Hydrogeological investigations at the Membach station, Belgium and application to correct long periodic gravity variations

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 ▲]

Geocenter motions from GPS: A unified observation model

in Journal of Geophysical Research (2006), 111(B05), 1-66

We test a unified observation model for estimating surface-loading-induced geocenter motion using GPS. In principle, this model is more complete than current methods, since both the translation and ... [more ▼]

We test a unified observation model for estimating surface-loading-induced geocenter motion using GPS. In principle, this model is more complete than current methods, since both the translation and deformation of the network are modeled in a frame at the center of mass of the entire Earth system. Real and synthetic data for six different GPS analyses over the period 1997.25–2004.25 are used to (1) build a comprehensive appraisal of the errors and (2) compare this unified approach with the alternatives. The network shift approach is found to perform particularly poorly with GPS. Furthermore, erroneously estimating additional scale changes with this approach can suggest an apparently significant seasonal variation which is due to real loading. An alternative to the network shift approach involves modeling degree-1 and possibly higher-degree deformations of the solid Earth in a realization of the center of figure frame. This approach is shown to be more robust for unevenly distributed networks. We find that a unified approach gives the lowest formal error of geocenter motion, smaller differences from the true value when using synthetic data, the best agreement between five different GPS analyses, and the closest (submillimeter) agreement with the geocenter motion predicted from loading models and estimated using satellite laser ranging. For five different GPS analyses, best estimates of annual geocenter motion have a weighted root-mean-square agreement of 0.6, 0.6, and 0.8 mm in amplitude and 21°, 22°, and 22° in phase for x, y, and z, respectively. [less ▲]

International Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2003

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

Results of the international comparison of absolute gravimeters in Walferdange (Luxembourg) of November 2003

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

Effects of atmospheric pressure loading and seven-parameter transformations on estimates of geocenter motion and station heights from space geodetic observations

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 ▲]

Superconducting gravimeters in seismology

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

Indication of the uplift of the Ardenne in long-term gravity variations in Membach (Belgium)

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 ▲]

Comparison between modeled and Observed Gravity Tidal Parameters at Ny-Alesund, Svalbard

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

Investigation of hydrological and atmospheric loading by space geodetic techniques

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 ▲]

Processing of the Absolute data of the ICAG01

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