From David Crossley:
The SG is capable of recording temporal gravity variations from seconds
to years and thus the GGP has application to large number of scientific
tasks. As indicated above, at long periods (months - years), we highly
recommend the use of a SG supplemented by an Absolute Gravimeter to fully
characterise secular trends in gravity.
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Earth tides and the nearly diurnal free wobble :
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the estimation of precise tidal parameters (e.g. gravitational delta factors)
will allow the development of better models for correcting for ocean loading
phenomena. In addition, the stacking of global delta factors provides important
information on the diurnal free wobble of the Earth which is essential
for theoretical work on the structure of the Earth's core.
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core modes :
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the search for internal gravity waves in the Earth's liquid core necessitates
global, long-period, long-duration recordings to separate local gravity
variations from a global coherent signal. The detection of these waves
will give direct information on the mechanical equilibrium of the fluid
in the core, and thus information on the operation of the geodynamo.
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atmospheric interactions :
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stacking global gravity and pressure data is essential to clarify the nature
of the long period phenomena in the atmosphere and for evaluating the effects
of global atmospheric surface pressure and mass redistribution on the Earth's
gravity field.
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Earth rotation and polar motion :
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the measurement of the gravity effect of polar motion (orientation of the
Earth's rotation axis) requires a global coverage of stations. It should
be possible to continuously monitor the location of the rotation pole on
the time scale of minutes and therefore provide an independent verification
of the same measurement now made with space techniques; connections with
the International Earth Rotation Service (IERS) service here will be valuable.
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gravity changes due to tectonic motions :
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the monitoring of long-term changes due to tectonic motions, sea-level
changes affecting the survival of coastal cities, post-glacial uplift and
the deformation associated with active tectonic events.
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enhancing absolute gravity measurements :
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SGs are a valuable aid to international programmes for the determination
of absolute gravity values on a global scale as they provide a short-term,
relative gravity reference level.
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general research tool :
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a high quality, continuous global data set will be a valuable resource
for future geodetic and geophysical studies that involve the Earth's gravity.
There are important connections between the above goals and other scientific
programmes of national concern. In particular, the geodetic community clearly
recognises the importance of simultaneous geodetic (positional) information
and gravity changes at fiducial stations which contain very high quality
instrumentation. There are two primary connections :
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space techniques :
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Two space techniques which require detailed models of Earth deformation
are satellite tracking and Very Long Baseline Interferometry (VLBI). At
the proposed sub-centimeter level of accuracy, for projects in the 1990's
such as the current Satellite Laser Ranging (SLR) and the proposed Geodynamics
Laser Ranging System (GLRS) mission, precise knowledge of the Earth's dynamics,
including resonances in the liquid core, are required. A global net of
SG's will give the required information on dynamics of the liquid core.
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sea level changes :
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A satisfactory solution to the problem of defining the origins of sea-level
changes requires input from different sources. The necessity of differentiating
between the effects of height variations caused by post glacial rebound
or plate tectonics and changes in sea level resulting from global warming
demands the establishment of a global geodetic/geophysical observatory
network, such as FLINN (Fiducial Laboratories for an International Natural
Science Network), an IUGG-sponsored project initiated at the Coolfront
Workshop in 1989. A central feature of such a network is the monitoring
of the gravity field at a smaller group of fiducial stations equipped with
SGs as well as precise positioning instrumentation (e.g. SLR, VLBI or GPS)
and having accurate connections to the reference tide gauges.
Additionally, the SG's can be used for valuable local studies in the following
areas :
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seasonal effects :
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long-period seasonal (annual, semi-annual) components have been observed
in gravity variations at some single SG stations. These variations cannot
be successfully modeled without comparisons with other SG stations.
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earthquakes :
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a SG with a bandwidth of 1 second to several years is the only instrument
capable of monitoring both earthquake activity and tectonic motions. At
intermediate time scales the SG is the ideal instrument for detecting slow
and silent earthquakes.
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seismic normal modes :
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the SGs have excellent noise characteristics for the observations of the
Earth's normal mode spectrum following a moderate to large earthquake.
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geodesy :
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as indicated above, single SGs, if located at strategic geodetic sites,
can considerably enhance local models used to reduce VLBI and other precise
measurements and if located near the coast, would provide data for estimating
true local sea level changes.
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