these displacements are determined by convolving National Meteorological Center (NMC) global values of atmospheric surface pressure with Farrell's elastic Green's functions. An analysis of the distances between radio telescopes determined by very long baseline interferometry (VLBI) between 1984 and 1992 reveals that in many of the cases studied there is a significant contribution to baseline length change due to atmospheric pressure loading. Our analysis covers intersite distances of between 1000 and 10,000 km and is restricted to those baselines measured more than 100 times. Accounting for the load effects (after first removing a best fit slope) reduces the weighted root-mean-square (WRMS) scatter of the baseline length residuals on 11 of the 22 baselines investigated. The slight degradation observed in the WRMS scatter on the remaining baselines is largely consistent with the expected statistical fluctuations when a small correction is applied to a data set having a much larger random noise. The results from all baselines are consistent with ∼60% of the computed pressure contribution being present in the VLBI length determinations. Site dependent coefficients determined by fitting local pressure to the theoretical radial displacement are found to reproduce the deformation caused by the regional pressure to within 25% for most inland sites. The coefficients are less reliable at near coastal and island stations. [less ▲]

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See detailDetection of atmospheric pressure loading using the Global Positioning System
van Dam, Tonie UL; Blewitt, Geoffrey; Heflin, M.

in Journal of Geophysical Research (1994), 99(B12), 23939-23950

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in ... [more ▼]

Earth deformation signals caused by atmospheric pressure loading are detected in vertical position estimates at Global Positioning System (GPS) stations. Surface displacements due to changes in atmospheric pressure account for up to 24% of the total variance in the GPS height estimates. The detected loading signals are larger at higher latitudes where pressure variations are greatest; the largest effect is observed at Fairbanks, Alaska (latitude 65°), with a signal RMS of 5 mm. Out of 19 continuously operating GPS sites (with a mean of 281 daily solutions per site), 18 show a positive correlation between the GPS vertical estimates and the modeled loading displacements. Accounting for loading reduces the variance of the vertical station positions on 12 of the 19 sites investigated. Removing the modeled pressure loading from GPS determinations of baseline length for baselines longer than 6000 km reduces the variance on 73 of the 117 baselines investigated. The slight increase in variance for some of the sites and baselines is consistent with expected statistical fluctuations. The results from most stations are consistent with ∼65% of the modeled pressure load being found in the GPS vertical position measurements. Removing an annual signal from both the measured heights and the modeled load time series leaves this value unchanged. The source of the remaining discrepancy between the modeled and observed loading signal may be the result of (1) anisotropic effects in the Earth's loading response, (2) errors in GPS estimates of tropospheric delay, (3) errors in the surface pressure data, or (4) annual signals in the time series of loading and station heights. In addition, we find that using site dependent coefficients, determined by fitting local pressure to the modeled radial displacements, reduces the variance of the measured station heights as well as or better than using the global convolution sum. [less ▲]

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See detailAtmospheric load response of the oceans determined using Geosat data
van Dam, Tonie UL; Wahr, John

in Geophysical Journal International (1993), 113(1), 1-16

Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure ... [more ▼]

Approximately one year's worth of altimeter-derived sea-surface heights are compared with global sea-level pressure fields to verify the open ocean inverted barometer response (-1 cm mb-1). When pressure is fit to the sea-surface height along individual altimeter tracks, the response is found to be only 60–70 per cent of the theoretical response or approximately -0.6 to -0.7 cm mb-1. Fits at fixed geographic locations show a clear dependence on latitude. There is a steady decrease in the absolute value o the regression coefficient between 70° and 20°, and then an abrupt increase again closer to the equator. A simple error analysis demonstrates that errors in the pressure data would reduce the along-track regression values, as is observed, and could produce a similar latitude dependence. But, the errors are unlikely to be large enough to explain the entire departure from inverted barometer. We estimate that pressure errors are apt to perturb the along-track track results by no more than about 0.1-0.2 cm mb-1. The possibility that the remaining disagreement is due to a global coherence between wind- and pressure-driven sea-surface height variability is considered. Winds driven by the pressure gradients of synoptic storms induce a sea-surface height response that is opposite in direction to that caused by the pressure cell. the wind-driven response is estimated for a stationary storm over a homogeneous barotropic ocean and for a moving storm over a two-layer baroclinic ocean by modeling the pressure cell as an idealized Gaussian distribution. the model results indicate that the wind-induced sea-surface height depends on both the radius and the translational velocity of the pressure cell. But, the winds associated with storms moving at average speeds of 10 ms-1 are apt to lower the theoretical pressure response in the model by only approximately 0.1 cm mb-1. the surface stress associated with those winds has the same latitudinal trend between 70° and 20° as the regression coefficients. But, the response of the ocean to that stress does not appear to exhibit the same trend. Nevertheless, the abrupt change in the regression coefficients near the equator suggests the apparent non-inverted barometer response may reflect a real change in sea-surface height related to atmospheric forcing (though the results near the equator are not as well defined as those at higher latitudes). [less ▲]

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