High-precision absolute gravimetry may be defined as obtaining repeatability of individual gravity station occupations with a scatter of ±3 µGal (3x10-8ms-2 from the station mean) or better. At sites which are free of temporal density variations and have low level microseismic noise, standard deviation of the individual, high-precision absolute gravity station values from the station mean is usually ±1.5 µGal or better [Peter et al., 1993, Sasagawa et al., 1993].
High-precision absolute gravity values have a number of theoretical and practical applications. Among the theoretical applications the most important are directed toward the refinement of models used to correct for environment-induced gravity effects. The improvement of Earth tide models, solid Earth response to tidal forces, the gravity effects due to ocean loading, and local and regional atmospheric attraction and loading are of key importance. Similarly, repeat measurements cannot be compared without appropriate models and corrections for groundwater table and soil moisture changes.
The use of high-precision absolute gravimetry for the detection of vertical crustal motion is both a theoretical and a practical application. Measurements of post-glacial rebound at a network of sites helps refining global post-glacial rebound models, which are then used to correct the measurements of long-term tidal trends at affected tide gauges. Vertical motion may be caused by men-induced effects, such as petroleum or water withdrawal from the ground, by global, plate tectonism related effects, or by post-glacial rebound. If the vertical land motion approaches several centimeters in a decade it can have significant effects on port facilities, levees, dams, and other structures, and could seriously threaten commerce and human lives.
Vertical land motion is also frequently associated with earthquakes. However, it is not known whether changes of stress or other crustal processes prior to certain earthquakes are accompanied or not by minute vertical motion as well. Monitoring potential vertical motion with high-precision gravimetry (continuous absolute or superconducting gravimetry) could provide further important clues to this important research area.
High-precision gravimetry could detect minute density variations, such as would accompany the emptying or recharge of aquifers, and could monitor motion of magma movement near currently active or some suspect dormant volcanoes. Both applications could have important societal impacts.
A nationally or globally distributed high-precision gravity reference network would serve commercial, navigation, and defense interests. Such network would help the calibration of high precision instruments, the definition of a more precise geoid, the calibration of relative gravity instruments, the definition of precise datums for relative gravity networks, the determination of orthometric heights, the monitoring of secular gravity variations, and it would provide comparison for other geodetic instrumentation, such as the Global Positioning System (GPS) and the Very Long Baseline Interferometry (VLBI).
The use of absolute gravimetry could provide specific improvements to relative gravity surveys. Because only few absolute gravimeters exist in the USA and in the world, most gravity monitoring in crustal dynamics studies involves the use of LaCoste and Romberg relative gravimeters. These instruments are subject to drift and tares, which are aggravated as the time and distance increase between observing stations. The use of absolute gravity stations intermixed with such relative gravity station monitoring network could provide significant accuracy improvement to the relative data. Long loop closures would not be necessary, as the relative measurements could be tied to the absolute stations without the need for returning to the starting point. Also, this combination of absolute and relative stations would reduce time and distance between reference stations, thus improving the attainable relative gravity measurement accuracy.
Relative gravimeters, such as the LaCoste and Romberg meters, are also in need of frequent instrument calibration for the determination of their scale constants. An important application of absolute gravity meters would be to place absolute gravity stations in strategic locations in the USA to establish several relative gravimeter calibration lines, allowing easy access to these by the research and exploration communities.
When absolute gravimetry is used for vertical crustal motion studies, absolute gravity results should be compared with high- precision leveling and, where available, with GPS and VLBI derived trends. With better than ±2 µGal repeatability, three to five repeat absolute gravity measurements in a three to five year period would generally be sufficient to derive a gravity trend with a sub-microgal precision. Without near-surface, crustal, or mantle density redistributions, the measured vertical gradient defines the relationship between gravity change and vertical motion (free-air relationship). To derive the ratio between gravity change and vertical motion when density changes do accompany vertical motion, careful comparisons of the absolute gravity trend with high-precision leveling, GPS, and VLBI (where available) are necessary to determine the exact ratio between gravity and elevation changes. Once this ratio is determined, absolute gravity can be used for monitoring vertical motion with a precision of better than 3 mm (<1 µGal).
The measurement of absolute gravity involves the measurement of fundamental standards, such as distance and time. The distance standard is defined by the wave length of the laser used to generate interference fringes during a measurement drop; the time standard used is a rubidium clock whose accuracy is based on atomic standards. Based on twice yearly calibration of the HeNe lasers used, and on other potential internal error sources, estimates of ±4 µGal and ±3 µGal were given as the absolute accuracy for the two different generation JILA instruments by Zumberge [1981] and Niebauer [1987], respectively. However, Klopping et al., [1991] reported that the systematic error terms related to the instrument's response to floor recoil are much larger than originally estimated, and in extreme cases may cause errors of around ń20 ęGal, if not measured and corrected for. Absolute accuracy of the latest FG5 AXIS instrument, using an iodine stabilized laser, is estimated at ±1.75 µGal. A large part of the systematic errors due to floor recoil have been eliminated by the FG5 optical path design (however, early measurements indicate that vibration of the air-vacuum interface between the interferometer and the dropping chamber require instrument response corrections in the order of around ń2 ęGal to be applied) [Sasagawa et al., 1993]. During international comparisons of absolute gravimeters of different designs, the measurements have a scatter of ±10 µGal on the same pier [Boulanger et al., 1991]. This suggests that either some instruments were not properly calibrated or adjusted, or were affected by systematic errors.
For most geophysical applications instrument repeatability is more important than absolute accuracy. At microseismically quiet sites, free from temporal density variations (such as groundwater table change), repeatabilities of better than ±3 µGal were reported [Torge et al., 1988, Peter et al., 1989, 1993]. The repeatability of the FG5 systems is better than ±1 µGal [Sasagawa et al., 1993]. In addition to improperly corrected or uncorrected temporal gravity changes, instrument adjustments and calibrations also affect repeatability. Repeated measurements at sites with low microseismic activity, built on bedrocks (so free from water-induced density variations), have been found most useful in checking for possible errors in instrument adjustment and calibration, which would result in a change of instrument bias.
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