Developments with Absolute Gravimetry

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Developments with Absolute Gravimetry

• Simon D.P. Williams, Trevor F. Baker,
• Daniel McLaughlin

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Carter reports on sea level monitoring
recommended the use of Absolute Gravity (AG) as a
complimentary (and independent) technique to CGPS
for VLM monitoring.
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Free fall the FG5 absolute gravimeter
Free falling mass
Interferometer

• At present, the fringes are produced from the
  light of a He-Ne laser (red light _at_ 633 nm)
• Free fall on 20 cm during 0.2 s
• 640.000 fringes, with frequency sweeping from 0
  to 6 MHz.
• In practice, one takes 1 fringe / 1000
• The time intervals between the occurrence of
  each fringe are measured by a Rb oscillator (or
  Cs if available)

Laser
The FG5 Gravimeter
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Why do we need both AG and CGPS?

• AG is absolute whereas CGPS is still really
  relative to the reference frame
• CGPS is less expensive per site and more robust
• CGPS is dependent on the US DoD and the IGS
  (cost?)
• AG is sensitive to mass changes as well as land
  movement (good and bad)
• AG is therefore a good calibration method for the
  more abundant GPS. It will also be useful for the
  new gravity missions.
• Other geodetic tools also important e.g SLR,
  VLBI, DORIS

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In Europe, various AG groups have measured at the
following sites near to tide gauges
Sweden, Spain, Ukraine, Russia, Poland,
Latvia, Croatia, Greece, Iceland, Norway,
Germany, France, UK, Belgium..
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AG Measurements at UK Tide Gauges
In the UK we have concentrated on AG measurements
at 3 UK tide gauges, considered to be the core UK
gauges, Newlyn, Lerwick and Aberdeen
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Methodology

• Measure for at least 3 days at each site, at
  least once per year (hopefully!)
• Data from each day are processed separately and
  correction made for solid-earth tides,
  ocean-loading effects, atmospheric pressure and
  polar motion.
• Single admittance factor and local pressure data
  are used to correct for atmospheric pressure.

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Intercomparisons
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Intercomparisons
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Results
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Post-glacial rebound
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Conclusions

• AG measurement spanning 5-10 years can be used to
  measure vertical land movements at tide gauges.
• Absolute Gravity is a useful complimentary
  technique to CGPS.
• Good site selection is essential to AG
  measurement success.
• Given a few more years of measurement, results
  can be used to test competing GIA models.