3.04 Gravimetric Methods – Superconducting Gravity Meters

94 Superconducting Gravity Meters
Merriam et al. (2001) reported on a series of nine
calibrations to achieve a precision of 0.13%.
Several authors have engaged in extensive campaigns
to intercompare the instruments. Francis
(1997) reported on 47 days of recording involving a
large number of AG drops. A precision of about 0.1%
was achieved both by linear regression and by comparing
tidal amplitude factors. A significant finding of
this study was that the precision of calibration was
not significantly improved for times longer than 5
days, suggesting that this is an optimum period for
such a comparison. Similar results were found for the
C024 instrument in Boulder (Francis et al., 1998) that
agreed very well at a level of 0.08% with the absolute
calibration using an acceleration platform.
Okubo et al. (1997) performed tests using FG5
#107 at two of the Japanese SG sites, T007 in
Esashi and T011 in Matsushiro. Their results pointed
out that the internal consistency of the SG was 0.3
and 0.62 mGal for Esashi and Matsushiro, respectively,
even better than the error in the AG
residuals at the two sites (1.55 and 1.52 mGal); the
overall precision was 0.2%.
AsamplingofotherresultsofAGcalibrationsis
given in Table 5. Amalvict et al. (2001a, 2002) showed a
series of 28 different calibration (totaling almost half a
million drops) runs over a 4 year period for ST, with an
average length of 5 days each. The regression method
gave an overall precision of 0.04% with a repeatability
of 0.1%. Imanishi et al. (2002) presented a study of the
various ways to cross-calibrate the AG and SG using a
simple regression, tidal factors, and a combined method
(Table 5). They found that the SG is not the only
instrument subject to ‘drift’, and the AG showed a bias
of several mGal over the 1 month of the experiment.
Nevertheless, they were able to extract a very precise
calibration of about 0.04%, which approaches the best
calibrations by an acceleration platform method.
Finally, Falk et al. (2001) and Harnisch et al. (2002)
have given useful results on a number of SGs and
AGs, together with some acceleration platform calibration
results.
Note that for BH the two spheres have different
calibration factors – this is true of all dual-sphere
instruments. Also, MC is one of many stations that
have been visited by several different AGs. The
‘errors’ are generally the precision of a least-squares
fit, and are known to underestimate uncertainties
obtained from differing sets of measurements. Thus,
some SGs have been DC-calibrated to better than
0.05%, but others are probably closer to 0.15%.
Francis and Hendrickx (2001) turned the calibration
issue around and used an SG as the reference signal in
the calibration of a spring gravimeter. This is possible
due to the extremely low drift and high precision of the
SGs and produces good results, especially when the
variable drift of the spring gravimeter is modeled along
with the calibration, as in Meurers (2002).
3.04.2.4.3 Amplitude calibration, absolute
methods
3.04.2.4.3.(i) Calibration using a moving
mass Warburton et al. (1975) first described the use
of a heavy mass calibration experiment. They rolled
a 321 kg hollow steel sphere filled with mercury to a
fixed position under the SG, thus generating a 10 mGal
test signal. Within the accuracy of the various parameters,
the most critical being the exact distance
between the center of the test sphere and the niobium
sphere, they found a precision of 0.2%. In such an
experiment, other factors limiting accuracy are the
homogeneity of the sphere, the exact geometry of the
position of the sphere with respect to the niobium
sphere, as well as loading and tilting effects that are
induced by the heavy mass itself.
A similar calibration was done by Goodkind et al.
(1993), who used a moving mass system to test the
inverse square law of Newtonian gravity. A spherical
mass was moved vertically under the SG in a specially
designed chamber; an accuracy of 0.09% was achieved
for two different types of sphere materials. The ultimate
limit in this type of calibration is the uncertainty
in the gravitational constant, though at 0.01% it is
below the precision of the above calibrations.
Achilli et al. (1995) implemented a unique moving
mass system for the full-sized T015 SG in Brasimone
as part of a test for constancy of the gravitational
constant. Their ring, weighing 272 kg, is raised and
lowered around the SG from strong supports in the
roof, but a limited range of about 1 m reduced the
mass effect to 6.7 mGal. During two calibrations, they
found a precision of 0.2% and a repeatability of 0.3%.
3.04.2.4.3.(ii) Calibration using a moving
platform The idea of using an acceleration platform
for the calibration of spring gravimeters was used
successfully for LCR gravimeters by Van Ruymbeke
(1989). The idea was applied to SGs by Richter (1991),
who pioneered the development of what is now known
as ‘The Frankfurt Calibration Platform’. The SG
instrument design, in which the dewar and coldhead
are mounted separately, limits the maximum vertical
range through which the gravimeter can be lifted. For a