viscous response to past ice mass changes. Computations of the temporal rate of change of
gravity in Antarctica based on the ICE-3G ice history model and the same solid earth
rheology used in the Laurentide calculation (above), predict a peak rate of over -3.5 J.lGal/yr
and rates of over -1.0 J.lGal/yr over a large area of western Antarctica (Figure 5). It has been
suggested that by measuring both gravity (using absolute gravimetry) and vertical
displacements (using GPS) in Antarctica and Greenland it is possible to separate the viscous
effects of past deglaciation from the elastic effects of present-day ice thickness changes (Wahr
et aI., 1995).
SUMMARY AND DISCUSSION
A precision of better than 0.4 J.lGal/yr in the measurement of the rate of change of gravity
(equivalent to about 2 mmlyr in height) has been demonstrated by eight years of annual
absolute gravity measurements at Churchill, Canada. Recent improvements in absolute gravity
instrumentation are expected to provide a precision equivalent to 1 mmlyr over similar
periods of time, depending on seasonal variations at the site.
The principal role of absolute gravity measurements is to provide observational constraints
at critical locations with respect to major intermediate-scale processes involving mass
redistribution on the Earth's surface. In most cases measurements should be made in
conjunction with geometric measurements of height. Repeated absolute gravity measurements
in Laurentia, Fennoscandia, Greenland, Antarctica and the Barents Sea with station spacings
of a few hundred kilometers are required to provide additional constraints on global
postglacial rebound and present-day ice mass effects. Improved models of these processes
are required to predict the long-term deformation of the ocean bottom for geodetic estimates
of ocean volume change. Combined absolute gravity and GPS measurements will also play
an important role in plate-boundary deformation and earthquake process studies. Absolute
gravity measurements provide an independent observational constraint which can be related
theoretically to geometric movement through a deformation model.
Regular measurements at global IAGBN stations with the new absolute gravity
instrumentation would be suitable for verifYing the stability of the geocenter at the equivalent
of the sub-centimeter level. However, estimates of global-scale temporal variations of the
Earth's gravity field from both satellite-tracking-derived, low-order harmonics and the global
IAGBN stations will contain aliased contributions from the intermediate-wavelength
processes described above. The challenge will be to correct the IAGBN station results for
intermediate-scale and local gravity effects without contaminating other global-scale, lowerdegree signals. It will be necessary to monitor the geometric heights of the IAGBN stations
by collocation with IGS stations or connection to the International Terrestrial Reference
Frame in order to be able to separate the vertical motion component and the massredistribution components of the gravity signal at each station. This will be particularly
important in studies of the stability of the geocentre (Mather et aI., 1977). Thus, regular
absolute gravity measurements at IAGBN stations will play an important role in the
maintenance of a self-consistent terrestrial reference system.
Acknowledgements. We gratefully acknowledge the Churchill absolute gravity data and
other assistance provided by G. Sasagawa, F. Klopping and the group at the NOAA Table
Mountain Gravity Observatory. We also acknowledge the support of the NOAA Global
Change Program Office. Geological Survey of Canada contribution number 27495.
27
gravity in Antarctica based on the ICE-3G ice history model and the same solid earth
rheology used in the Laurentide calculation (above), predict a peak rate of over -3.5 J.lGal/yr
and rates of over -1.0 J.lGal/yr over a large area of western Antarctica (Figure 5). It has been
suggested that by measuring both gravity (using absolute gravimetry) and vertical
displacements (using GPS) in Antarctica and Greenland it is possible to separate the viscous
effects of past deglaciation from the elastic effects of present-day ice thickness changes (Wahr
et aI., 1995).
SUMMARY AND DISCUSSION
A precision of better than 0.4 J.lGal/yr in the measurement of the rate of change of gravity
(equivalent to about 2 mmlyr in height) has been demonstrated by eight years of annual
absolute gravity measurements at Churchill, Canada. Recent improvements in absolute gravity
instrumentation are expected to provide a precision equivalent to 1 mmlyr over similar
periods of time, depending on seasonal variations at the site.
The principal role of absolute gravity measurements is to provide observational constraints
at critical locations with respect to major intermediate-scale processes involving mass
redistribution on the Earth's surface. In most cases measurements should be made in
conjunction with geometric measurements of height. Repeated absolute gravity measurements
in Laurentia, Fennoscandia, Greenland, Antarctica and the Barents Sea with station spacings
of a few hundred kilometers are required to provide additional constraints on global
postglacial rebound and present-day ice mass effects. Improved models of these processes
are required to predict the long-term deformation of the ocean bottom for geodetic estimates
of ocean volume change. Combined absolute gravity and GPS measurements will also play
an important role in plate-boundary deformation and earthquake process studies. Absolute
gravity measurements provide an independent observational constraint which can be related
theoretically to geometric movement through a deformation model.
Regular measurements at global IAGBN stations with the new absolute gravity
instrumentation would be suitable for verifYing the stability of the geocenter at the equivalent
of the sub-centimeter level. However, estimates of global-scale temporal variations of the
Earth's gravity field from both satellite-tracking-derived, low-order harmonics and the global
IAGBN stations will contain aliased contributions from the intermediate-wavelength
processes described above. The challenge will be to correct the IAGBN station results for
intermediate-scale and local gravity effects without contaminating other global-scale, lowerdegree signals. It will be necessary to monitor the geometric heights of the IAGBN stations
by collocation with IGS stations or connection to the International Terrestrial Reference
Frame in order to be able to separate the vertical motion component and the massredistribution components of the gravity signal at each station. This will be particularly
important in studies of the stability of the geocentre (Mather et aI., 1977). Thus, regular
absolute gravity measurements at IAGBN stations will play an important role in the
maintenance of a self-consistent terrestrial reference system.
Acknowledgements. We gratefully acknowledge the Churchill absolute gravity data and
other assistance provided by G. Sasagawa, F. Klopping and the group at the NOAA Table
Mountain Gravity Observatory. We also acknowledge the support of the NOAA Global
Change Program Office. Geological Survey of Canada contribution number 27495.
27