HR: 0800h
AN: G21A-0112    [Abstracts]
TI: Accuracy of absolute gravity measurements when measuring crustal deformation
AU: * Van Camp, M
EM: mvc@oma.be
AF: Royal Observatory of Belgium, Avenue Circulaire 3, Bruxelles, BE-1180 Belgium
AU: Williams, S D
EM: sdwil@pol.ac.uk
AF: Proudman Oceanographic Laboratory, Joseph Proudman Building, 6 Brownlow Street , Liverpool, L35DA United Kingdom
AU: Francis, O
EM: olivier@ecgs.lu
AF: Universit‚ de Luxembourg, rue Coudenhove Kalergi, 6, Luxembourg, LU-1359 Luxembourg
AU: Camelbeeck, T
EM: thierry.camelbeeck@oma.be
AF: Royal Observatory of Belgium, Avenue Circulaire 3, Bruxelles, BE-1180 Belgium
AB: To observe secular land movements of the order of a few millimetres per year, a very precise instrument with long-term stability is required. This can be achieved using absolute gravimeters, which do not depend on a reference frame. Vertical land movements would modify the gravity at a rate of about -10 nms-2 (1 $\mu$Gal) for 5 mm of uplift. Repeated absolute gravity (AG) measurements have now been performed at several sites for five or more years to constrain, for example, tectonic deformation and post-glacial rebound. It is often assumed that these time series contain only white noise. However, many geodetic data sets have now provided evidence for error sources that introduce large temporal correlations into the data. One common statistical model for many types of geophysical signal (which may contribute to the noise) maybe described as a power-law process. Accounting for the type of noise is very important when estimating gravity variations and their uncertainties. Moreover this can contribute to identify the noise source(s) affecting AG measurements We present an analysis of the error model using 10 years of AG data taken at the Proudman Oceanographic Laboratory (POL) and using 96 AG gravity measurements recorded over a period of 8 years at the Membach station (Belgium). A superconducting gravimeter (SG) is also continuously monitoring gravity variations at this site. The AG set-up white noise is estimated by comparison with the SG series. The coloured environmental noise is estimated using the Maximum Likelihood Estimation (MLE) technique to fit two types of stochastic model to the SG time series, power-law noise and first order Gauss Markov (FOGM) noise. The gravity rate of change and the associated uncertainties as a function of the noise structure are computed. Then we investigate the noise of AG values at frequencies higher than 1 cpd, where a white noise component usually dominates. Finally the POL and Membach experiments are applied to estimate the uncertainties of AG campaigns repeated once or twice a year to monitor crustal deformation. The results from repeated AG campaigns along a profile across the Ardenne and along the UK coastline are presented. Such repeated AG measurements should allow one to constrain gravity rate of change with an uncertainty of 1 nms-2 (or 0.5 mm) after 8 to 15 years. The conclusion is that long-term measurements using absolute gravimeters are appropriate for monitoring slow vertical tectonic deformation.
DE: 1200 GEODESY AND GRAVITY
DE: 1206 Crustal movements--interplate (8155)
DE: 1208 Crustal movements--intraplate (8110)
DE: 1244 Standards and absolute measurements
DE: 1294 Instruments and techniques
SC: Geodesy [G]
MN: 2004 AGU Fall Meeting