3.04 Gravimetric Methods – Superconducting Gravity Meters

72 Superconducting Gravity Meters
weaker. This effect was first observed in 1981 when the
TT30 was installed in the cellar vault in the ROB. In
contrast, trains did not affect operation of the LCR
gravity meter operating in a nearby vault.
The SG is supplied with an automatic leveling
system consisting of two tiltmeters mounted orthogonally
on top of the gravimeter vacuum can and two
thermally activated levelers that are placed under
two of the dewar support points. After tilt-minimizing
the SG sensor, the tiltmeters are ‘aligned’ to the
same null by electronically setting their output voltages
to zero. In feedback, the tiltmeters continuously
adjust the power controlling the expansion of the
levelers to keep alignment better than 1 mrad. This
leveling precision is essential in gravity studies where
apparent tilt-induced gravity changes must be kept
less than 1 nGal (1 nGal ¼ 0.01 nm s 2 ). The tilt
minimum adjustment is made on initial installation
and checked every year or so by the operator. A
recent study (Iwano and Fukuda, 2004) on SG data
from the Syowa station shows the clear advantage of
the tilt compensation system in reducing the noise in
gravity, especially in the tidal range.
3.04.1.2.5 Sphere and sphere resonance
The sphere is a hollow superconducting shell that is
manufactured with a slight mass asymmetry so that it
has a preferred orientation when levitated. Various
manufacturing processes are discussed in Warburton
and Brinton (1995). A small hole is drilled on the top
of the sphere to allow the helium gas to enter and to
prevent a differential pressure from developing when
it is cooled to 4 K. Just as important, the volume of
the shell displaces 15 times less helium gas than the
volume of the sphere; so the hole reduces buoyancy
forces that result from changes in the surrounding
helium gas pressure.
When the gravimeter is tilted, particularly impulsively,
the horizontal displacement of the sphere
turns into an orbital motion (precession) with an
associated vertical component in the feedback output.
This mode appears as a sphere ‘resonance’ that
has a period of 60–120 s depending on the particular
instrument. In the absence of trapped magnetic fields
and helium gas in the chamber, the Q of this mode is
several thousand, so it is always excited making the
instrument not usable. Slow damping of the mode is
provided by adding helium gas to the chamber, but
the resonance remains underdamped and is clearly
visible in some of the instruments’ data. By comparison,
the vertical resonance of the sphere is heavily
damped with a period close to 1 s. Further technical
details on the instrument design can be found in
Goodkind (1991, 1999).
3.04.1.3 Development of the Dual-Sphere
Design
In the early commercial SGs manufactured up to
1990, offsets (or ‘tares’) occurred in the SG gravity
records that affected both long-term stability and
measurement of tidal factors (Seama et al., 1993;
Harnisch and Harnisch, 1995; Hinderer et al., 1994).
The offsets could be quite large (up to 100 mGal) if
caused by mechanical shock from transferring liquid
helium, power failures, or earthquakes. Small instrumental
offsets less than 5 mGal could occur at random
intervals that were not associated with outside disturbances.
Rapid offsets larger than 0.2 mGal and
occurring in less than 1 min could be easily detected
and corrected (Merriam et al., 2001). However, there
was a concern that the residual data would depend
arbitrarily on the threshold value chosen in automatic
offset detection programs (Harnisch and
Harnisch, 1997). When two SGs were operated side
by side, the difference in recordings provided a much
clearer determination of the occurrence of offsets
(Klopping et al., 1995).
On the basis that random offsets will seldom occur
in two sensors simultaneously, a dual-sphere SG was
manufactured to solve the instrument offset problem
(Richter and Warburton, 1998). The two spheres are
mounted one above the other and separated by about
20 cm. The lower sensor is manufactured exactly like
previous single-sphere sensors, and the temperature
and tilt control remain the same. Small differences in
the sphere masses, superconducting shield, coil windings,
and machining tolerances produce magnetic
asymmetries that are not identical in the two sensors.
These asymmetries produce slightly different tilt
minima and require more complicated electronics
to align the tilt minimum of the upper sensor with
the lower sensor.
The complications of a dual-sphere system are
justified by providing a built-in instrumental offset
detector. Because the outputs are treated as signals
from two different gravimeters, the user can combine
the processed data sets, select the least disturbed
sphere output for any one time period, or convert
the two signals into a gravity gradient by using the
known vertical separation. CD029 was the first dualsphere
SG produced and it was tested at Bad
Homburg beginning July 1998 before being moved