Results of the Sixth International Comparison of Absolute Gravimeters, ICAG-2001

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 ▲]

Evaluation of the precision of using absolute gravimeters to calibrate superconducting gravimeters

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 ▲]

Geodetic measurements in Greenland and their implications

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 ▲]

Beyond Tides - Determination of Core Properties from Superconducting Gravimeter Observations

in Journal of the Geodetic Society of Japan (2001), 47(1), 364-372

Gravity Changes due to Continental Water Storage

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 ▲]

About Time Variations of Gravity

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 ▲]

Using GPS and Gravity to Infer Ice Mass Changes in Greenland

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

Results of relative gravimeter measurements at the ICAG97 intercomparison

in Bulletin d'information du Bureau Gravimétrique International (2000), (85), 61-72

Intercomparison between the FG5 202 and FGG 206 at the site of the superconducting gravimeter C021 in Membach (Belgium)

in Bulletin d'Information du Bureau Gravimétrique International (1999), (84), 36-39

Proper Usage of the ICET Data Bank - Comparison with theoretical applications on Earth Models

in Ducarme, Bernard; Pâquet, Paul (Eds.) Proceedings of the Thirteenth International Symposium on Earth Tides (1998)