Bruno Meurers

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

[1] 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.

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

Like all the previous International Comparisons of Absolute Gravimeters (ICAGs) the sixth, ICAG-2001, was held at the Bureau International des Poids et Mesures (BIPM). Major improvements in the 2001 campaign were a new measurement strategy using the absolute gravimeters to measure the ties of the gravity network, new sites constructed at the BIPM, improved relative measurements of the ties and gravity gradients, and combined adjustment of the absolute and relative data, realized using new software with a novel data weighting and rejection scheme. The g-values at four sites of the BIPM were measured with an uncertainty of 6 μGal. Good agreement was obtained between the results of the absolute and relative measurements of the ties of the gravity network. The final mean gvalue obtained at the reference site A was 7 μGal less than that obtained in the previous comparison, ICAG-97.

Characterizing long-time scale hydrological effects on gravity for improved distinction of tectonic signals

The influence of the hydrological noise on repeated gravity measurements has been investigated on the basis of the time series of 18 superconducting gravimeters (SGs) and on predictions inferred from the Land Dynamics (LaD) world-Gascoyne land water-energy balances model. Presently, the global hydrologic models are not precise enough to fulfill the geodetic requirements and are not efficient enough to separate the hydrology from tectonic motion in the land-based gravity time series. However, although the LaD model predictions and the gravity observations present significant differences in the time domain, it is shown that they have similar amplitudes in the frequency domain in most of the cases. The time series of the Global Geodynamics Project make it possible to investigate phenomena of a few years in the best case. Given the similarity between the power spectral densities (PSDs) of the LaD model predictions and the SG measurements when taken at the same epoch, it makes sense to use the LaD model to study the spectral behavior of the hydrological effects down to the decadal time scale, which is not yet possible with land-based measurements. It is shown that the PSDs of the hydrological effects flattens at low frequency and is characterized by a generalized Gauss-Markov structure. With such a noise level, the time necessary to measure a gravity rate of change of 1 nm/s(2)/a, at the 1 sigma level should not extend any longer than 17 years at the locations where the hydrological effects play a major role

Superconducting Gravimeter Calibration by CoLocated Gravity Observations: Results from GWR C025

In autumn 2007 the superconducting gravimeter GWR C025 was transferred from Vienna (VI) to the new Conrad observatory (CO) 60 km SW of Vienna. It is one of few instruments which were operated at different stations. This aspect motivated a reanalysis of all calibration experiments at VI and CO, focused on drift and noise effects. Considering the drift even of absolute gravimeters in a common adjustment reduces the root mean square error of the averaged calibration factor essentially. Also spring type gravimeters have some potential to contribute to the SG calibration factor determination. The calibration factor of GWR C025 did not significantly change during the transfer from VI to CO. The final calibration factor is calculated as weighted average over in total 9 JILAg and FG5 experiments with an accuracy of better than ±0.5‰. The calibration factor is temporarily stable with maximum variation less than ±0.1‰. Based on these results the gravity time series of VI and CO have been analyzed. The respective amplitude factors for O1, K1, and M2 agree almost perfectly at both stations after correcting for ocean loading effects. The maximum deviation from the numbers provided by the nonhydrostatic-body-tide models DDW and MAT01 is 0.8‰.

Modelling of global mass effects in hydrology, atmosphere and oceans on surface gravity

We present a MatlabTM/Octave-based software tool mGlobe to compute the effect of atmospheric, continental water storage, and non-tidal ocean mass variations on surface gravity. These effects must be considered or reduced prior to any analysis of geophysical phenomena using observations of superconducting gravimeters. Contrary to the alternative providers, mGlobe allows the computation for an arbitrary location worldwide, supports a larger number of input models and offers more flexibility in terms of computation settings. The high number of supported models is important for assessment of model uncertainties. Discrepancies exceeding 75% were found. The continental water storage effect showed low sensitivity to spatial and temporal resolution. The deficient temporal resolution affects the non-tidal loading and atmospheric effects significantly. The same holds true for the influence of the spatial resolution on atmospheric effects. To compensate this effect, we introduce a site-specific correction factor based on differences between the real topography and models orography. HighlightsA comprehensive tool for the analysis of gravity effects induced by large scale mass variations.Default support of majority of freely available hydrological, oceanic and atmospheric models.Novel approach for reducing impact of low resolution of atmospheric models on surface gravity effect.

