M. Van Camp

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.

Use of microtremor measurement for assessing site effects in Northern Belgium – interpretation of the observed intensity during the MS = 5.0 June 11 1938 earthquake

To provide quantitative information on site effects in the northern part of Belgium, forty-seven H/V microtremor measurements were performed with 5 second seismometers over an area of about 15.000 km2. Most of the results show a northward regular increase of the fundamental period in agreement with the augmentation of the Mesozoic and Cainozoic soft sedimentsthickness from a few meters 40 km south of Brussels to 900 m at the Netherlands-Belgium border. The measured resonance frequency values wereconsistent with theoretical computations performed at different sites onthe basis of existing information and shallow seismic experiments. At oneparticular site (Uccle) where borehole data were available, microtremor measurements using an array of four seismological stations with differentapertures allowed to obtain the low frequency part of the Rayleigh wave dispersion curve, extending the range covered by the analysis of surfacewaves artificially generated. The Vs profile derived from the surface waveinversion corroborates the 1 Hz natural frequency of the site. Comparison of these results with the macroseismic information concerning the MS = 5.0 1938 earthquake which occurred 50 km west of Brussels,confirmed the hypothesis that the geological structure of the Brabant massifis likely to control damage distribution during such an earthquake. Comparisonbetween the intensity map of the 1938 earthquake and the resonance period ofsediments obtained by our microtremor study shows a clear relation betweenthe two parameters. During the 1938 earthquake, site effects played a prominent role due to the dimension of the source whose corner frequency wasabout 1 Hz.

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

Results of the European Comparison of Absolute Gravimeters in Walferdange (Luxembourg) of November 2007

The second international comparison of absolute gravimeters was held in Walferdange, Grand Duchy of Luxembourg, in November 2007, in which twenty absolute gravimeters took part. A short description of the data processing and adjustments will be presented here and will be followed by the presentation of the results. Two different methods were applied to estimate the relative offsets between the gravimeters. We show that the results are equivalent as the uncertainties of both adjustments overlap. The absolute gravity meters agree with one another with a standard deviation of 2 μgal (1 gal = 1 cm/s2).

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

The results of an international comparison of absolute gravimeters held in Walferdange, Luxembourg, in November 2003 are presented here in detail. The absolute meters agreed with one another to within a standard deviation less than 2 μGal (1 Gal = 1 cm/s), where we have excluded the results from a single prototype instrument from the analysis. This result, represents the best agreement ever obtained in a comparison of absolute gravimeters. In addition, for the first time, we were able to quantify the effect of the operators on the instrument agreement. The result indicates that the contribution to the errors in the observations due to the operator are less than 1 μGal, i.e. within the observational errors. We also demonstrate that there are no systematic differences between observations taken with FG5’s incorporating the bulk interferometer and those using the fiber optic version of the interferometer.

Final report on the Seventh International Comparison of Absolute Gravimeters (ICAG 2005)

The Bureau International des Poids et Mesures (BIPM), S?vres, France, hosted the 7th International Comparison of Absolute Gravimeters (ICAG) and the associated Relative Gravity Campaign (RGC) from August to September 2005.ICAG 2005 was prepared and performed as a metrological pilot study, which aimed: To determine the gravity comparison reference values; To determine the offsets of the absolute gravimeters; and As a pilot study to accumulate experience for the CIPM Key Comparisons. This document presents a complete and extensive review of the technical protocol and data processing procedures. The 1st ICAG?RGC comparison was held at the BIPM in 1980?1981 and since then meetings have been organized every 4 years.In this paper, we present an overview of how the meeting was organized, the conditions of BIPM gravimetric sites, technical specifications, data processing strategy and an analysis of the final results. This 7th ICAG final report supersedes all previously published reports.Readings were obtained from participating instruments, 19 absolute gravimeters and 15 relative gravimeters. Precise levelling measurements were carried out and all measurements were performed on the BIPM micro-gravity network which was specifically designed for the comparison.

Experiment to evaluate crustal motions across the Ardenne and the Roer Graben (north-western Europe) using absolute gravity measurements

Absolute gravity measurements using the FG5-202 gravimeter have been conducted twice a year since September 1999 along a 120 km long profile across the Belgian Ardenne and the Roer Graben. As in this region there is a difference of an order of magnitude between the inferred vertical crustal movements deduced from the difference of levelling (about 1 mm/year) and geological information (0.1 mm/year), our goal is to better constrain the present-day deformation. The absolute gravity measurements are performed at Rochefort, Humain, Werpin, Manhay, Sprimont, Membach (Belgium) and Monschau, Julich and Bensberg (Germany). A complete description of the network is given, and specific difficulties at some stations are presented. The precision achieved, generally less than 2.0 µGal integrated over more than 24 h of observation, is quite satisfactory at all stations but Julich. The mining activities around Julich cause an increase in the noise up to 7 µGal integrated over more than 72 h. We also discuss possible causes of gravity variations in this region. At present, it is still not possible to detect any trend in the available data. However, after five years of observations, we should be able to detect or constrain any possible long-term trend with a limit of 2.5 mm/year, assuming that a height change of 1 mm causes a change in gravity of 0.2 µGal. To ensure that the absolute gravimeter is giving accurate results, it has been regularly compared with the superconducting gravimeter installed at the Membach reference station.

Separating climate‐induced mass transfers and instrumental effects from tectonic signal in repeated absolute gravity measurements

We estimate the signature of the climate-induced mass transfers in repeated absolute gravity measurements based on satellite gravimetric measurements from the Gravity Recovery and Climate Experiment (GRACE) mission. We show results at the globe scale and compare them with repeated absolute gravity (AG) time behavior in three zones where AG surveys have been published: Northwestern Europe, Canada, and Tibet. For 10 yearly campaigns, the uncertainties affecting the determination of a linear gravity rate of change range 3–4 nm/s^2/a in most cases, in the absence of instrumental artifacts. The results are consistent with what is observed for long-term repeated campaigns. We also discuss the possible artifact that can result from using short AG survey to determine the tectonic effects in a zone of high hydrological variability. We call into question the tectonic interpretation of several gravity changes reported from stations in Tibet, in particular the variation observed prior to the 2015 Gorkha earthquake.

Results of the Seventh International Comparison of Absolute Gravimeters ICAG-2005 at the Bureau International des Poids et Mesures, Sèvres

The International Comparison of Absolute Gravimeters ICAG-2005 was held at the Bureau International des Poids et Mesures (BIPM), Sevres, France in September 2005. The organization of ICAG-2005, measurement strategy, calculation and presentation of the results were described in a technical protocol pre-developed to the comparison. Nineteen absolute gravimeters carried out 96 series of measurements of free-fall acceleration g at the sites of the BIPM gravity network. The vertical gravity gradients were measured by relative gravimeters. For the first time the budgets of uncertainties were presented.