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NGS, October 2009

land-based gravity measurements using absolute and superconducting gravimeters. M. Van Camp 1 & O. Francis 2. 1 Royal Observatory of Belgium 2 U. Luxembourg. NGS, October 2009. Example of repeated AG measurements. 40 3 nm/s²/yr. Slow oscillations ? Cause: Hydrology? (see poster).

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NGS, October 2009

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  1. land-based gravity measurements using absolute and superconducting gravimeters M. Van Camp1 & O. Francis2 1Royal Observatory of Belgium 2U. Luxembourg NGS, October 2009

  2. Example of repeated AG measurements 403 nm/s²/yr Slow oscillations? Cause: Hydrology? (see poster) • Average (Jülich not included) : 1.21.4 nm/s²/yr • Subsidence of 0.60.7 mm/yr • (1 nm/s²  0.5 mm) (1 s)

  3. The Membach Geodynamic Station SG: continuously since 1995 AG: since 1996: 1 measurement /1 month

  4. PSDs of AG and SG (Membach) 100 days 10 days 1 day Fractional Brownian noise: k = -1.25 AG & SG provide same information Toward FOGM? (see poster) AG meets SG at 1 day Probably white AG instrumental noise : 100 nms-2 Hz-0.5 @ 5 s period : resolution = (1.0E4/(2*5))0.5 =30 nm/s² • White SG instrumental noise : 2.2 nms-2 Hz-0.5 • e.g. : • @ 1 h period : resolution = (5/(2*3600))0.5 = 0.03 nm/s² • @ 100 s period : resolution = (5/(2*100))0.5 = 0.2 nm/s² Van Camp et al., JGR, 2004

  5. Conclusions  Based on collocated SG/AG measurements : AG “set-up” noise ~16 nm/s²;  At periods longer than ~1-2 months : both the AG and SG tell the same story;  In Belgium and Germany : no significant gravity rates of change > 3 nm/s²/yr  1.5 mm/yr (2 s);   Hydrological effects should not prevent one to measure slow tectonic processes (unless climate changes?) (Van Camp et al., JGR, in review);  Worst case: 1 nm/s²/yr needs 16 years Conservative result based on 18 short SG time series (spanning 5-13 years); This may be revised when similar analyze performed on longer time series;    Measuring slow processes using AGs should not be hopeless (the AG profile in Belgium and Germany seems to confirm this).

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