Geophysics From Terrestrial Time-Variable Gravity Measurements

In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. Plain Language Summary: In a context of global change and increased human vulnerability to terrestrial hazard, monitoring the Earth system is one of the key challenges of geoscience. In particular, terrestrial gravimetry, with its precision at the level of one part of a billion, allows the monitoring of many phenomena, from water resource availability to volcanic activity. This paper reviews the technique, its advantages and limitations, how it has been used in the Earth monitoring, and the next challenges to be met by ground gravimetry.

The reduction of hydrology-induced gravity variations at sites with insufficient hydrological instrumentation

The hydrology-induced gravity variation is a limiting factor in the study of geophysical phenomena with superconducting gravimeters. The goal of this paper is to analyse and reduce the hydrological effects on gravity at the Vienna (Austria) station that is a typical example of a site insufficiently equipped with hydro-meteorological sensors. The hydrological effects are studied in a local as well as a global scale. A new method for computing the local soil moisture effect is presented. This approach overcomes the lack of in situ soil moisture observations and utilizes gravity residuals in the calibration process of a local conceptual 1D soil moisture model. In addition, only a priori soil moisture variations, provided by a global hydrological model, in situ temperature, precipitation and snow height time series are required in this approach. The coupling of the calibration process to gravity residuals increases the sensitivity of the modelled soil moisture to corrections that are applied within the processing of the gravity observations. This is shown in this study using different global hydrological corrections. The differences between these corrections are reflected in the modelled soil moisture so that the total hydrological effect (local plus global) is almost identical. The total hydrological effects reduce the observed gravity variation by 30%. Moreover, both seasonal as well as shortterm variations clearly related to observed hydro-meteorological parameters are minimized. On the other hand, the sensitivity of the modelled soil moisture to gravity corrections implies that the long-term gravity residuals are not suitable for local hydrological studies unless the significant differences between the global hydrological corrections are resolved.

Comparison of Superconducting Gravimeter and CHAMP Satellite Derived Temporal Gravity Variations

The operational Superconducting Gravimeter (SG) network can play an important role for validation of satellite-derived temporal gravity field variations. A comparison shows a quite good agreement between SG and CHAMP results within their estimated error bars. It could be proved that the SG-derived temporal gravity variations are representative for a large area within the µgal accuracy, if the local gravity effects are removed.. The long-periodic tidal waves are well determined by ground measurements, therefore they can be applied as a reference for validation. For further validation, field SG measurements should be carried out in representative areas with large gravity variations (e.g. Amazon area).

Potential‐field continuation between irregular surfaces—Remarks on the method by Xia et al.

Xia et al. (1993) offer an excellent method for potential‐field continuation between irregular surfaces by applying the equivalent source technique. This method has proven to be the fastest and most stable procedure for solving the problem of reducing potential‐field data to a constant datum (e.g., Pail, 1995) as long as no sources exist between observation surface and the equivalent stratum. The authors suggest using special equations for the continuation of magnetic fields. Theoretically this is correct, but neither necessary nor well suited, because of the characteristics of the operator for magnetic fields applied in the wavenumber domain.

Atmospheric Corrections for Superconducting Gravimeters Using Operational Weather Models

Atmospheric pressure fluctuations are a major source of noise in precise gravimetric measurements and must be corrected carefully. A big portion of this effect can be eliminated using the local air pressure and a single admittance factor, which reduces up to 90–95 % of the atmospheric signal. However, modern superconducting gravimeters require an even better atmospheric correction if small signals are to be identified. For this task the three-dimensional modeling of atmospheric mass attraction based on operational numerical weather models has shown promising results. Similar strategies are realized and applied successfully for de-aliasing measurements of satellite gravity missions, such as GRACE (Gravity Recovery and Climate Experiment). In this study we show that such models, here called AGC (Atmospheric Gravity Coefficients), can also be used to correct atmospheric effects on superconducting gravimeter (SG). The Conrad Observatory near Vienna and the SG in Membach are taken as example stations for the SG corrections. The resulting residuals using AGC are in both cases smaller than the traditional single admittance or Green’s function approach and the performance can also be compared to the more sophisticated model used by ATMACS (Atmospheric Attraction Computation Service).