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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)
10 days
1 day
100 days
Fractional Brownian noise k -1.25
Toward FOGM? (see poster)
Probably white AG instrumental noise 100 nms-2
Hz-0.5 _at_ 5 s period resolution
(1.0E4/(25))0.5 30 nm/s²

• White SG instrumental noise 2.2 nms-2 Hz-0.5
• e.g.
• _at_ 1 h period resolution (5/(23600)) 0.5
  0.03 nm/s²
• _at_ 100 s period resolution (5/(2100))0.5
  0.2 nm/s²

Van Camp et al., JGR, 2004
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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 gt 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).